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Heino, M., Baulier, L., Boukal, D. S., Ernande, B., Johnston, F. D., Mollet, F. M., Pardoe, H., Therkildsen, N. O., Uusi-Heikkilä, S., Vainikka, A., Arlinghaus, R., Dankel, D. J., Dunlop, E. S., Eikeset, A. M., Enberg, K., Engelhard G. H., Jørgensen, C., Laugen, A. T., Matsumura, S., Nusslé, S., Urbach, D., Whitlock, R., Rijnsdorp, A. D., and Dieckmann, U. 2013. Can fisheries-induced evolution shift reference points for fisheries management? – ICES Journal of Marine Science, 70: 707–721. Biological reference points are important tools for fisheries management. Reference points are not static, but may change when a population's environment or the population itself changes. Fisheries-induced evolution is one mechanism that can alter population characteristics, leading to “shifting” reference points by modifying the underlying biological processes or by changing the perception of a fishery system. The former causes changes in “true” reference points, whereas the latter is caused by changes in the yardsticks used to quantify a system's status. Unaccounted shifts of either kind imply that reference points gradually lose their intended meaning. This can lead to increased precaution, which is safe, but potentially costly. Shifts can also occur in more perilous directions, such that actual risks are greater than anticipated. Our qualitative analysis suggests that all commonly used reference points are susceptible to shifting through fisheries-induced evolution, including the limit and “precautionary” reference points for spawning-stock biomass, Blim and Bpa, and the target reference point for fishing mortality, F0.1. Our findings call for increased awareness of fisheries-induced changes and highlight the value of always basing reference points on adequately updated information, to capture all changes in the biological processes that drive fish population dynamics.

Turbot and brill are widely distributed in the Northeast Atlantic but occur at low abundance. They are ecologically very similar and closely related. The low abundance and the similarities make them particularly interesting to study the population dynamics because it raises the questions how the populations can sustain themselves at low abundances and how turbot and brill avoid strong interspecific competition. Knowledge of both species is hampered by lack of analysed data. The main objective of this study is therefore to increase the knowledge of turbot and brill and in particular to compare the two species in order to address the above questions. Based on biological samples collected in the North Sea, we calculated seasonal von Bertalanffy growth parameters, maturity ogives, monthly gonado-somatic indices (GSI) and condition factors (Fulton's K) and indices of inter- and intraspecific mean crowding and compared the results for turbot and brill. The main differences between the two species were found in their spawning period, with brill having a more protracted spawning period. Brill also showed an earlier peak in their GSI values, suggesting an earlier start of their spawning period. The mean crowding showed that interspecific competition was lower than intraspecific competition. The exploitation pattern was also studied. Turbot and brill are exploited as a bycatch species in the mixed demersal fishery. We found that productivity is highest in areas where the maximum temperature is close to the optimal temperature for growth (16–18 °C) and landings decrease where salinity falls below ~ 5 psu (turbot) and ~ 15 psu (brill). Recent fishing mortality rates of North Sea turbot are around 0.5–0.7, but there is no indication that recruitment is impaired at low levels of spawning stock biomass. We conclude that although both species have similar ecological characteristics, differences may reduce inter-specific competition.

Dover sole (Solea solea) is an obligate ectotherm with a natural thermal habitat ranging from approximately 5 to 27°C. Thermal optima for growth lie in the range of 20 to 25°C. More precise information on thermal optima for growth is needed for cost-effective Dover sole aquaculture. The main objective of this study was to determine the optimal growth temperature of juvenile Dover sole (Solea solea) and in addition to test the hypothesis that the final preferendum equals the optimal growth temperature. Temperature preference was measured in a circular preference chamber for Dover sole acclimated to 18, 22 and 28°C. Optimal growth temperature was measured by rearing Dover sole at 19, 22, 25 and 28°C. The optimal growth temperature resulting from this growth experiment was 22.7°C for Dover sole with a size between 30 to 50 g. The temperature preferred by juvenile Dover sole increases with acclimation temperature and exceeds the optimal temperature for growth. A final preferendum could not be detected. Although a confounding effect of behavioural fever on temperature preference could not be entirely excluded, thermal preference and thermal optima for physiological processes seem to be unrelated in Dover sole.

Bottom trawls are a globally used fishing gear that physically disturb the seabed and kill non-target organisms, including those that are food for the targeted fish species. There are indications that ensuing changes to the benthic invertebrate community may increase the availability of food and promote growth and even fisheries yield of target fish species. If and how this occurs is the subject of ongoing debate, with evidence both in favour and against. We model the effects of trawling on a simple ecosystem of benthivorous fish and two food populations (benthos), susceptible and resistant to trawling. We show that the ecosystem response to trawling depends on whether the abundance of benthos is top-down or bottom-up controlled. Fishing may result in higher fish abundance, higher (maximum sustainable) yield and increased persistence of fish when the benthos which is the best-quality fish food is also more resistant to trawling. These positive effects occur in bottom-up controlled systems and systems with limited impact of fish feeding on benthos, resembling bottom-up control. Fishing leads to lower yields and fish persistence in all configurations where susceptible benthos are more profitable prey. Our results highlight the importance of mechanistic ecosystem knowledge as a requirement for successful management.

Annual length increments of female North Sea plaice were back-calculated from distances between rings in otoliths. Growth of the smaller size classes ( 35 cm) increased from 1970. Growth changes of the smaller size classes were significantly correlated with indices of plaice density, eutrophication, and seabed disturbance by beam trawling. Moreover, they could be related to spatial and temporal patterns in variations in eutrophication and beam trawling. No correlation was observed with temperature. The analysis led to a consistent interpretation suggesting that eutrophication and beam trawling have both affected the growth rate of plaice. The contributions of these factors differed in space. Effects of eutrophication dominated in the shallow coastal waters, whereas beam trawling dominated in the waters further offshore.

This paper summarizes the developments in flatfish recruitment studies over the last decade with emphasis on the general patterns that have emerged from the contributions to the two Flatfish Symposia of 1990 and 1993. Recruitment variability is largely generated by density-independent factors acting during the pelagic egg and larval phases. Effects of variability generating processes tend to be amplified towards the edges of the distribution range and appear to be related to abiotic conditions. Density-dependent feedback processes occur in the demersal juvenile phase, when flatfish become highly specialized benthic feeding fish. There is evidence that density-dependent feedback processes may also occur during the adult phase in not or lightly exploited populations. Areas of future research that emerge from this symposium are: 1. the classification of flatfish populations in ‘ecological equivalents’, including the habitat requirements of the successive life history stages; 2. comparative studies among species and populations of recruitment processes; 3. analysis of environmental factors determining the survival of pelagic eggs and larvae; 4. density-dependent habitat selection of demersal juveniles in relation with growth and mortality; 5. analysis of the habitat characteristics of species for pelagic eggs and larvae, demersal juveniles and adults, including drift of eggs and larvae, transport from spawning towards nursery areas and adult migration; and 6. simulation studies of relevant population dynamical processes to explore quantitatively the importance of the various processes and the necessary precision with which these should be known.

Blooms of the marine microalga Fibrocapsa japonica (Raphidophyceae) are suggested to lead to the diminution of fish stocks, but the exact mechanism(s) involved in ichthyotoxicity is still unclear. In the present study fish tests were performed, using larvae of the common flat fish sole (Solea solea), to further investigate properties and variability of F. japonica toxicity. Sole larvae were directly and indirectly exposed to a density range of a Dutch Wadden Sea F. japonica strain, as well as F. japonica cell extracts, for a maximum of 7 days, during which larval mortality was monitored. Most experiments were done at 16 degrees C, however some experiments were performed at 20 degrees C. Furthermore, larvae were exposed to concentration ranges of the alleged toxic compounds, e.g. the brevetoxins PbTx-2 and PbTx-3, and three poly-unsaturated fatty acids (PUFAs): M, EPA and OTA.The larvae showed highly variable responses to the F. japonica treatments. At the late exponential growth phase of F. japonica highest sole mortality was found: 100% mortality after 7 days of direct exposure. Indirect and cell extract exposures as well as direct exposure to other growth phases caused lower larval mortality (up to 40%) after the same incubation period. Temperature had a distinct effect on sole mortality, showing significantly enhanced toxicity at 20 degrees C, compared with 16 degrees C. Exposure to PbTx-2 showed an LC50 of 28.7 ng mL(-1) after 48 h, while exposure to PbTx-3 caused no 100% mortality up to 10 ng mL(-1). Exposure to pure PUFAs showed the fastest mortality observed in all experiments, with an LC50 ofOur research suggests pronounced ichthyotoxicity in late exponential F. japonica cells, related primarily with the combination of endo- and exotoxins, with most probably haemolytic PUFAs as the main endotoxins, and other haemolysins and/or ROS as the main exotoxins. Implications are given for present and future risks in the already warming Dutch coastal waters. (C) 2012 Elsevier B.V. All rights reserved.

European anchovy Engraulis encrasicolus increased its abundance and distribution in the North Sea during the mid-1990s and may consume similar zooplankton to and/or compete with other occupants of the North Sea like herring Clupea harengus and sprat Sprattus sprattus. The diets of adult anchovy, sprat and juvenile herring of comparable sizes, sampled close in time and space, were compared to understand how the 3 species prey on zooplankton and establish whether their diets overlap or not. Anchovy was found to be more generalist, consuming a higher diversity of prey items. Herring was more specialized, with low diversity of food items. Sprat was intermediate between anchovy and herring. The dietary overlap between anchovy and sprat was highest, followed by herring and sprat before anchovy and herring. The mean weight of stomach contents did not differ between species. We conclude that of the 3 species, anchovy is likely to be the least affected by changing plankton communities.

As a contribution to the ecosystem approach to fisheries management, we estimated the effects of spawning closures on stock status, ecosystem impacts and economic performance. We focused on the flatfish fishery in the North Sea and explored how spawning closures for plaice and sole contribute to sustainable management of 4 target species (sole, plaice, turbot and brill). Seasonal patterns in fishing effort and catchability by age group and area were estimated to quan- tify the effect of different spawning closure scenarios on the selection pattern. The scenario per- formance was evaluated using indicators of stock status (spawning stock biomass), economic per- formance of the fishery (yield, revenue) and ecosystem impact (discards, bycatch of cod and rays, seabed integrity, fisheries-induced evolution). In a single-species context, spawning closures may be beneficial for the target species, while in a mixed fisheries and ecosystem context, negative effects may occur. A spawning closure for plaice combines positive effects on the plaice stock and the revenue with reductions of the negative impact for several ecosystem indicators and only a small negative effect on sea bed integrity. The effects did not differ when evaluated at current lev- els of effort or at maximum sustainable yield (MSY) effort. Tailor-made solutions are required that need to be developed in stakeholder consultation to trade-off the ecological and economic objec- tives. Mixed-species MSY was lower than the sum of the single-species MSYs.

Concern about the impact of fishing on ecosystems and fisheries production is increasing (1, 2). Strategies to reduce these impacts while addressing the growing need for food security (3) include increasing selectivity (1, 2): capturing species, sexes, and sizes in proportions that differ from their occurrence in the ecosystem. Increasing evidence suggests that more selective fishing neither maximizes production nor minimizes impacts (4–7). Balanced harvesting would more effectively mitigate adverse ecological effects of fishing while supporting sustainable fisheries. This strategy, which challenges present management paradigms, distributes a moderate mortality from fishing across the widest possible range of species, stocks, and sizes in an eco-system, in proportion to their natural productivity (8), so that the relative size and species composition is maintained. Summary: http://www.sciencemag.org/cgi/content/summary/335/6072/1045?ijkey=cjKQs4eSejTM.&keytype=ref&siteid=sci Full Text: http://www.sciencemag.org/cgi/content/full/335/6072/1045?ijkey=cjKQs4eSejTM.&keytype=ref&siteid=sci

Egg size of the common sole Solea solea (L.) was studied from ichthyoplankton surveys conducted between 1987 and 1991 in seven different areas ranging from the Bristol Channel and western English Channel to the German Bight of the North Sea, encompassing central as well as peripheral parts of the distribution range of the species. Egg size showed a seasonal decline of 0.00179 mm·d −1 . Egg size at peak spawning decreased from 1.37 mm in central areas in the south-west to 1.13 mm in peripheral areas in the northeast, corresponding to a decrease in egg volume of 44%. The difference in egg size between areas coincided with differences in the timing of spawning, but a significant small geographical difference in egg size remained after removing the seasonal effect. Egg size was largest in the southwestern areas and decreased to the northeast. The close relationship between spawning time, egg size and egg number (fecundity) suggest that these are three aspects of a common process of ovarian development which is influenced by the photoperiod and temperature. Egg size increased between fertilization and hatching, suggesting that the mortality process is size-selective with a higher mortality rate for smaller eggs. A negative relationship between egg mortality and egg size was observed among the seven sole stocks studied and also in a study of literature data on egg mortality rate of various species spawning in the North Sea and adjacent areas. The ecological implications of these results are discussed.

Flatfish support major fisheries and the study of regulatory processes are of paramount importance for evaluating the resilience of the resource to exploitation. This paper reviews the evidence for processes operating during the adult phase that may 1. generate interannual variability in recruitment; 2. contribute to population regulation through density-dependent growth, density-dependent ripening of adults and density-dependent egg production. With regard to (1), there is evidence that in the adult phase processes do occur that may generate recruitment variability through variation in size-specific fecundity, contraction of spawning season, reduction in egg quality, change in sex ratio and size composition of the adult population. However, time series of recruitment do not provide support for this hypothesis. With regard to (2), there is ample evidence that exploitation of flatfish coincides with an increase in growth, although the mechanisms involved are not always clear. The presence of density-dependent growth in the adult phase of unexploited populations appears to be the most likely explanation in some cases. From the early years of exploitation of flatfish stocks inhabiting cold waters, evidence exists that adult fish do not spawn each year. Fecundity schedules show annual variations, but the available information suggests that size-specific fecundity is stable over a broad range of population abundance and may only decrease at high population abundance. The analysis is complicated by the possibility of a trade-off between egg numbers and egg size. Nevertheless, a density-dependent decrease in growth will automatically result in a decrease in absolute fecundity because of the reduced body size. The potential contribution of these regulatory effects on population regulation is explored. Results indicate that density-dependent ripening and absolute fecundity, mediated through density-dependent growth, may control recruitment at high levels of population abundance. The effect of a density-dependent decrease in size-specific fecundity seems to play a minor role, although this role may become important at extremely high levels of population abundance.

Ulf Dieckmann, Mikko Heino, and Adriaan Rijnsdorp suggest that we are incurring a "Darwinian debt" that will have to be repaid by future fishers and consumers.

Heavy fishing and other anthropogenic influences can have profound impact on a species' resilience to harvesting. Besides the decrease in the census and effective population size, strong declines in mature adults and recruiting individuals may lead to almost irreversible genetic changes in life-history traits. Here, we investigated the evolution of genetic diversity and effective population size in the heavily exploited sole (Solea solea), through the analysis of historical DNA from a collection of 1379 sole otoliths dating back from 1957. Despite documented shifts in life-history traits, neutral genetic diversity inferred from 11 microsatellite markers showed a remarkable stability over a period of 50 years of heavy fishing. Using simulations and corrections for fisheries induced demographic variation, both single-sample estimates and temporal estimates of effective population size (N(e) ) were always higher than 1000, suggesting that despite the severe census size decrease over a 50-year period of harvesting, genetic drift is probably not strong enough to significantly decrease the neutral diversity of this species in the North Sea. However, the inferred ratio of effective population size to the census size (N(e) /N(c) ) appears very small (10(-5) ), suggesting that overall only a low proportion of adults contribute to the next generation. The high N(e) level together with the low N(e) /N(c) ratio is probably caused by a combination of an equalized reproductive output of younger cohorts, a decrease in generation time and a large variance in reproductive success typical for marine species. Because strong evolutionary changes in age and size at first maturation have been observed for sole, changes in adaptive genetic variation should be further monitored to detect the evolutionary consequences of human-induced selection.

During an effort-regulated period from 1996 to 2002, unregistered annual increases of 0.3% of hooks fished per day were demonstrated for the Faroe Islands longline fishery. However, annual increases were higher (1.5%) during a preceding total allowable catch regulated period, thereby invalidating an expectation of the 1996 shift in regulations (output to input control) to have induced increases in the number of hooks set per day. Underlying this result is a substantial increase in total yearly effort (fishing days) and a shift in targeting behaviour from secondary to primary (high value) target species in response to the transition from output to input control. Interview data on technology were combined with logbook data and analysed with generalized linear modelling to demonstrate haddock ( Melanogrammus aeglefinus ) and Atlantic cod ( Gadus morhua ) catch-per-unit-effort increases of 51% and 26%, respectively, following the introduction of skewed hooks and swivel line. The technological introductions were not correlated to regulation shifts. So, rather than the management system in force, an ongoing technological development seems to be the principal driver of fishing power trends. The results highlight the need to explicitly address technological development and targeting behaviour when attempting to meet conservation objectives through input control of fisheries.

Patch exploitation dynamics, based on individual tow data, provided new insights into the fishing behavior of mixed fisheries. Fishing grounds were determined and their geometry quantified based on the proximity of tow positions. Tows were classified as being part of either searching, sampling, or exploitation behavior based on the intertow distance. Fishers can detect patches of flatfish on a scale of ∼25 nautical miles2. Catch rate during exploitation was 24%–36% above the catch rate while searching, and decreased at a rate of 20%·day–1. Once a patch was found, exploitation occurred until the catch rate dropped below a threshold level. The optimal giving-up catch rate was estimated based on the observed search time, catch rate decline, and range of fishing ground quality. The observed giving-up catch rate was 6%–11% higher than predicted by the marginal value theorem. The discrepancy between the observed and predicted patch leaving decision was consistent with the bias expected in an individual transferable quota (ITQ) management system. Our results provide a basis for interpreting vessel monitoring system (VMS) data and studying the interaction among fishers and between fishers and their resources at the appropriate time and spatial scale.

ter Hofstede, R., and Rijnsdorp, A. D. 2011. Comparing demersal fish assemblages between periods of contrasting climate and fishing pressure. – ICES Journal of Marine Science, 68: 1189–1198. Fish communities are dynamic and their structure is known to change over time. Traditionally, these changes were considered to be fisheries-induced, but recent analyses also suggest that global warming could affect the distribution, abundance, and assemblage composition of marine fish. However, disentangling the effects of fisheries and those resulting from climate change is difficult, because both potential drivers act simultaneously. In our study, we distinguished between the effects of fisheries and climate change on the fish assemblage of the southern North Sea by comparing survey catch data for that region during four unique periods throughout the past century, characterized by (i) low fishing pressure during a cold period (1902–1908), (ii) low fishing pressure during a warm period (1950–1956), (iii) high fishing pressure during a cold period (1978–1984), and (iv) high fishing pressure during a warm period (2002–2008). Our analysis indicates that the demersal fish community in the southern North Sea has changed in response to changes in both climate and fishing pressure. Our results suggest both a relatively higher richness of Lusitanian (warm-favouring) species compared with boreal (cool-favouring) species, and a lower mean body size of the fish community during times of warming, independent of fishing pressure.

The relationship between juvenile growth and the onset of sexual maturation of female North Sea plaice, Pleuronectes platessa, is explored on the individual and population level by analysing individual growth curves back-calculated from otoliths of immature and mature fish and by analysing maturity–length ogives of individual cohorts sampled between 1960 and 1990 from commercial landings from three areas in the North Sea. Back-calculated growth rate of females sampled as mature fish was higher than that of immature females up to 3 yr prior to sampling but lower in the last year. The probability of maturation was significantly related to the length reached as well as to the growth rate in previous years. Maturity–length relationships from market samples showed significant differences in slopes and intercepts between age groups, geographical areas, and year classes. Length at maturation (Lmat) decreased with age and increased with latitude. The differences in Lmatof individual year-classes corresponded to differences in the growth rate and temperature during early life. Both the evidence obtained from the individual and population analysis suggest a 2- to 3-yr maturation time. A model of the juvenile–adult transition process is proposed that provides a consistent interpretation of the observations.

Poos, J. J., Bogaards, J. A., Quirijns, F. J., Gillis, D. M., and Rijnsdorp, A. D. 2010. Individual quotas, fishing effort allocation, and over-quota discarding in mixed fisheries. – ICES Journal of Marine Science, 67: 323–333. Many fisheries are managed by total allowable catches (TACs) and a substantial part by individual quotas. Such output management has not been successful in mixed fisheries when fishers continue to fish while discarding marketable fish. We analyse the effects of individual quotas on spatial and temporal effort allocation and over-quota discarding in a multispecies fishery. Using a spatially explicit dynamic-state variable model, the optimal fishing strategy of fishers constrained by annual individual quotas, facing uncertainty in catch rates, is studied. Individual fishers will move away from areas with high catches of the restricted quota species and, depending on the cost of fishing, will stop fishing in certain periods of the year. Individual vessels will discard marketable fish, but only after their individual quota for the species under consideration has been reached. These results are in line with observations on effort allocation and discarding of marketable fish, both over-quota discarding and highgrading, by the Dutch beam-trawl fleet. The models we present can be used to predict the outcomes of management and are therefore a useful tool for fisheries scientists and managers.

We present a new methodology to estimate rates of energy acquisition, maintenance, reproductive investment and the onset of maturation (four-trait estimation) by fitting an energy allocation model to individual growth trajectories. The accuracy and precision of the method is evaluated on simulated growth trajectories. In the deterministic case, all life history parameters are well estimated with negligible bias over realistic parameter ranges. Adding environmental variability reduces precision, causes the maintenance and reproductive investment to be confounded with a negative error correlation, and tends, if strong, to result in an underestimation of the energy acquisition and maintenance and an overestimation of the age and size at the onset of maturation. Assuming a priori incorrect allometric scaling exponents also leads to a general but fairly predictable bias. To avoid confounding in applications we propose to assume a constant maintenance (three-trait estimation), which can be obtained by fitting reproductive investment simultaneously to size at age on population data. The results become qualitatively more robust but the improvement of the estimate of the onset of maturation is not significant. When applied to growth curves back-calculated from otoliths of female North Sea plaice Pleuronectes platessa, the four-trait and three-trait estimation produced estimates for the onset of maturation very similar to those obtained by direct observation. The correlations between life-history traits match expectations. We discuss the potential of the methodology in studies of the ecology and evolution of life history parameters in wild populations.

Regional warming causes changes in local communities due to species extinctions and latitu- dinal range shifts. We show that the species richness of fish in 3 regional seas in the eastern North Atlantic Ocean has changed over time (1997 to 2008), and we relate this to higher water temperatures and the biogeographic affinity of the species. In the North and Celtic Seas, species richness increased due to in- creases in the number of warm-favouring Lusitanian species. In the area west of Scotland, species rich- ness decreased because the number of cold-favouring Boreal species decreased. Additional analyses of trends in fishing effort imply that the observed changes in species richness are unlikely to have been induced by fisheries in the North and Celtic Seas, thereby strengthening the idea that climate change affects spe- cies richness of marine fish. However, in the area west of Scotland, a potential effect of fisheries in addition to temperature change on the observed change in spe- cies richness could not be ruled out.

Rijnsdorp, A. D., van Damme, C. J. G., and Witthames, P. R. 2010. Implications of fisheries-induced changes in stock structure and reproductive potential for stock recovery of a sex-dimorphic species, North Sea plaice. – ICES Journal of Marine Science, 67: 1931–1938. A key assumption in stock assessment and stock forecasts often is that spawning-stock biomass (SSB) and egg production are proportional and that the reproductive potential is independent of stock structure (age composition and sex ratio). Based on a 60-year time-series of total egg production (TEP) of North Sea plaice, we demonstrate that this assumption could result in a biased perception of the temporal trend in reproductive potential. The time-series incorporates: (i) annual observations on maturity, growth, and condition, (ii) a predictive model for interannual variations in fecundity caused by variations in body condition and by the probability of being a recruit spawner, and (iii) a cohort analysis of sex-specific landings-at-age since 1948. Following an increase in fishing mortality rate, TEP declined by a factor of 7–8 from a peak in the 1970s to a minimum in 1999–2000. Concurrent with this decline, the contribution of recruit spawners and the size difference between spawning males and females decreased. The implications of phenotypic plasticity and fisheries-induced evolutionary changes in growth and maturation for the recovery potential of the plaice stock are discussed.

Poos, J. J., Quirijns, F. J., and Rijnsdorp, A. D. 2010. Spatial segregation among fishing vessels in a multispecies fishery. – ICES Journal of Marine Science, 67: 155–164. Individual decisions of fishers on where to fish in heterogeneous environments may shape the relation between fishing effort and fishing mortality. Fishers may be viewed as individual foragers, whose decisions are aimed at optimizing short-term gain, as in ideal free distribution (IFD) theory. Although IFD assumes all foragers to be equal, they are likely to differ in competitive abilities for different prey types or target species. Here, we present an IFD-like model of a fishing fleet consisting of two components with different competitive abilities for two target species, showing that spatial segregation can result from unequal interference competition, but also in the absence of interference competition. Differences in catch efficiency between vessels for target species in combination with differences in the spatial distribution of target species can result in spatial segregation among vessel groups. The theoretical results are used to interpret the observed spatial segregation of two components within the Dutch beam trawl fleet using observations from a vessel monitoring by satellite system. However, this study cannot pinpoint which of the processes included in the theoretical model is the prime cause of the segregation within the Dutch beam trawl fleet.

The level of recruitment and its variability were analysed in sole stocks using results of virtual population analysis (VPA) of commercial landings and pre-recruit surveys. Stocks were studied in the North Sea, eastern and western English Channel, Bristol Channel and the Irish Sea, Variability of recruitment increased from 34–55% in the central areas to 97–127% in the northern ones: the Irish Sea and the North Sea. Winter temperatures explained part of the additional variability in recruitment in the North Sea but not in the Irish Sea. In the North Sea level of recruitment appeared to be determined before the end of the first year of life, although additional mortality of older fish during cold winters may further reduce recruitment. In general there was no similarity in the pattern of recruitment variability between populations, except in adjacent populations, suggesting that the factors determining recruitment differ substantially over rather small distances. The level of recruitment in the five populations was related to the surface area of the nursery grounds.

This is a pre-copy-editing, author-produced PDF of an article accepted for publication in ICES Journal of Marine Science following peer review. The definitive publisher-authenticated version Bogaards, J. A., Kraak, S. B. M., and Rijnsdorp, A. D. 2009. Bayesian survey-based assessment of North Sea plaice (Pleuronectes platessa): extracting integrated signals from multiple surveys. – ICES Journal of Marine Science, 66: 665–679 is available online at: http://icesjms.oxfordjournals.org/content/66/4/665, peer-reviewed, Dependence on a relatively small sample size is generally viewed as a big disadvantage for survey-based assessments. We propose an integrated catch-at-age model for research vessel data derived from multiple surveys, and illustrate its utility in estimating trends in North Sea plaice abundance and fishing mortality. Parameter estimates were obtained by Bayesian analysis, which allows for estimation of uncertainty in model parameters attributable to measurement error. Model results indicated constant fishing selectivity over the distribution area of the North Sea plaice stock, with decreased selectivity at older age. Whereas separate analyses of survey datasets suggested different biomass trends in the southeast than in the western and central North Sea, a combined analysis demonstrated that the observations in both subareas were compatible and that SSB has been increasing over the period 1996- 2005. The annual proportion of fish that dispersed in a northwesterly direction was estimated to increase from about 10% at age 2 to 33% at age 5 and older. We also found higher fishing mortality rates than reported in ICES assessments, which could be the consequence of inadequate specification of catchability-at-age in this study or underestimated fishing mortality by the conventional ICES assessment, which relies on official landings figures.

Atlantic herring ( Clupea harengus ) have contrasting spawning strategies, with apparently genetically similar fish “choosing” different spawning seasons, different egg sizes, and different spawning areas. In the North Sea, both autumn- and winter-spawning herring share the same summer feeding area but have different spawning areas. Females of both spawning types start their oocyte development in April–May. Oocyte development is influenced by the body energy content; during the maturation cycle, fecundity is down-regulated through atresia in relation to the actual body condition. Hence, fecundity estimates must account for the relative time of sampling. The down-regulation over the whole maturation period is approximately 20% in autumn- and 50% in winter-spawning herring. The development of the oocytes is the same for both spawning strategies until autumn when autumn spawners spawn a larger number of small eggs. In winter spawners, oocyte development and down-regulation of fecundity continues, resulting in larger eggs and lower number spawned. In theory, autumn and winter spawners could therefore switch spawning strategies, indicating a high level of reproductive plasticity.

The changes in growth of plaice and sole between 1957 and 1988, as estimated from samples of the commercial fishery and pre-recruit surveys, were analysed in order to study possible density dependent effects. Indices of potentially competitive biomasses of plaice and sole, based on Lloyd's index of mean crowding, were estimated from the average spatial distribution of various age groups during the summer growing period and from the population age structure as estimated by virtual population analysis. Growth of all age groups of sole increased in the 1960s and was stable in the 1970s and 1980s. In plaice only age groups 1 to 3 showed a similar increase in the 1960s, whereas the growth of 1-year-old plaice tended to decrease in the 1980s. Growth did not show a negative correlation with mean crowding, except in age group 1 of plaice and in age group 3+ of sole. It is concluded that these negative correlations do not provide unequivocal evidence for density dependent growth in plaice and sole, since they could equally well be caused by parallel but unrelated trends in time of one or more other factors. The simultaneous increase in growth in the 1960s of age groups of sole and plaice in the southern North Sea, and the absence of such an increase in age groups in the central North Sea, suggests that food availability must have increased in the Southern North Sea. This inference is supported by several macrobenthos studies. Whether the reduced growth of 1-group plaice in the 1980s, when recruitment was well above the average level, is caused by density dependent growth or to a reduced food availability remains an open question.

Shallow waters along the North Sea coast provide nursery areas for juveniles of commercially exploited species and natural habitat for resident species and seasonal visitors. The areas have gone through major changes in the last decades due to climate change and human activities such as fishing and eutrophication and changes in abundance of apex predators. Using a long-term dataset we present trends from 1970 to 2006 in 34 fish species in three coastal areas in the Netherlands: the Dutch Wadden Sea, the Westerschelde and the Dutch coastal zone. The patterns varied widely among individual species as well as between the three areas. Total fish biomass showed a dome shape pattern with an increase from 1970 to 1985 and a subsequent decline until the early 2000s. Based on multivariate and time series analyses we explore possible correlations of fish density with a predefined set of three categories of environmental variables: abiotic, biotic and fisheries related variables. Dynamic factor analysis (DFA) identified one common trend for every area: for the Wadden Sea and Westerschelde increasing from the 1970s to the early 1980s followed by a steep decrease until the mid 1990s, a temporary period (until 2002) of increase for the Wadden Sea, and a continuing increase for the Westerschelde. The common trend in the Dutch coastal zone shows a similar increase but a time lag compared to the estuarine areas, while the distinct decline was absent here. The species that showed the strongest correlation with this common trend differed between the areas, and explains the difference between the common trend in the coastal zone with that in the estuarine areas. Common trends were best described by models containing variables from all categories of environmental variables (only maximum 2 tested at a time).

The effect of rising seawater temperature on growth of 0-group sole Solea solea and plaice Pleuronectes platessa in the southeastern North Sea was investigated for the period 1970 to 2004 using annual autumn pre-recruit survey data and frequent surveys on a nursery ground. Autumn length showed an increasing trend in sole but not in plaice. Increasing winter temperatures significantly increased the growing period of sole, a warm-water species that spawns in spring, but not of plaice, a temperate species that spawns in winter. Growth rate increased with higher summer temperatures in sole and to a lesser degree in plaice. Compared to experimental growth rates at ambient temperatures and unlimited food, observed growth rates were close to experimental values until mid-June but were much lower in July to September, suggesting food limitation in summer. The higher temperatures observed since 1989 positively affected the quality of the shallow coastal waters as a nursery area for sole but not for plaice. A further increase may negatively affect the nursery quality if the production rate of benthic food cannot meet the increase in energy requirements of 0-group flatfish.The effect of rising seawater temperature on growth of 0-group sole Solea solea and plaice Pleuronectes platessa in the southeastern North Sea was investigated for the period 1970 to 2004 using annual autumn pre-recruit survey data and frequent surveys on a nursery ground. Autumn length showed an increasing trend in sole but not in plaice. Increasing winter temperatures significantly increased the growing period of sole, a warm-water species that spawns in spring, but not of plaice, a temperate species that spawns in winter. Growth rate increased with higher summer temperatures in sole and to a lesser degree in plaice. Compared to experimental growth rates at ambient temperatures and unlimited food, observed growth rates were close to experimental values until mid-June but were much lower in July to September, suggesting food limitation in summer. The higher temperatures observed since 1989 positively affected the quality of the shallow coastal waters as a nursery area for sole but not for plaice. A further increase may negatively affect the nursery quality if the production rate of benthic food cannot meet the increase in energy requirements of 0-group flatfish.

Fishery closures and marine protected areas are increasingly being used as tools to achieve sustainable fisheries. The ''plaice box'', a gear restriction area in the North Sea that was established to reduce the bycatch of undersized plaice (Pleuronectes platessa), is considered ineffective because there has been a shift in the distribution of juvenile plaice to the waters that remained open to bottom trawlers. Here we examine the hypothesis that bottom trawling benefits the small benthic invertebrates that form the food source for plaice and that the plaice box had a negative impact on food production for plaice. A size-based model of benthic communities indicates that the production of prey was low without trawling and maximal in areas that are trawled once to twice a year. Therefore, bottom disturbance may improve the feeding conditions for species that feed on small invertebrates. As plaice aggregate at the locations with the highest benthic biomass, this may explain the observed redistribution to areas outside the plaice box. We conclude that the plaice box may not have been the most appropriate measure to protect plaice from discarding and that the species' ecology should be considered when choosing the most appropriate management measure to achieve an objective. Resume´ : Les fermetures de la peche et les zones de protection marine servent de plus en plus d'outils pour obtenir des peches durables. Le « cantonnement a plies », une zone de restriction des engins de peche dans la mer du Nord etablie pour reduire la capture accessoire de plies (Pleuronectes platessa) de taille inferieure ala limite, est consideree inefficace car il y a eu un deplacement dans la repartition des plies vers les eaux qui restent disponibles aux navires de chalutage de fond. Nous examinons ici l'hypothese selon laquelle le chalutage de fond favorise les petits invertebres benthiques qui constituent la source de nourriture des plies et que le cantonnement a plies a un impact negatif sur la production de nourri- ture des plies. Un modele des communautes benthiques basesur la taille indique que la production de proies est basse en l'absence de chalutage et qu'elle est maximale dans les zones ouil y a du chalutage une ou deux fois par an. C'est pour- quoi, la perturbation du fond peut ameliorer les conditions alimentaires pour les especes qui se nourrissent de petits inver- tebres. Comme les plies se rassemblent dans les sites de biomasse benthique maximale, cela peut expliquer le redeploiement des plies vers les zones exterieures au cantonnement a plies. Nous concluons que le cantonnement a plies ne s'est peut-etre pas avereetre la methode la plus appropriee pour proteger les plies du rejet ala mer; il faut donc tenir compte de l'ecologie de l'espece lors du choix de la mesure de gestion la plus appropriee pour realiser un objectif. (Traduit par la Redaction)

The objective identification of targeting behavior in multispecies fisheries is critical to the development and evaluation of management measures. Here, we illustrate how the statistical distribution of commercial catches can provide information on species preference that is consistent with economic data but not a simple function of price. Using the Dutch beam trawl fishery from 1998 to 2003, we show that the distribution of the log10-transformed catch rates of preferred species exhibit greater negative skews than less preferred species. Furthermore, subsets of the fleet employing spatially distinct strategies generate the expected patterns in the skews of their catch distributions. A simple model is presented to illustrate a behavioral mechanism for variation in skews and identify circumstances where it could apply. As a result of this analysis we propose that (i) catch distributions should be examined by species when investigating targeting behavior and (ii) changes in error structure over time can be expected in comparisons of catch statistics such as those used to create abundance indices or estimate fishing power.

An analysis of the changes in the Dutch demersal fishing fleet since the 1950s revealed that competitive interactions among vessels and gear types within the constraints imposed by biological, economic and fisheries management factors are the dominant processes governing the dynamics of fishing fleets. Double beam trawling, introduced in the early 1960s, proved a successful fishing method to catch deep burying flatfish, in particular sole. In less than 10 years, the otter trawl fleet was replaced by a highly specialised beam trawling fleet, despite an initial doubling of the loss rate of vessels due to stability problems. Engine power, size of the beam trawl, number of tickler chains and fishing speed rapidly increased and fishing activities expanded into previously lightly fished grounds and seasons. Following the ban on flatfish trawling within the 12 nautical mile zone for vessels of more than 300 hp in 1975 and with the restriction of engine power to 2000 hp in 1987, the beam trawl fleet bifurcated. Changes in the fleet capacity were related to the economic results and showed a cyclic pattern with a period of 6–7 years. The arms race between fishers was fuelled by competitive interactions among fishers: while the catchability of the fleet more than doubled in the ten years following the introduction of the beam trawl, a decline in catchability was observed in reference beam trawlers that remained the same. Vessel performance was not only affected by the technological characteristics but also by the number and characteristics of competing vessels. © 2008 Elsevier B.V. All rights reserved.

Catch per unit effort (CPUE) is commonly used as an indicator for monitoring developments in stock size. To ensure proportionality between average CPUE and total stock size, two processes that should be accounted for are the degree of targeting behaviour of the fleet and the management-induced responses in fishing behaviour. We studied the effect of restrictive individual quotas and targeting behaviour on average CPUE in the Dutch beam trawl fleet. Fishing opportunities varied in time and across species due to changes in quotas. Using catch and effort data by fishing trip of the total fleet and haul-by-haul data from a reference fleet, targeting behaviour of the beam trawl fleet was quantified for sole and plaice, at various space and time scales. Sole was targeted on all scales examined, whereas plaice was only targeted on a micro-scale of 10 × 10 nautical miles. When sole quota restrictions were relaxed, the fleet increasingly targeted sole instead of plaice. Targeting indices for sole and plaice were negatively correlated. Our findings indicate that catch and effort data by fishing trip are sufficient to characterise targeting behaviour on a macro-scale, whereas haul-by-haul data are needed to quantify the targeting on a micro-scale (30 × 30 nautical miles). The micro-scale targeting index can be used to standardize macro-scale CPUE data for bias due to variations in directed fishing among local fishing grounds.

This paper summarizes the results of survival experiments with plaice and sole discards caught by commercial beam-trawl vessels and a research vessel fishing with an otter trawl, and with soles escaped through the meshes in covered-cod-end experiments. In the commercial beam-trawl fishery the survival of both plaice and sole discards was estimated to be less than 10%. The survival of soles escaped through the meshes was estimated at 60%. The survival of discards was negatively affected by the catching process. During the catching process mortality was caused through the action of the tickler chains and the injuries imposed during the stay in the net. Under present-day commercial practice the processing of the catch on deck hardly affects the survival of discards.

The allometric relationship between body size and otolith size (length and width) was determined for male and female plaice over the entire length range. Growth rate, geographical area and month significantly affected the relationship, but their contributions to the total variance were negligible. Between June and September, otolith growth lags behind body growth by up to 5% and catches up again between September and January. Annulus formation occurs between January and June, and the back-calculated length reflects on average the length on 1 April at the start of the growing season. The validity of the back-calculation method was corroborated by: (1) the close correspondence of back-calculated and observed length at age in survey data and commercial landings; (2) the close correspondence between the back-calculated and observed length increment of tagged females recaptured after 11–13 months; (3) the agreement between back-calculated lengths from the symmetrical and asymmetrical otolith of the same fish; (4) the proportionality of otolith growth and body growth as observed in a tagging experiment in which plaice otoliths were chemically labelled with tetracycline. The precision of the back-calculation method, expressed as the relative deviation from the mean, decreases from 25% in Age Group I to 5% in Age Group III–VI. The precision of back-calculated annual length increment or volume increase was shown to be less precise, with relative errors of 25–45%. It is concluded that the growth history of individual plaice can be back-calculated from otoliths at a precision that is well within the natural variability in growth between individual plaice.

Skates (Rajidae) are characterized by slow growth rate, low fecundity, and late maturity and are thus considered to be vulnerable to exploitation. Although understanding mating systems and behavior are important for long-term conservation and fisheries management, this aspect of life history is poorly understood in skates. Using 5 highly polymorphic microsatellite loci, we analyzed egg clutches collected from 4 female Raja clavata captured in the wild to test for multiple paternity. Using the reconstructed multilocus genotypes method to explain the progeny genotype array, we showed that all 4 clutches were sired by a minimum of 4-6 fathers and, thus, female thornback rays are polyandrous. Whether polyandry in R. clavata is natural or a consequence of overexploitation remains uncertain. This is the first report of multiple paternity in a rajiform species and any oviparous elasmobranch.

Segers, F. H. I. D, Dickey-Collas, M., and Rijnsdorp, A. D. 2007. Prey selection by North Sea herring (Clupea harengus), with special reference to fish eggs. ICES Journal of Marine Science, 64: 60–68. he herring stock in the North Sea in recent years has recovered to a relatively high biomass, and here we investigate prey selection of individual North Sea herring when population numbers are high. The diet composition, and specifically pelagic fish eggs, was investigated in February 2004. Samples of herring from the International Bottom Trawl Survey were used for stomach analysis, and ichthyoplankton samples from the southern North Sea were used to investigate selection. Crustaceans were the main diet component. The average diameter of the fish eggs recovered from the stomachs was significantly larger than that of the eggs collected in the field. In addition, the frequency at which the latest developmental stages occurred in the herring stomachs was significantly different from the frequency at which these stages were found in the field. This shows selective foraging. There was a relationship between the amount of food and the number of eggs in a herring stomach: the fullest stomachs tended to contain fewer fish eggs. This suggests that herring forage on eggs when other prey are not available. Hence, it is likely that the dynamics of multiple trophic levels influence the ecological impact of a large herring stock on the North Sea ecosystem.

Les baies du golfe de Gascogne (France) hébergent des habitats où la sole, Solea solea, est le plus abondant des poissons plats. Cette étude vise à évaluer le fonctionnement des habitats pour leur rôle de nourriceries, en analysant la croissance et la condition des juvéniles de sole pendant leur première année. Les données de deux séries, 1999 et 2000, de chalutages mensuels sont comparées à celles acquises lors d'une expérience de 6 mois en mésocosme impliquant l'élevage de poissons originaires de la même nourricerie. La croissance observée est comparée à la croissance potentielle, en utilisant un modèle expérimental décrivant la croissance en fonction de la température. Les soles du groupe d'âge 0 capturées dans la nourricerie atteignent une taille moyenne de 130 mm en septembre. Le taux de croissance est de 0,7-0,9 mmjour-1 en été et, Tidal embayments in the Bay of Biscay (France) host nursery grounds where common sole, Solea solea, is the most abundant flatfish species. This study aimed to appraise the way those habitats function as nurseries through juvenile sole's responses in somatic growth and condition (Fulton's K) during their first year of occupancy. Field data, two yearly trawling series, taken monthly, were compared with a 6-month-long mesocosm experiment involving reared fish of the same wild origin. Growth rates were compared with predicted maximum growth according to an experimentally established model in relation to temperature. In the field, 0-group sole total length (TL) averaged 130 mm from September onwards in 1999 and 2000. Mean growth rates were 0.7-0.9 mmday-1 in summer and

To protect the main nursery area of plaice, an area called the ‘Plaice Box’ was closed to trawl fisheries with large vessels in 1989, with the expectation that recruitment, yield and spawning stock biomass would increase. However, since then the plaice population has declined and the rate of discarding outside the Plaice Box has increased, suggesting an offshore shift in spatial distribution of juvenile plaice. Using research vessel survey data collected since 1970, the change in distribution of juvenile age groups was analysed in relation to the distance to the coast. Further, a comparison of the distribution of different length classes of plaice between three historic periods was made (1902–1909; 1983–1987; 1999–2003). A shift towards deeper water of larger-sized plaice (20–39 cm) is apparent already before the 1980s and may be related to the decrease in the number of competitors or predators. An offshore shift in the distribution of young plaice occurred in the 1990s most likely in response to higher water temperatures that may have exceeded the maximum tolerance range or increased the food requirements above the available food resources. A decrease in competition with larger plaice offshore, possibly in combination with increased inshore predation by cormorants and seals, may also have played a role. The offshore shift in distribution has reduced the effectiveness of the Plaice Box as a technical measure to protect the under-sized plaice from discarding, since an increased proportion of the population of undersized plaice is moving to the more heavily exploited offshore areas.

The phylogeography of thornback rays (Raja clavata) was assessed from European waters, using five nuclear microsatellite loci and mitochondrial cytochome b sequences. Strong regional differentiation was found between the Mediterranean basin, the Azores and the European continental shelf. Allelic and haplotype diversities were high in Portuguese populations, consistent with the existence of a refugium along the Iberian Peninsula. Unexpectedly, high diversity was also found in the English Channel/North Sea area. The lowest genetic diversity was found in the Black Sea. Populations sampled from the Mediterranean, Adriatic and Black Seas were characterized by a single mitochondrial haplotype. This haplotype was also the most ancestral and widespread outside of the Mediterranean basin except for the Azores. Populations from the Azores were dominated by a second ancestral haplotype which was shared with British populations. Results from multidimensional scaling, amova and nested clade analysis indicate that British waters are a secondary contact zone recolonized from at least two refugia--one around the Iberian Peninsula and one possibly in the Azores. Links to a potential refugium known as the Hurd Deep, between Cornwall and Brittany, are discussed. Finally, a historical demographic analysis indicates that thornback ray populations started to expand between 580,000 and 362,000 years ago, which suggests that the Last Glacial Maximum (20,000 years ago) had mainly affected the distribution of populations rather than population size.

Effort management has been proposed as an alternative for quota management in mixed demersal fisheries. It requires a metric to estimate the fishing mortality imposed by a given quantity of nominal fishing effort. Here, we estimate the partial fishing mortality rate imposed by one unit of fishing effort (Fpue) during individual fishing trips and explore the usefulness of this indicator for managing North Sea beam trawlers >300 hp targeting sole (Solea solea) and plaice (Pleuronectes platessa). Fpue is positively related to vessel engine power, and increased annually by 2.8% (sole) and 1.6% (plaice). The positive trend was due to an increase in skipper skills and investment in auxiliary equipment, the replacement of old vessels by new ones and, to a lesser extent, to upgrade engines. The average Fpue imposed per day at sea by a 2000 hp beam trawler was estimated to be 1.0 × 10−5 (sole) and 0.6 × 10−5 (plaice), and it showed substantial seasonal and spatial variations. The Fpue of sole and plaice were negatively related in summer and showed no relationship in winter. The existence of predictive seasonal and spatial patterns in Fpue opens up the possibility of fine-tuning management by directed effort restrictions and uncoupling management of plaice and sole.

Life history theory predicts that fishing may select for increased reproductive investment. A model of the reaction norm for reproductive investment in a capital breeder was developed to disentangle changes in reproductive investment from changes in growth rate in North Sea plaice (Pleuronectes platessa). Trends in reproductive investment since 1960 were estimated as (i) the decrease in body weight of mature males and females between the start and end of the spawning period, (ii) the difference in weight of ripe and spent females, and (iii) the ovary weight of prespawning females. These estimates were related to somatic growth estimated by back-calculation of otoliths and temperature. The ovary weight and weight loss of females that had just started and just finished spawning did not reveal any trends. There was a significant increase in weight loss over the spawning season in both sexes, but much of this increase was likely due to changes in environmental conditions. Evidence for a fisheries-induced change in reproductive investment from our analyses thus remained inconclusive. However, fecundity and ovary-weight data from previous studies tentatively suggest that an increase in reproductive investment occurred between the late 1940s and the 1960s. Such an increase is consistent with a fisheries-induced evolutionary change.

For more than a century, scientists have used mark-recapture techniques to describe the spatial dynamics of marine demersal fish species in the North Sea. Although such experiments have provided extensive data sets, the information is limited to the date and position at release and at recapture. Furthermore, these data may be biased due to the distribution of fishing effort. Recently, electronic (archival) data storage tags (DSTs) have successfully been used to reconstruct the movements of free-ranging demersal fish between release and recapture. Data from DST experiments allow the calculation of fisheries independent migration parameters, and thereby provide a means of evaluating conventional tagging data. We compared the migration patterns of North Sea plaice (Pleuronectes platessa L.) as inferred from a database of twentieth century conventional tagging experiments (CT), with data from 132 plaice tagged with DST. In general, the CT experiments allowed a reliable interpretation of migration patterns, although for certain release areas the migration distances were biased due to the heterogeneous distribution of fishing effort. (C) 2004 International Council for the Exploration of the Sea. Published by Elsevier Ltd. All rights reserved.

Overexploitation and subsequent collapse of major worldwide fisheries has made it clear that marine stocks are not inexhaustible. Unfortunately, the perception remains that marine fishes are resilient to large population reductions, as even a commercially ‘collapsed’ stock will still consist of millions of individuals. Coupled with this notion is the idea that fisheries can, therefore, have little effect on the genetic diversity of stocks. We used DNA from archived otoliths collected between 1924 and 1972 together with 2002 juvenile's tissue to estimate effective population size (Ne) in plaice (Pleuronectes platessa).Ne was estimated at 20 000 in the North Sea and 2000 in Iceland. These values are five orders of magnitude smaller than the estimated census size for the two locations. Populations examined between 1924 and 1960 were in Hardy–Weinberg equilibrium, whereas populations examined after approximately 1970 were not. Extensive testing was performed to rule out genotyping artefacts and Wahlund effects. The significant heterozygote deficiencies found from 1970 onward were attributed to inbreeding. The emergence of inbreeding between 1950 and 1970 coincides with the increase in fishing mortality after World War II. Although the biological mechanisms remain speculative, our demonstration of inbreeding signals the need for understanding the social and mating behaviour in commercially important fishes.