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In this study, we present an innovative framework to obtain essential biological knowledge on exploited populations, based on fine scale spatio-temporal resolution in biological and fishery data. The framework uses species sales notes by commercial size category, the length and age composition of commercial sizes from onshore sampling and the fishing ground and fishing effort from vessel monitoring system records. We apply the framework to horse mackerel (Trachurus trachurus), one of the most important commercially exploited stocks throughout the Northeast Atlantic, using data collected during 2017 in Portuguese waters. We show that high-resolution mapping of the species annual landings by commercial size category and age group can already reveal different distribution patterns by life stage. We further examined, with daily resolution, the variability in the proportion of adult fish by trip, finding differences along the coast and a seasonal pattern indicating that spawning occurred mainly over the slope and adult horse mackerel dominated in the South zone over the year. The framework can be applied to an extended stock area and for several years to investigate migration patterns across areas as well as intra- and inter-annual changes in the species overall distribution. In addition, its application on a trip basis for the species ensemble landed can provide overriding information for the accuracy of mixed fisheries models and management.

Early life history (ELH) traits are key to understand variable recruitment success and hence the stock size of marine fish. One of the currently most puzzling ecosystems in this regard is the northern part of the Benguela Current upwelling system off Namibia. Here, populations of the formerly dominant pelagic species, sardine and anchovy, failed to recover during the last three decades after a dramatic decline. In contrast, Cape horse mackerel, Trachurus capensis, maintained a constant population size. Warming of the system and shoaling of hypoxic zones together with feedback loops within an altered foodweb are discussed to be responsible for this regime shift. In this study, we address the role of larval traits for the successful performance of the T. capensis population under the present environmental conditions with the focus on feeding ecology. We investigated seasonal variations of the geographical distribution, growth rate, feeding ecology, and nutritional condition of their ELH stages and examined relationships with water temperature, dissolved oxygen concentration, and micro-zooplankton composition. T. capensis' ELH stages showed a wide spatial and seasonal distribution, a preference for higher water temperatures (18–21°C) and presence over a wide range of dissolved oxygen concentrations (0.13–6.35 ml O2 l−1). Feeding success was high and mainly different groups of Copepoda were targeted, which were strongly size selected. The high dietary importance of micro-copepods during large parts of the larval phase indicates successful exploitation of this food source, which has increased in abundance during the last decade. It also explains observed best nutritional conditions at temperatures between 18 and 21°C, since these small copepods are commonly associated with warmer temperatures. Altogether, these traits enhance the species' probability to encounter suitable environments for the survival of their ELH stages, which is likely to lead to their high recruitment success in the northern Benguela ecosystem.

Zimmermann, F., and Heino, M. 2013. Is size-dependent pricing prevalent in fisheries? The case of Norwegian demersal and pelagic fisheries. – ICES Journal of Marine Science, 70: . It is commonly acknowledged that body weight of fish is a key factor in determining market value of landed catch, thus influencing optimal harvest strategies. However, in management strategy evaluations and bioeconomic modelling, body size is often an overlooked economic parameter, and there are no systematic studies on the prevalence of size-dependent pricing. Here we assess the presence and magnitude of size-dependent pricing in ex-vessel prices of fish in Norwegian fisheries. The data encompass landings of four pelagic and four demersal stocks in Norway in 2000–2010. Linear mixed models and generalized additive models were used to determine the dependence of unit price on weight class as well as on total yield and time (year). The results show a significant positive relationship between weight class and price for seven out of the eight examined fish stocks. The relative effect of body weight on price was the strongest for cod, Greenland halibut, Norwegian spring-spawning herring and mackerel, lesser for North Sea herring and saithe, and negligible for horse mackerel. These findings demonstrate that size-dependent pricing is common in Norwegian fisheries, and is therefore of high relevance for resource economics and fisheries management.

Bachiller, E. and Irigoien, X. 2013. Allometric relations and consequences for feeding in small pelagic fish in the Bay of Biscay. – ICES Journal of Marine Science, 70:232–243. The body size of fish is an important factor in determining their biology and ecology, as predators eat prey smaller than themselves. Predator mouth size restricts the availability of possible prey. In this paper we provide the allometric relationships of eight common, small pelagic fish species in the Bay of Biscay. In addition, we describe the predator-prey size ratios for different species, and we determine changes in their ratio-based trophic-niche breadth with increasing body size. Results suggest that gape size does not totally determine the predator-prey size ratio distribution, but predators use the entire available prey size range, including the smallest. As they grow they simply incorporate larger prey as their increased gape size permits. Accordingly, a large degree of overlap was found in the diet composition in terms of size and predator-prey ratios, even between fish of different sizes. Of the species studied, only horse mackerels seem to be clearly specialized in relatively large prey.

Farrell, E. D., Hüssy, K., Coad, J. O., Clausen, L. W., and Clarke, M. W. 2012. Oocyte development and maturity classification of boarfish (Capros aper) in the Northeast Atlantic. – ICES Journal of Marine Science, 69: 498–507. This study presents the first detailed investigation of the oocyte development and maturity classification of boarfish, Capros aper, which has recently become the target of an industrial fishery in the Northeast Atlantic. A total of 2014 boarfish were collected from January to December 2010. Mature male and female boarfish were sexually dimorphic and could be readily identified based on external characteristics. A comprehensive maturity scale was developed, which indicated that the length at 50% maturity for males and females was 9.7 cm total length. Female boarfish were observed to spawn in Irish waters in June and July. Once spawning ceased the remaining mature oocytes were resorbed. Preliminary analysis of reproductive strategy indicates that the boarfish is likely an asynchronous batch spawner with indeterminate fecundity.

A non-hierarchical classification technique (clustering large applications, CLARA) was used to identify four fishing tactics of Spanish otter trawlers in the Cantabrian Sea (ICES Division VIIIc) from 1983 to 2004: mixed fishing, blue whiting fishing, horse mackerel fishing, and mackerel fishing. There were no significant differences in the fishing tactics employed by two trawl fleets identified using a non-hierarchical classification technique (partition around medoids). There was, however, a decline in the use of the blue whiting fishing tactic from 2000 on, perhaps as a result of competition with pairtrawls, a gear whose main target species is blue whiting. There was an increase in the number of trips using the mackerel fishing tactic from 1996, a change possibly caused by improved market conditions. Between 2000 and 2004, the fleets had two distinct behaviour patterns, identified depending on the area in which they operated. The study area could therefore be subdivided into two areas based on the prevalence of the fishing tactic followed. The horse mackerel fishing tactic was more commonly used in the west, and the mixed fishing tactic in the east., Sí

De Oliveira, J. A. A., Darby, C. D., and Roel, B. A. 2010. A linked separable–ADAPT VPA assessment model for western horse mackerel (Trachurus trachurus), accounting for realized fecundity as a function of fish weight. – ICES Journal of Marine Science, 67: 916–930. The western horse mackerel stock covers a large area of the Northeast Atlantic and is characterized by the sporadic occurrence of strong year classes. The only fishery-independent data available for its assessments are the estimates of total annual egg production from triennial egg surveys. Horse mackerel are indeterminate spawners, so a direct conversion of total annual egg production to spawning-stock biomass using prespawning-season estimates of fecundity is not viable. There is also evidence that potential fecundity per kg spawning female increases with fish weight. A linked separable–ADAPT VPA (SAD) model was developed that combines data on total catch, catch-at-age, and total annual egg production with data on potential and realized fecundity to provide population estimates for western horse mackerel. The model accounts for potential fecundity as a function of fish weight and for the development of a targeted fishery on the strong 1982 year class. Simulations confirm that the SAD model is able to reproduce population estimates without bias under a range of scenarios, except where there is a trend in realized fecundity. This underscores the need for improved information on realized fecundity, or alternatively the need to develop management plans that are robust to this source of uncertainty.

Gonçalves, P., Costa, A. M., and Murta, A. G. 2009. Estimates of batch fecundity, and spawning fraction for the southern stock of horse mackerel (Trachurus trachurus) in ICES Division IXa. – ICES Journal of Marine Science, 66: 617–622. Since 1995 the annual egg production method has been applied triennially to the southern stock of horse mackerel (Trachurus trachurus) in the Northeast Atlantic (ICES Division IXa). This method assumes that fecundity is determinate, but increasing evidence indicates that horse mackerel are indeterminate spawners. The daily egg production method (DEPM) does not rely on the assumption of determinate fecundity, making it the appropriate method for this species. Therefore, we reanalysed samples collected from previous surveys (2002, 2004, 2005, and 2007) to obtain estimates for batch fecundity and spawning fraction, which are important DEPM parameters. The estimates of batch fecundity are around 200 oocytes g−1 of female (total ovary-free weight). Several criteria were used to estimate spawning fraction (migratory nucleus stage, hydrated oocytes, and post-ovulatory follicles) and all showed the same trend among years, varying between 0.10 and 0.30 d−1. The estimates were significantly different among methods, but those differences were similar across surveys, indicating that a consistent bias would be reflected in the final spawning-stock biomass (SSB) estimates obtained from the DEPM. Until further information is available regarding the accuracy of the criteria used to estimate spawning fraction, the southern horse mackerel SSB estimates from the DEPM should only be taken as indicative of trends rather than measures of absolute abundance.

Rijnsdorp, A. D., Peck, M. A., Engelhard, G. H., Möllmann, C., and Pinnegar, J. K. 2009. Resolving the effect of climate change on fish populations. – ICES Journal of Marine Science, 66: 1570–1583. This paper develops a framework for the study of climate on fish populations based on first principles of physiology, ecology, and available observations. Environmental variables and oceanographic features that are relevant to fish and that are likely to be affected by climate change are reviewed. Working hypotheses are derived from the differences in the expected response of different species groups. A review of published data on Northeast Atlantic fish species representing different biogeographic affinities, habitats, and body size lends support to the hypothesis that global warming results in a shift in abundance and distribution (in patterns of occurrence with latitude and depth) of fish species. Pelagic species exhibit clear changes in seasonal migration patterns related to climate-induced changes in zooplankton productivity. Lusitanian species have increased in recent decades (sprat, anchovy, and horse mackerel), especially at the northern limit of their distribution areas, while Boreal species decreased at the southern limit of their distribution range (cod and plaice), but increased at the northern limit (cod). Although the underlying mechanisms remain uncertain, available evidence suggests climate-related changes in recruitment success to be the key process, stemming from either higher production or survival in the pelagic egg or larval stage, or owing to changes in the quality/quantity of nursery habitats.

Ndjaula, H. O. N., Hansen, T., Krüger-Johnsen, M., and Kjesbu, O. S. 2009. Oocyte development in captive Atlantic horse mackerel Trachurus trachurus. – ICES Journal of Marine Science, 66: 623–630. This 13-month experimental study focused on developing oocyte production in 101 adult female Atlantic horse mackerel. In addition, proxies of energy patterns in relation to oocyte development were investigated. The fish were captured off western Norway and fed to satiation from October 2005 to November 2006 in two replicate adjacent circular tanks. Monthly histological examination of the ovaries indicated that vitellogenic oocytes were present at all times throughout the study period, but that oocyte development did not progress past the incipient migratory nucleus stage. The oocyte diameter (OD) threshold between pre-vitellogenic and vitellogenic oocytes and the mean OD of the leading cohort were investigated, and no hiatus was observed within the OD size distribution around the 185-µm pre-vitellogenic and vitellogenic threshold. Variation in gonadosomatic index, hepatosomatic index, condition factor, and fat content (as measured by the use of a Distell Fish Fatmeter) increased with oocyte development. The observed development of oocyte recruitment and the absence of a hiatus in the oocyte distribution are characteristics of an asynchronous spawner. The species seems to have a prolonged spawning season, judging from the advanced and variety of maturity stages throughout the study period.

Costa, A. M. 2009. Macroscopic vs. microscopic identification of the maturity stages of female horse mackerel. – ICES Journal of Marine Science, 66: 509–516. The consequences to fisheries management of erroneous macroscopic identification of maturity stages are evaluated. The percentage error in macroscopic identification of maturity stages of female horse mackerel (Trachurus trachurus) from the Portuguese coast was determined and analysed by maturity stage, between the periods January–June and July–December, between research surveys and commercial landings, and between per year and month and by length class, to assess its implications in determination of spawning season, maturity ogives, and spawning-stock biomass. The highest percentage errors in macroscopic identification were between late ripening and partly spent stages, in fish >30 cm, and for all fish collected in 1998 and 2005. Late ripening was generally macroscopically underestimated, and the partly spent stage overestimated. The implications for the information provided to the ICES assessment working group are analysed.

Garrido, S., Murta, A. G., Moreira, A., Ferreira, M. J., and Angélico, M. M. 2008. Horse mackerel (Trachurus trachurus) stomach fullness off Portugal: index calibration and spatio-temporal variations in feeding intensity. – ICES Journal of Marine Science, 65: 1662–1669. An index for stomach fullness used in the routine biological sampling of horse mackerel from Portuguese waters was calibrated about the correspondent stomach mass. The fullness scale comprises four levels (1, empty; 2, 50% filled; 4, bursting) that are easily attributed to whole stomachs during the sampling process. Significant differences were found among the four levels of the fullness scale in their relative stomach masses, indicating that the index can be used for the semi-quantitative analysis of feeding intensity in this species. The fullness index was applied to study spatio-temporal variations in horse mackerel feeding off the Portuguese coast, using data collected from 1985 to 2006 on commercial vessels and research cruises. There was no significant difference in feeding intensity between sexes. Smaller horse mackerel and those in the resting stage of reproduction fed more intensively than larger and spawning fish. Feeding intensities of fish from the southern and northwest coasts of Portugal were greater than for those captured along the southwest coast. Horse mackerel feeding intensity was greater at dawn and varied seasonally, being higher during spring and autumn and lower during winter. Interannual variation in feeding intensity of horse mackerel captured along the west coast of Portugal revealed a significant but slight decrease in recent years.

Borges, L., van Keeken, O. A., van Helmond, A. T. M., Couperus, B., and Dickey-Collas, M. 2008. What do pelagic freezer-trawlers discard? – ICES Journal of Marine Science, 65: 605–611. Estimates of discards by the Dutch pelagic freezer-trawler fleet were derived based on data from observers on board commercial vessels. In all, 27 fishing trips of duration 2–5 weeks each were sampled between 2002 and 2005, covering the North Sea and the western waters of the British Isles. Different methods to estimate discards were compared, and raising these estimates by number of trips or by total landings did not greatly influence the annual estimates of total discards. Some 30 000 t of fish were discarded annually by the fleet, the commercial target species mackerel, herring, and horse mackerel being discarded most, with 41%, 19%, and 15% of the total discards by weight, respectively. The most commonly discarded non-commercial species was boarfish, accounting for 5% of total discards. Slippage accounts for 10% of all discards, and the most common species slipped is herring. There is extensive discarding of juvenile mackerel in the horse mackerel fishery, and to a lesser extent highgrading of mackerel in the same fishery.

Roel, B. A., and De Oliveira, J. A. A. 2007. Harvest control rules for the Western horse mackerel (Trachurus trachurus) stock given paucity of fishery-independent data. – ICES Journal of Marine Science, 64: 661–670. The Western horse mackerel stock, widely distributed in EU waters, is characterized by spasmodic recruitment. Currently, the strength of a year class cannot be confirmed before it is fully recruited to the fishery and has reached the age of 5 years. The only fishery-independent information available is an estimate of egg abundance made every third year. The state of the stock is considered uncertain, and there is no agreed management plan. Following EU requests, a set of harvest control rules (HCRs) that allows for the increased proportion of juvenile fish taken by the fishery in recent years was tested by simulation. The proposed HCRs are based either on the results of a full assessment (Constant-proportion strategy) or simply on the egg estimate used as an indicator of stock status (Slope strategy). Biological risk is compared for scenarios where: (i) uncertainty regarding stock dynamics and in the relationship between egg data and spawning-stock biomass is high (current situation); (ii) variable fractions of the total allowable catch (TAC) are taken in the areas occupied by juveniles and adults; and (iii) there is an implementation error. Results suggest that taking a larger portion of the TAC in the area occupied by juveniles increases the risk of impaired recruitment. Comparison of the Constant-proportion and Slope strategies suggests that the former is more conservative, if the assessment is unbiased or if bias is low. Given the paucity of fishery-independent data, a strategy resulting in an approximately constant catch may be appropriate for this stock.

A refined fish consumption model for lactating Cape fur seals in Namibia during the eight-month lactation period, which allows for spatio-temporal variation in the diet as determined by scat analyses, has been developed. Previous estimates of prey consumption by Cape fur seals have been based mostly on coarse diet composition models. Sensitivity analyses showed that the energetic requirement and mass of lactating females (bioenergetic variables), as well as the energetic density of prey (diet variable), contributed most to the uncertainty in consumption estimates. Uncertainty in the remaining input variables had minimal effects on the estimates of food consumption. The consumption of commercial prey (horse mackerel, hake and pelagic fish) was greatest by the colony at Cape Cross. The model estimated that a female of average mass 55 kg ingested, on average, 11% of her body mass per day. This model is easily applied to other age/sex classes of the seal population. It permits improvement of the estimates of prey consumption by seals, which are useful for assessing levels of competitive interactions between seals and fisheries or other predators, or the impacts of seals on prey species.

Turan, C. 2004. Stock identification of Mediterranean horse mackerel (Trachurusmediterraneus) using morphometric and meristic characters. e ICES Journal of MarineScience, 61: 774e781.Morphologic differentiation among stocks of Mediterranean horse mackerel, Trachurusmediterraneus, throughout the Black, Marmara, Aegean and Eastern Mediterranean Seas,was investigated using morphometric and meristic characters. Discriminant functionanalysis of both morphometric and meristic characters suggested that there is restrictedmigration of mackerel among the adjacent seas. Overlapping of four Black sea samples onthe discriminant space in morphometric and meristic characters suggested that there is oneself-recruiting population in the area. The Marmara sea samples were the most isolatedsamples from all others for both morphometric and meristic characters, which may indicateexistence of a distinguishable mackerel stock in the area. The sample from the Aegean Seawas grouped with one geographically close Mediterranean sample based on morphometrics,and separated from all other Mediterranean samples based on meristic characters,suggesting some degree of intermingling between these areas. Examination of thecontribution of each morphometric variable to canonical functions indicated that differencesamong samples seemed to be associated with the anterior part of the body. In meristicanalyses, highest contributions to canonical functions were associated with the number ofgill rakers and pectoral fin rays.

The biomass of horse mackerel (Trachurus trachurus) in the North Sea was estimated for the period 1991-1997. Calculation was based on a ratio estimate that related catch per unit effort (cpue) data from scientific surveys of horse mackerel to the cpue data of a reference species from the same surveys. It was assumed that both horse mackerel and the reference species were equally available to the survey gear. For the reference species, estimates of absolute biomass per age-class were available from North Sea Multispecies Virtual Population Analysis (MSVPA). That biomass was used to scale the cpue ratios and to obtain estimates of absolute biomass for horse mackerel. The method used was size-structured and spatially explicit. The inherent precision of the methodology, the choice of reference species and the effects of different spatial and body size resolutions were investigated. The final results were achieved by applying 1 cm size-classes and whiting (Merlangius merlangus) as the reference species. The spatio-temporal distribution of horse mackerel biomass was visualized using a Geographic Information System (GIS). Horse mackerel in the North Sea are split into two distinct components. The estimated biomass of the northern component is underestimated by the method described because the surveys do not cover the full extent of the stock. The biomass of the southern component in the third quarter of the year was estimated as 63 000, 163 000, 213 000, 248 000, 124 000, 147 000 and 57 000 t for the years 1991-1997 respectively.

Two methods were used to validate age determination of horse mackerel otoliths. In the first, whole otoliths were examined with a light microscope. Age was determined by identifying and counting annuli. Marginal increment widths were also measured and served to estimate ages, which ranged from 0.6–4.3 years. In the second method, otoliths were examined with a scanning electron microscope and fish ages were determined by daily increment counts. Sixty-four otolith specimens from the Southeast Atlantic and 24 from the Northeast Atlantic were aged by counting the daily growth increments. The number of daily increments was highly variable within each growth zone. Ages exceeding 365 days in the first annual growth zone indicated that the first annulus was not always deposited in some fish. The variable daily increment counts in the first year’s growth zones may be related to the very protracted spawning season of horse mackerel. Examination of subsequent growth zones indicated that false rings and annuli are often of a similar visual appearance and that true annuli can only be identified if concurrent measurements of growth zone widths are available. Estimated ages agreed with ages derived by counting daily increments, thus validating the ages of horse mackerel up to four years. 2001 International Council for the Exploration of the Sea

The results of preliminary investigations on the possible relationships between coastal upwelling variability observed with satellite remote sensing, and sardine and horse mackerel recruitment dynamics in the west coast of Portugal are presented. The analysis of a sardine-recruitment time-series for the period 1976–1998 shows that there has been a decreasing trend since 1983 (with the exceptions of 1991 and 1992), the years from 1993–1995 representing the lowest values in that time series. Horse mackerel exhibited a decrease from 1986 until 1990, and from 1992–1995, the latter representing the lowest value for the period 1986–1998. Sea surface temperature (SST) derived from satellite data were used to compute monthly SST upwelling indices along the Portuguese west coast over a period from 1987–1997. The analysis focused on seasonal and annual upwelling variability off Portugal, and its possible effects on sardine and horse mackerel recruitment to the fishery. An increasing trend in SST upwelling indices during the winter (January–March), especially between 1992 and 1995 was found. The upwelling events observed off Portugal during winter months, which correspond to the spawning season for sardine and horse mackerel, had a negative impact on the recruitment of these fish species. This effect on recruitment could be due to an increase in conditions favourable to the offshore transport of larvae and consequently an increase in their mortality. Prior to 1992, when no upwelling events occurred during winter, recruitment dynamics was closely linked with the spring–summer seasonal upwelling variability off the Portuguese west coast. 2001 International Council for the Exploration of the Sea

The analysis of catch data from groundfish surveys (1985–1988) conducted on the Portuguese shelf and upper slope (20–500 m) revealed five major geographic areas, each characterized by the presence of a typical fish assemblage. These areas of distinct fish assemblages were found to be closely aligned with depth, in a way that resembles spatial patterns previously described for other continental shelves in the North Atlantic. The sharpest biological transition on the Portuguese shelf takes place as one moves from areas shallower than 120 m (‘‘Shallow Groups’’) towards deeper locations offshore (‘‘Deep Groups’’). Beyond the 150 m isobath, the biomass was dominated by blue whiting, whereas inshore variability in assemblage composition was much greater. Species such as sardine, horse mackerel, mackerel (to the north of Lisbon) and sparids (to the south) comprised significant and highly variable proportions of the population abundance inshore. There are similarities between the trophic and spatial organization of the marine community on the Portuguese shelf and that of other coastal upwelling ecosystems that are briefly reviewed here. 2001 International Council for the Exploration of the Sea

The current delimitation of the Atlanto–Iberian stock of horse mackerel, Trachurus trachurus (Linnaeus, 1758), is based on scant biological evidence. Here, the location of the southern boundary of this stock is investigated through the analysis of several morphometric and meristic characteristics. A total of 384 horse mackerel were sampled from three areas off the Portuguese, one off the Spanish (Gulf of Cadiz) and another off the Moroccan coast, and compared using multivariate techniques. Some 14 morphometric and five meristic characters were analysed using hierarchical cluster analysis, multivariate discriminant analysis and randomization tests. Morphometric characteristics showed considerably greater discriminatory power to distinguish individuals from different areas than did the meristic characters. All morphometrical analyses showed similar results: the greatest differences were found between the Gulf of Cadiz and all other groups, while fish from the Portuguese coast appeared more similar to each other than to those from Moroccan waters. Meristic characters did not allow a clear distinction between geographical areas, with the Mahalanobis distances among areas being an order of magnitude smaller than those for the analysis of the morphometrics. If these results are corroborated by further biological evidence (such as genetical or parasitological data), there is a strong argument for the southern limit of the Atlanto–Iberian stock of horse mackerel to be revised. 2000 International Council for the Exploration of the Sea

Pelagic fish distributed near the surface oV Angola were sampled with a medium-sized pelagic trawl having a vertical opening of about 20 m. Numerous trawls targeting on dense sardinella schools (Sardinella maderensis and Sardinella aurita) failed to yield large catches. Schools that were at the trawl depth as the vessel passed over them were often observed far below the foot rope of the trawl, indicating vertical avoidance. The net sonde transducer often recorded schools in the net mouth for long periods: it was assumed that these fast-swimming fish were able to swim along with the trawl, and escaped when the towing speed was reduced during hauling. No correlations were found, however, between towing speed (2.7‐5.0 knots) and the total catch of sardinella. Increasing the two duration during the day to more than 2 h also failed to yield consistently higher catches. A negative correlation at night between tow duration (15‐60 min) and catch size arose simply because the trawl was towed for a longer time to obtain adequate samples in areas with scattered recordings. During the day, sardinella was caught both inshore and further oVshore, while horse mackerel (Trachurus trecae and Trachurus capensis) was caught further oVshore only. Both species were well represented in catches made at night. There were substantial day and night diVerences in the length distributions of sardinella, with small sardinella being caught only at night. ? 1999 International Council for the Exploration of the Sea

Egg production methods have been used successfully in the provision of advice for fisheries management. These methods need accurate and unbiased estimates of fecundity. We explore the reproductive strategy of horse mackerel and estimation of fecundity. Fecundity and fecundity regulation in relation to condition was investigated over a number of years. Fulton's K, lipid content, and hepatosomatic index increased after the start of spawning, though decreased again at the end of spawning. The increase in the gonadosomatic index, fecundity, and body condition after the onset of spawning suggests that horse mackerel utilizes food resources during the spawning season and might be an income breeder. However, the decline in K and lipid before the spawning season suggests that the first batch of oocytes is developed on stored energy. Fecundity varied between years and within a spawning season. Over latitude, variations in fecundity were small. K and lipid content are not reliable indices as proxy for fecundity. Batch fecundity appears to be heterogeneous across the spawning season but homogeneous across latitude. The homogeneity of batch fecundity over latitude could indicate that the daily egg production method is an appropriate approach for estimating the abundance of a wide ranging species, as horse mackerel., batch fecundity body condition egg production method indeterminate lipid content Trachurus trachurus, SI

The paper reports the results of species-recognition-rate measurements on caged aggregations of mackerel, horse mackerel, saithe, haddock, and two sizes of cod. Data on the acoustic backscattering coeYcients were collected in eight contiguous bandwidth intervals covering the frequency band between 27 and 54 kHz. The measurements were made during two to six periods of 24 h for each aggregation of fish. Replicate experiments were carried out for mackerel, horse mackerel, and two sizes of cod. The data were processed to give average frequency spectra. The number of independent observations used to establish the mean was varied to examine the species-recognition dependence on the number of independent observations. The mean spectra were analysed using two recognition methods: neural network and discriminant analysis. A neural network was trained on subsets of the data and recognition rates established for the diVerent numbers of samples used to calculate the mean spectra. Classical discriminant analysis was applied using the same data sets. The results of the two identification methods are presented and show that recognition rates of about 95% are possible using average spectra. The diVering recognition rates by species and fish sizes are discussed and the two identification methods compared. Implications for the future development of these methods are considered. ? 1996 International Council for the Exploration of the Sea

In situ measurements of target strength (TS) at 38 kHz have been taken from night-time scattering layers of anchovy, pilchard, and Cape horse mackerel in the course of routine surveys. These species have diVerent numerical packing densities, which necessitates treating the TS data diVerently in each case, as the performance of single-target detectors is strongly dependent on target densities. Evidence for the presence of multiple targets in their TS distributions is presented, and empirical methods of extracting mean target strengths from them described. After careful assessment of the quality of each data point, and the removal of unwanted influences where possible, the data were used to fit TS/length expressions for the three species. ? 1996 International Council for the Exploration of the Sea

The extraction of highly discriminant features is crucial for successful species identification of fish shoals if backscattered narrowband signals do indeed contain discriminant information. Four different methods of feature extraction are described and applied to the same data, providing new descriptors expected to improve species identification. Echograms, amplitude probability density function (PDFs) and spectral features are used to describe acoustic images of single shoals. Image processing is used to improve signal shoal description, by taking into account the shoal structure and species-related spatial distribution. Three pelagic species are considered: sardine (Sardina pilchardus (Walbaum)), anchovy (Engraulis encrasicolus (L.)), and horse mackerel (Trachurus trachurus (L.)) detected during fisheries acoustics surveys conducted in the Bay of Biscay. A correct classification rate of 57% overall was found for data covering a mesoscale oceanographic environment and including seasonal variability. If space and time scales are reduced this value increased to 98%, emphasizing the value of non-acoustic and a priori information.

Fish schools of sardine, anchovy, and horse mackerel can be discriminated from each other, under given conditions, using a set of parameters extracted from echo-integration data. Trawl sampling and hydroacoustic data were collected in 1992 and 1993 in the Thermaikos Gulf by using a towed dual-beam 120 kHz transducer. The parameters extracted from the available schools were used to train multi-layered feed-forward artificial neural networks. Various applied networks easily generated associations between school descriptors and species identity, providing a powerful tool for classification. The expertise of the trained network was tested with data from identified schools not used in training. The use of neural networks cannot replace classical statistical procedures, but offers an alternative when there are significant overlaps in the school characteristics and the parametric assumptions are not satisfied.

Biomass estimates are made by combining ICES working group VPA biomass estimates of 11 commercial fish species with catch rates in the ICES International Young Fish Surveys (1YFS) in the first quarter of the year and in the English Groundfish Surveys (EGFS) in the third quarter of the year. Data from 1983, 1984, and 1985 are used. The total biomass of fish is calculated as the mean of the three years. In the first three months of the year the mean total biomass was 8.6 million tonnes, of which the 11 commercial species constituted 5.9 million tonnes. In the third quarter it was 13.1 million tonnes, of which the 11 commercial species constituted 8.5 million tonnes. The difference between the first and third quarters was mainly caused by the migration of 0.5 million tonnes of Western stock mackerel and 1.6 million tonnes of horse mackerel into the North Sea during the third quarter. Comparisons are made between the productivity of the North Sea and other shelf regions in temperate latitudes.

Pellets of Cape Cormorants (Phalacrocorax capensis) are cast daily and can be used to determine prey species through identification of otoliths and other indigestible parts. Experiments in captivity indicate that some otoliths may be partially or entirely digested, making it impossible to estimate the original size or number of fish ingested per meal or per day. Birds of various species regurgitate casts or pellets of indigestible material (Knight 1964). Pellets of cormorants could be useful for study- ing diets of these birds because sampling is non-destructive and large samples can be ob- tained with little disturbance to the birds (Ain- ley et al. 198 1). Crystals of fish eyes taken from pellets might be counted to estimate daily food intake (Jordan 1959, Schlatter and Moreno 1976) and otoliths could be useful in identi- fying the species and sizes of fish consumed by the birds. However, certain aspects of pellet produc- tion require investigation before pellets can be used for either quantitative or qualitative stud- ies. How frequently are pellets cast? Does a pellet (accurately) reflect a meal or a bird's daily consumption of food? Do otoliths remain unchanged while residing in a bird's stomach, so that otoliths can be used to estimate the sizes of the fish eaten? How long do otoliths persist in a bird's stomach? We report here on studies designed to answer these questions, us- ing information obtained from pellets pro- duced by captive Cape Cormorants (Phalacro- corax capensis). METHODS Seven adult Cape Cormorants were taken from Dyer Island (34"4 1 'S, 19"15'E), Cape Province, South Africa, on 19 November 198 1. The birds were transported overnight by sea to Cape Town and placed in cages. They were kept in the cages for six weeks until their weights sta- bilized and they became accustomed to cap- tivity. Birds were grouped in two cages of two individuals and one cage of three, because they refused to eat if kept separately. During the experimental period, the birds were fed com- mercially obtained horse mackerel (Trachurus cupensis; 140 mm) and cape anchovy (En- gruulis capensis; 120 mm). Birds were fed twice daily (10:00 and 16:OO) by throwing them fish, until satiated. Cages were carefully searched for pellets at 08:00, after which the birds were weighed. Recovered pellets were measured to the nearest millimeter for maximum length and their volumes determined by displacement in water. Each pellet was then cut open and any otoliths and eye crystals extracted and count- ed. Three experiments were carried out: Experiment one. Does the composition of pellets accurately reflect daily food intake? We counted the number of otoliths and eye crystals in pellets in relation to the number of fish con- sumed. The size and number of fish fed to each bird in each cage were noted. The number of pellets produced one day later was recorded for each cage. We combined the number of fish fed and number of pellets produced within each cage, since we did not know which bird pro- duced each pellet within a cage. If pellets per- fectly reflected fish intake, and if pellets are produced daily, the number of otoliths and eye crystals should have been twice the number of fish consumed the day before. Experiment two. Can otolith size be used to