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Reproductive investment is a central trait for population dynamics and productivity. Fishing and environmental variations are major drivers affecting population structure, dynamics, and adaptation of life-history and behavioural traits. However, those factors are often considered independently, and few studies take into account their joint effect. In this study, we investigate the contribution of environment, fishing pressure, and intra-specific competition to variation in the reproductive investment of the Norwegian spring-spawning herring (Clupea harengus), a stock that has been fished for centuries, and monitored for decades. Reproductive investment and post-spawning weight were affected differently by growth rate (measured as mean age-at-length), sea surface temperature, North Atlantic Oscillation, and spawning stock biomass in periods with no fishing, unselective fishing, and low but size-selective fishing. We hypothesize that those changes can be explained by direct effects of exploitation such as age truncation and changes in migration patterns. Our results highlight how fishing, by affecting population-level dynamics, can modify the impact of environmental variations on life-history traits.

Norway is the world’s largest producer of farmed Atlantic salmon and is home to ∼400 rivers containing wild salmon populations. Farmed escapees, a reoccurring challenge of all cage-based marine aquaculture, pose a threat to the genetic integrity, productivity, and evolutionary trajectories of wild populations. Escapees have been monitored in Norwegian rivers since 1989, and, a second-generation programme was established in 2014. The new programme includes data from summer angling, autumn angling, broodstock sampling, and snorkelling surveys in >200 rivers, and >25 000 scale samples are analysed annually. In 2014–2017, escapees were observed in two-thirds of rivers surveyed each year, and between 15 and 30 of the rivers had >10% recorded escapees annually. In the period 1989–2017, a reduction in the proportion of escapees in rivers was observed, despite a >6-fold increase in aquaculture production. This reflected improved escape prevention, and possibly changes in production methods that influence post-escape behaviour. On average, populations estimated to experience the greatest genetic introgression from farmed salmon up to 2014 also had the largest proportions of escapees in 2014–2017. Thus, populations already most affected are those at greatest risk of further impacts. These data feed into the annual risk-assessment of Norwegian aquaculture and form the basis for directing mitigation efforts.

Growth, onset of maturity and investment in reproduction are key traits for understanding variation in life-history strategies. Many environmental factors affect variation in these traits, but for fish, hypoxia and size-dependent mortality have become increasingly important because of human activities, such as increased nutrient enrichment (eutrophication), climate warming and selective fishing. Here, we study experimentally the effect of oxygen availability on maturation and growth in guppies (Poecilia reticulata) from two different selected lines, one subjected to positive and the other negative size-dependent fishing. This is the first study to assess the effects of both reduced ambient oxygen and size-dependent mortality in fish. We show that reduced ambient oxygen led to stunting, early maturation and high reproductive investment. Likewise, lineages that had been exposed to high mortality of larger-sized individuals displayed earlier maturation at smaller size, greater investment in reproduction and faster growth. These life-history changes were particularly evident for males. The widely reported trends towards earlier maturation in wild fish populations are often interpreted as resulting from size-selective fishing. Our results highlight that reduced ambient oxygen, which has received little experimental investigation to date, can lead to similar phenotypic changes. Thus, changes in ambient oxygen levels can be a confounding factor that occurs in parallel with fishing, complicating the causal interpretation of changes in life-history traits. We believe that better disentangling of the effects of these two extrinsic factors, which increasingly affect many freshwater and marine ecosystems, is important for making more informed management decisions.

To improve assessments of the environmental risks of aquaculture, a series of simulated escapes of farmed Atlantic salmon (Salmo salar L.) from seawater netpens were performed. Individually tagged post-smolts and adult Atlantic salmon were released from various locations at different times of the year. Post-smolts that escaped during their first summer were capable of rapid migration towards the open sea. A small fraction returned to spawn and were recaptured after 1.3 years at sea (0.4%, range 0.0.1.1%). A total of 13% of the post-smolts that escaped during autumn were reported in nearby fisheries during subsequent months, partly because they had grown large enough to be caught in the gillnets used, but more importantly because migratory behaviour diminished towards the end of the year. The mean recapture rate of adult salmon was high after releases in fjords (7.33%), lower after coastal releases (4.7%), and zero on the outer coast. Most of these recaptures were immature fish recaptured in sea relatively close to the release site during their first months post-release. Recaptures of adult escapees after 1.2 years in the wild were very rare (0.09%), probably because of their low survival. A Monte-Carlo method was developed to estimate the annual numbers of escapees from Norwegian fish farms based on reported catches of escaped farmed salmon in te sea and in rivers and the recapture probabilities reported here. The model provides a tool to estimate numbers of escapees independently from the reported numbers. Importantly, our analysis suggests that the total numbers of post-smolt and adult escapees have been two- to fourfold as high as the numbers reported to the authorities by fish farmers, depending on whether the incomplete sea fishery statistics are compensated for.

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.

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, B_lim and B_pa, and the target reference point for fishing mortality, F_0.1. 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.

Understanding the challenges of sustainable fisheries management is not easy for non-specialists, and even many specialists fail to appreciate the potential evolutionary consequences of harvest. We propose candy-fish experiments as a savoury approach to teaching and disseminating the key principles of applied ecology and evolution to students, practitioners and the general public. We performed a simple experiment where the resource was represented by fish-shaped candy of distinct colours and flavours (strawberry and liquorice). Typically, harvesting was neither ecologically sustainable (55% of the populations were extinct by the end of the experiment) nor evolutionarily sustainable (most surviving populations had liquorice fish only). This harvest-induced evolution went apparently unnoticed. Somewhat encouragingly, the harvest was most likely ecologically sustainable when a person spontaneously took the role of a stock manager.

We study the dynamics of Atlantic cod (Gadus morhua L.), pollack (Pollachius pollachius L.), and saite (Pollachius virens L.) in Masfjorden, a small fjord in western Norway. Annually, cohorts of pollack and cod grow in size and decline in abundance, as expected of closed populations, whereas saithe virtually disappear before maturity. Seasonally, in contrast, the dynamics of cod and pollack differ. Pollack shows a regular growth pattern, with most of the growth taking place in summer. Its abundance-at-age shows strong seasonal variations, with a marked increase from spring to summer followed by a decline through autumn and winter. These patterns relate partly to seasonal changes in depth distribution and catchability. Combined with the observation that spawning-stage pollack are abundant in spring, we interpret these patterns to suggest that pollack in Masfjorden represent a dynamically independent unit. Seasonal changes in abundance are less marked for cod, and seasonal changes in depth distribution seem insignificant. However, cod shows an unusual pattern in length at age, with no apparent growth in summer and fast growth in autumn. These patterns suggest that the population structure of cod may be more open than that of pollack and may involve mixing of more than one population component.

Material collected in summer 2004 from the Mid-Atlantic Ridge between Iceland and the Azores with three pelagic trawls was used to estimate relative catchabilities of common fish, cephalopod, decapod, and jellyfish species. Catchability is defined as the ratio of numbers caught between two trawls, standardized for towed distance. Taxon-specific catchability coefficients were estimated for two large pelagic trawls with graded meshes, using a smaller pelagic trawl with a uniform mesh size as the reference trawl. Two of the trawls were equipped with multiple opening-closing codends that allowed sampling of different depth layers. Generalized linear and mixed models suggest that most of the taxa have catchabilities much lower than expected from the area of opening alone, indicating that only a few species are herded by the large mesh at the mouth of larger trawls. Catchability coefficients across taxa show a very large spread, indicating that the sampled volume for the larger trawls with graded meshes was highly taxon-specific. Part of this variability can be explained by body size and taxonomic group, the latter probably reflecting differences in body form and behaviour. The catchability estimates presented here form the basis for combining data for quantitative analyses of community structure.

Blue whiting (Micromesistius poutassou) are distributed throughout the North Atlantic, including the Norwegian and Barents Seas. In recent years, both abundance and distribution of blue whiting in the Barents Sea have increased dramatically. Therefore, to evaluate the trophic impact of this increase, we analysed the diet of the species. In all, 54 prey species or taxa were identified, the main prey being krill. However, the diet varied geographically and ontogenetically: the proportion of fish in the diet was higher in large blue whiting and in the north of the range. Blue whiting overlap geographically with other pelagic species at the edge of their distribution in the Barents Sea, with juvenile herring in the south, with polar cod in the north, and with capelin in the northeast. The overlap in diet between blue whiting and these other pelagic species ranged from 6 to 86% and was greatest with capelin in areas where both species feed on hyperiids and krill. The importance of blue whiting as prey for predatory fish was highest in the areas of greatest abundance, but overall, blue whiting were seemingly unimportant as prey of piscivorous fish in the Barents Sea.

To interpret long-term trends in age and size at maturation, new statistical methods have recently been devised for estimating probabilistic maturation reaction norms based on data collected for the management of fisheries. Here we apply these methods to three Newfoundland stocks of American plaice (NAFO Divisions 2J3K, 3LNO, 3Ps) and report a clear long-term shift in the maturation reaction norms of these stocks towards maturation at younger age and smaller size. Theory predicts that such trends could result from fishing acting as a selective force, inducing evolutionary changes in the life histories of exploited populations. Matching long-term trends in maturation reactions norms have already been documented for several stocks that have experienced high fishing pressures (Gulf of Maine and Georges Bank cod, Northeast Arctic cod, North Sea plaice). Our results add a new dimension to these earlier findings: since fishing pressures for two stocks of Newfoundland plaice (2J3K and 3Ps) have been relatively low, our results imply that fishing is likely to result in the evolution of life histories even when fishing mortality is low, or that natural mortality has played an important role in determining selective pressures in these populations. Both options suggest that conditions for rapid life-history evolution in exploited stocks are less restrictive than previously appreciated.

The 1992 Convention on Biological Diversity has established an international framework for broader conservation objectives for the management of ocean use activities (cf. Sainsbury and Sumaila, 2003). The Convention calls for preservation of biological diversity, including genetic, species and ecosystem diversity, thereby creating a demand for developing management forms that can cope with this issue. A number of initiatives have been developed to address this and other related international agreements. Notably, Ecosystem-Based Fishery Management (EBFM; cf. Brodziak and Link, 2002; Sainsbury and Sumaila, 2003) has emerged as a holistic approach to maintaining ecosystems and sustainable fisheries. EBFM should incorporate all levels of diversity, but in practice has focused on species and ecosystem diversity (e.g., Brodziak and Link, 2002). The application of EBFM must be broadened to include conservation of genetic diversity, including intraspecific diversity, which are not necessarily protected by maintaining diversity at higher levels (cf. Kenchington, 2003). Scientific justification for conserving genetic diversity stems from several sources including: (1) maintaining adaptability of natural populations; (2) the future utility of genetic resources for medical and other purposes; and (3) changes in life history traits and behaviour that influence the dynamics of fish populations, energy flows in the ecosystem, and ultimately, sustainable yield. The challenge is to formulate appropriate management actions for the preservation of genetic diversity (e.g., Sainsbury and Sumaila, 2003). This will require consensus on what it is we are trying to preserve (e.g., alleles, traits, population structure) and some means of assessing genetic status. This paper endeavours to outline a process to develop management advice for marine genetic diversity. We challenge fish conservation geneticists to consider their work in a more applied context so that management actions can be deve

Reaction norms for age and size at maturation describe the probability of immature fish maturing at a certain age and size. Knowledge of such reaction norms is increasingly important, both for observing and understanding changes in maturation and for calibrating size- and age-structured population models. Estimating the reaction norms for age and size at maturation by logistic regression requires data on the size and age distribution of immature and maturing fish. To permit such estimation when measurements of the size and age distribution of immature fish are not available, the information can be reconstructed by means of a back-projection procedure. For the reconstruction, only the size and age distribution of maturing fish, or first-time spawners, together with the age-dependent proportion of mature fish, given in the form of maturity ogives, have to be known. The method of Gulland (1964) is used to generate maturity ogives in the absence of data on immature fish. The robustness of the estimation method is demonstrated by analysing artificial data generated with a known reaction norm. Application of the reconstruction method to a set of measurements on Northeast Arctic cod (Gadus morhua) collected between 1933 and 1944 illustrates how the approach allows new information to be extracted from real data.