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Highly resolved general circulation models (GCMs) now generate realistic flow fields, and have revealed how sensitive larval drift routes are to vertical positioning in the water column. Sensible representation of behavioural processes then becomes essential to generate reliable patterns of environmental exposure (growth and survival), larval drift trajectories and dispersal. Existing individual-based models involving larval fish allow individuals to vary only in their attributes such as spatial coordinates, and not in their inherited behavioural strategies or phenotypes. We illustrate the interaction between short-term behaviour and longer-term dispersal consequences applying a model of larval cod Gadus morhua drifting in a GCM, and show how variations in swimming behaviour influence growth and dispersal. We recommend a deep integration of oceanography and behavioural ecology. First, we need to understand the causes and survival value of behaviours of larval fish, framed in terms of behavioural ecology. Second, we need practices to address how drift and dispersal of offspring are generating spawning strategies (timing and location) of adults, using life history theory. Third, the relative importance of local growth and mortality versus the need to drift to particular areas depend strongly on the mobility of organisms at the time of settling, or the spatial fitnesslandscape. The field of 'individual-based ecology' provides sound methods to approach this interface between evolutionary theory and physical oceanography.

Atlantic cod have been a primary target for marine stock enhancement since the 1880s. In the early part of this period, hatched larvae were released in Norway, the USA and Canada. The last larval releases were conducted in Norway in 1971, and a century of cod larvae releases were halted without any clear evidence of benefit. Since the early 1980s, the focus has been on production of larger, more viable juvenile cod. Emphasis has been given to the design of tag–release programmes involving large‐scale releases and ecosystem analysis in selected ecosystems. Most of this research has been carried out in Norway, where more than one million tagged juvenile cod have been released. Smaller stocking experiments have also been performed in Denmark, Sweden, the Faroe Islands and the USA. This paper reviews the major findings from these programmes. We include summaries and evaluations of rearing techniques for juvenile cod, methods of tagging and recapture, experimental fishing, migration, mortality and growth rates in the different habitats, genetic analysis, and ecosystem studies that have tried to describe the variation in the cod carrying capacity of selected release areas. Despite relatively large variation in environmental conditions, in cod production and in fishing mortality along the Norwegian coast, results indicate that, under the conditions experienced during the 1980s and 1990s, releases of juvenile cod did not significantly increase cod production and catches. The biological limitations and future prospects of Atlantic cod stock enhancement are addressed.

Due to vertical variations in ocean circulation, larval Northeast Arctic cod Gadus morhua may influence their own drift routes by migrating vertically. By coupling a larval individualbased model and a general circulation model, we simulated larval vertical positioning according to simple rules based on individual risk sensitivity. This enabled us to investigate how larval growth, survival and horizontal distribution vary between individuals following different rules. Immediate depth selection follows from the rules, with implications for environmental exposure and instantaneous growth rates. The behavioural rules had long-term and large-scale consequences, since vertical positioning influences the drift trajectory of the larva, and thereby the physical environment the larva experiences along its way. Two alternative rule formulations were explored, each containing the full range of strategies, from maximising immediate growth to maximising immediate survival. Fitness was defined as accumulated survival probability up to 18 mm for larvae released at 2 important spawning grounds in the Lofoten area. Both rules gave better fitness than for individuals drifting at fixed depths. The most successful individuals performed active vertical migration and had an intermediate risk sensitivity. When risk sensitivity was allowed to change with ontogeny, larvae that first emphasised growth and then changed to intermediate risk sensitivity were the most successful ones, although improvements were minor compared to fixed sensitivities. The 2 spawning grounds led to slight differences in fitness, but success as a result of risk sensitivity was similar at both, suggesting that optimal larval strategies may be robust across different spawning grounds.

BackgroundFisheries exploitation, habitat destruction, and climate are important drivers of variability in recruitment success. Understanding variability in recruitment can reveal mechanisms behind widespread decline in the abundance of key species in marine and terrestrial ecosystems. For fish populations, the match-mismatch theory hypothesizes that successful recruitment is a function of the timing and duration of larval fish abundance and prey availability. However, the underlying mechanisms of match-mismatch dynamics and the factors driving spatial differences between high and low recruitment remain poorly understood.Methodology/Principal FindingsWe used empirical observations of larval fish abundance, a mechanistic individual-based model, and a reanalysis of ocean temperature data from 1960 to 2002 to estimate the survival of larval cod (Gadus morhua). From the model, we quantified how survival rates changed during the warmest and coldest years at four important cod spawning sites in the North Atlantic. The modeled difference in survival probability was not large for any given month between cold or warm years. However, the cumulative effect of higher growth rates and survival through the entire spawning season in warm years was substantial with 308%, 385%, 154%, and 175% increases in survival for Georges Bank, Iceland, North Sea, and Lofoten cod stocks, respectively. We also found that the importance of match-mismatch dynamics generally increased with latitude.Conclusions/SignificanceOur analyses indicate that a key factor for enhancing survival is the duration of the overlap between larval and prey abundance and not the actual timing of the peak abundance. During warm years, the duration of the overlap between larval fish and their prey is prolonged due to an early onset of the spring bloom. This prolonged season enhances cumulative growth and survival, leading to a greater number of large individuals with enhanced potential for survival to recruitment.

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Several factors lead to expectations that the scale of larval dispersal and population connectivity of marine animals differs with latitude. We examine this expectation for demersal shorefishes, including relevant mechanisms, assumptions and evidence. We explore latitudinal differences in (i) biological (e.g. species composition, spawning mode, pelagic larval duration, PLD), (ii) physical (e.g. water movement, habitat fragmentation), and (iii) biophysical factors ( primarily temperature, which could strongly affect development, swimming ability or feeding). Latitudinal differences exist in taxonomic composition, habitat fragmentation, temperature and larval swimming, and each difference could influence larval dispersal. Nevertheless, clear evidence for latitudinal differences in larval dispersal at the level of broad faunas is lacking. For example, PLD is strongly influenced by taxon, habitat and geographical region, but no independent latitudinal trend is present in published PLD values. Any trends in larval dispersal may be obscured by a lack of appropriate information, or use of 'off the shelf' information that is biased with regard to the species assemblages in areas of concern. Biases may also be introduced from latitudinal differences in taxa or spawning modes as well as limited latitudinal sampling. We suggest research to make progress on the question of latitudinal trends in larval dispersal.

Arctic and Antarctic marine systems have in common high latitudes, large seasonal changes in light levels, cold air and sea temperatures, and sea ice. In other ways, however, they are strikingly different, including their: age, extent, geological structure, ice stability, and foodweb structure. Both regions contain very rapidly warming areas and climate impacts have been reported, as have dramatic future projections. However, the combined effects of a changing climate on oceanographic processes and foodweb dynamics are likely to influence their future fisheries in very different ways. Differences in the life-history strategies of the key zooplankton species (Antarctic krill in the Southern Ocean and Calanus copepods in the Arctic) will likely affect future productivity of fishery species and fisheries. To explore future scenarios for each region, this paper: (i) considers differing characteristics (including geographic, physical, and biological) that define polar marine ecosystems and reviews known and projected impacts of climate change on key zooplankton species that may impact fished species; (ii) summarizes existing fishery resources; (iii) synthesizes this information to generate future scenarios for fisheries; and (iv) considers the implications for future fisheries management. Published studies suggest that if an increase in open water during summer in Arctic and Subarctic seas results in increased primary and secondary production, biomass may increase for some important commercial fish stocks and new mixes of species may become targeted. In contrast, published studies suggest that in the Southern Ocean the potential for existing species to adapt is mixed and that the potential for the invasion of large and highly productive pelagic finfish species appears low. Thus, future Southern Ocean fisheries may largely be dependent on existing species. It is clear from this review that new management approaches will be needed that account for the changing dynamics in these regions under climate change.