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In this paper, we present a simple theoretical, steady-state equilibrium, predator-prey model for the joint management of marine mammals and a fish species. As an empirical case, we choose cod and grey seals in the Nordic-Baltic Sea countries, and several benefits and costs related to the latter are considered. We show that the optimal grey seal population is much lower than the actual population, and this result is robust to variations in relevant parameter values. This result can be explained by the fact that the profit from harvesting cod is much higher than the net benefits from grey seals.

We develop the age-structured fishery model by including endogenous harvesting selectivity and stochastic recruitment, as a growing body of fishery ecological evidence suggests these factors to be critical for fisheries management. Optimal harvesting selectivity aims to direct fishing towards age classes that are preferable to catch given information on fish growth, natural mortality, and recruitment in addition to implications on harvesting cost. We analytically show that maximum sustainable yield (MSY) leads to potentially serious and previously unrecognized deviation from economic optimality, as it neglects the dependence of harvesting costs on gear selectivity. We further show that the steady-state population level may fall below the MSY population even with zero discounting and stock-dependent harvesting costs. We quantify our results using empirical data for Baltic cod. Applying the age-structured model with endogenous harvesting selectivity, we find large differences between maximum sustainable yield and the economic optimum, although the classic biomass model suggests that these differences should be unimportant. Stochastic recruitment implies a threefold increase in young age classes, but the stochastic solution can be accurately approximated by the certainty equivalence principle.

International audience; Climate change is rapidly modifying marine fish assemblages in the Arctic. As fish eggs and larvae have a narrower thermal tolerance than nonreproductive adults, their response to increasing temperatures is likely one of the main drivers of these changes. In this study, we described ichthyoplankton assemblages in West Greenland between 62 and 73 °N, during summers 2017–2019, and investigated the relationship between sea surface temperature in the spring and summer and the survival of Arctic cod (Boreogadus saida (Lepechin, 1774)) early life stages over the hatching season. Warm years were associated with partial recruitment failures resulting from thermal stress to the eggs and larvae hatched late in the season. Using past environmental conditions, we forecasted an imminent decline in Arctic cod recruitment in the regions of Uummannaq and Disko Bay. Observations from fjords suggested that glacial meltwater could create a subsurface thermal refuge allowing Arctic cod larvae to survive despite very high summer sea surface temperature (ca. 10 °C). As the Greenland ice sheet is melting at an unprecedented speed, the mechanism underlying the “glacial meltwater summer refuge hypothesis” could curb some of the negative effects of ocean warming on the survival of young Arctic cod in West Greenland and other Arctic fjord systems.

Specific changes identified in the otolith macrostructure of Northeast Arctic cod as “spawning zones” are presumed to represent spawning events, but recent experimental studies have challenged this relationship. Because these zones are not routinely recorded outside of Norway, otoliths from multiple Atlantic cod populations with different life history and environmental traits were first examined to see if spawning zones could be identified as a general characteristic of cod. Then, a large archival collection of cod otoliths was used to investigate temporal changes in the occurrence of spawning zones and test for correlations between maturity at age derived from otolith spawning zones and gonad maturity stages. This study shows that spawning zones likely are a universal trait of Atlantic cod and not limited to certain environments or migratory behaviors as previously proposed. Maturity at age derived from spawning zone data showed trends consistent with those from gonad examinations. However, spawning zones appear to form with a one- or two-year lag with sexual maturity, which is suspected to reflect a stabilizing of energy partitioning after the first spawning events. Our results illustrate the potential for use of spawning zones, for example in species or populations with limited available maturity data, and highlights the need for addressing the physiological processes behind their formation.

Place: Hoboken Publisher: Wiley WOS:000531167600001; Many deep-water fish populations, being K-selected species, have little resilience to overexploitation and may be at serious risk of depletion as a consequence. Sea warming represents an additional threat. In this study, the condition, or health, of several populations of common ling (Molva molva), blue ling (Molva dypterygia) and Mediterranean or Spanish ling (Molva macrophthalma) inhabiting different areas in the North Atlantic and the Mediterranean was evaluated, to shed light on the challenges these deep-water species are facing in the context of fishing activity and a warming climate. The data on the condition of Molva populations which are analysed here have been complemented with data on abundance and, for the southernmost species (Mediterranean ling), with two other health indicators (parasitism and hepato-somatic index). Despite some exceptions (e.g., common ling in Icelandic waters), this study shows that the condition of many populations of Molva species in the northeastern Atlantic and the Mediterranean Sea has worsened, a trend which, in recent decades, has usually been found to be accompanied by a decline in their abundance. In addition, the poor health status of most populations of common ling, blue ling and Mediterranean ling considered in this analysis points to a lower sustainability of these populations in the future. Overall, the health status and abundance of Molva populations in the northeastern Atlantic and the Mediterranean suggest that only some populations located in the North Atlantic may be able to rebuild, whereas the populations in southern North Atlantic and the Mediterranean, which are probably most at risk from sea warming, are facing serious difficulties in doing so. In the context of fisheries and global warming, this study's results strongly indicate that management bodies need to consider the health status of many of the populations of Molva species, particularly in southern European waters, before implementing their decisions.

An experiment was conducted to assess the selectivity in a typical Mediterranean bottom trawl, equipped with a square-mesh panel inserted in front of the cod end, for Atlantic horse mackerel (Trachurus trachurus), European hake (Merluccius merluccius), red mullet (Mullus barbatus), poor cod (Trisopterus minutus), broadtail shortfin squid (Illex coindetii), and deep-water rose shrimp (Parapaeneus longirostris). The release efficiency of undersized individuals through the panel was low. The differences in selectivity between the gear with and without the panel were very small. The low release efficiency of the square-mesh panel was caused by the lack of fish contact with the panel as they drifted towards the cod end, since the average contact probability was estimated not to exceed 9% for any of the species investigated. A low probability of contact with the selection device was thus found to be the reason for the low efficiency of the square-mesh panel.

The importance of survival and growth variations early in life for population dynamics depends on the degrees of compensatory density dependence and size dependence in survival at later life stages. Quantifying density- and size-dependent mortality at different juvenile stages is therefore important to understand and potentially predict the recruitment to the population. We applied a statistical state-space modelling approach to analyse time series of abundance and mean body size of larval and juvenile fish. The focus was to identify the importance of abundance and body size for growth and survival through successive larval and juvenile age intervals, and to quantify how the dynamics propagate through the early life to influence recruitment. We thus identified both relevant ages and mechanisms (i.e. density dependence and size dependence in survival and growth) linking recruitment variability to early life dynamics. The analysis was conducted on six economically and ecologically important fish populations from cold temperate and sub-arctic marine ecosystems. Our results underscore the importance of size for survival early in life. The comparative analysis suggests that size-dependent mortality and density-dependent growth frequently occur at a transition from pelagic to demersal habitats, which may be linked to competition for suitable habitat. The generality of this hypothesis warrants testing in future research.

This paper examines variations in gender-specific body growth, morphology and reproduction of the bearded goby (Sufflogobius bibarbatus) across the Namibian shelf, built on previous work on male reproductive modes. The results indicate a spatial variation in the size composition, condition factor and maturation of gobies across the shelf area. Low oxygen (

Catecholamines protect the fish heart during hypoxia. However, the humoral adrenergic stress response may only be invoked in extremis. We investigated the hypothesis that endogenous (e.g., neuronal) myocardial catecholamines may also impact cardiac performance during hypoxia in a hypoxia-tolerant tropical fish, the red-bellied piranha (Pygocentrus nattereri). First, we measured endogenous tissue catecholamines and in vitro catecholamine release from piranha myocardium using ultraperformance liquid chromatography. Ventricle homogenates contained detectable levels of both adrenaline (7.27 ng/g) and noradrenaline (14.48 ng/g), but only noradrenaline was released from ventricular tissue incubated in Ringer’s solution. Noradrenaline released in this assay was not affected by hypoxia but was promoted by the catecholamine releasing agent tyramine. Our second series of experiments explored cardiac contractile performance in vitro using tyramine, exogenous noradrenaline or adrenaline, and propranolol (a β-adrenoceptor antagonist). In ventricular strip preparations, β-adrenergic blockade with propranolol had no effects on twitch force or contraction kinetics in either normoxia or hypoxia, confirming that spontaneous endogenous catecholamine release did not impact cardiac performance. However, in the absence of propranolol, tyramine mimicked the positive inotropic effect of noradrenaline (10 µM) during hypoxia, although adrenaline was capable of generating larger effects. Our results suggest that, although it is not spontaneously released, inducible endogenous noradrenaline release may have a significant β-adrenoceptor-dependent impact on hypoxic performance in the fish heart.

Vertical migration is the most widespread migration in the aquatic world, yet the mechanisms limiting the extent of this behavior are largely unknown. In the Baltic Sea, some Atlantic cod Gadus morhua perform vertical foraging migrations into severely hypoxic demersal zones where aerobic metabolism is insufficient to cover energy requirements. After foraging, the fish return to better oxygenated waters for physiological recovery and digestion. To test the influence of phenotypic variation in aerobic scope (AS; the difference between the maximum and the minimum metabolic rate) on the capacity to migrate into severely hypoxic zones, we incorporated AS into a state-dependent individual-based model simulating vertical foraging migrations of G. morhua. We found little effect of individual variation in AS on the capacity for vertical migration when the zone used for physiological recovery was normoxic. In contrast, when there was moderate hypoxia (30% air saturation, O-2sat) in the zone used for physiological recovery, the high AS phenotype had a clear advantage because it could forage 3-4 times longer in the severely hypoxic (16% O-2sat, i.e. below the threshold for aerobic metabolism of the species) demersal zone compared to the low AS phenotype. Thus, phenotypic variation in AS is only important when there is moderate hypoxia in the zone used for physiological recovery, suggesting that the influence of AS variation on the capacity for vertical migration is context dependent. We propose that elevated AS may be evolutionarily favorable when hypoxia prevails in the water column.

WOS:000426336600006; International audience; Thyroid hormones (THs) are key regulators of growth, development, and metabolism in vertebrates and influence early life development of fish. TH is produced in the thyroid gland (or thyroid follicles) mainly as T4 (thyroxine), which is metabolized to T3 (3,5,3'-triiodothyronine) and T2 (3,5-diiodothyronine) by deiodinase (DIO) enzymes in peripheral tissues. The action of these hormones is mostly exerted by binding to a specific nuclear thyroid hormone receptor (THR). In this study, we i) cloned and characterized thr sequences, ii) investigated the expression pattern of the different subtypes of thrs and dios, and iii) studied how temperature affects the expression of those genes in artificially produced early life history stages of European eel (Anguilla anguilla), reared in different thermal regimes (16, 18, 20 and 22 degrees C) from hatch until first-feeding. We identified 2 subtypes of thr (thr alpha and thr beta) with 2 isoforms each (thr alpha A, thr alpha B, thr beta A, thr beta beta) and 3 subtypes of deiodinases (dio1, dio2, dio3). All thr genes identified showed high similarity to the closely related Japanese eel (Anguilla japonica). We found that all genes investigated in this study were affected by larval age (in real time or at specific developmental stages), temperature, and/or their interaction. More specifically, the warmer the temperature the earlier the expression response of a specific target gene. In real time, the expression profiles appeared very similar and only shifted with temperature. In developmental time, gene expression of all genes differed across selected developmental stages, such as at hatch, during teeth formation or at first-feeding. Thus, we demonstrate that thrs and dios show sensitivity to temperature and are involved in and during early life development of European eel.

WOS:000407548800031; International audience; Temperature is important for optimization of rearing conditions in aquaculture, especially during the critical early life history stages of fish. Here, we experimentally investigated the impact of temperature (16, 18, 20, 22 and 24 degrees C) on thermally induced phenotypic variability, from larval hatch to first-feeding, and the linked expression of targeted genes [heat shock proteins (hsp), growth hormone (gh) and insulin-like growth factors (igf)] associated to larval performance of European eel, Anguilla anguilla. Temperature effects on larval morphology and gene expression were investigated throughout early larval development (in real time from 0 to 18 days post hatch) and at specific developmental stages (hatch, jaw/teeth formation, and first-feeding). Results showed that hatch success, yolk utilization efficiency, survival, deformities, yolk utilization, and growth rates were all significantly affected by temperature. In real time, increasing temperature from 16 to 22 degrees C accelerated larval development, while larval gene expression patterns (hsp70, hsp90, gh and igf-1) were delayed at cold temperatures (16 degrees C) or accelerated at warm temperatures (20-22 degrees C). All targeted genes (hsp70, hsp90, gh, igf-1, igf-2a, igf-2b) were differentially expressed during larval development. Moreover, expression of gh was highest at 16 degrees C during the jaw/teeth formation, and the first-feeding developmental stages, while expression of hsp90 was highest at 22 degrees C, suggesting thermal stress. Furthermore, 24 degrees C was shown to be deleterious (resulting in 100% mortality), while 16 degrees C and 22 degrees C (similar to 50 and 90% deformities respectively) represent the lower and upper thermal tolerance limits. In conclusion, the high survival, lowest incidence of deformities at hatch, high yolk utilization efficiency, high gh and low hsp expression, suggest 18 degrees C as the optimal temperature for offspring of European eel. Furthermore, our results suggest that the still enigmatic early life history stages of European eel may inhabit the deeper layer of the Sargasso Sea and indicate vulnerability of this critically endangered species to increasing ocean temperature.

WOS:000397021200006; High-density (HD) rotifer culture systems have been recently commercialized, but are not commonly used by the aquaculture industry. The aim of this study was to determine if HD systems could be used in cod hatcheries. An enrichment strategy using the commercial products, ArteMac and Protein Selco Plus (Com), was compared with the manufacturer's suggested enrichment, using Pavlova-DHA (Pav). The Pav enrichment increased the eicosapentaenoic acid (EPA) levels in rotifers, but reduced the docosahexaenoic acid (DHA) and 22: 5n-6 (n-6DPA, docosapentaenoic acid) levels. Larvae EPA levels in both polar and neutral lipids were relatively stable in the larvae fed with Com-rotifers; while they were higher in early stages, they were progressively reduced through ontogeny in the Pav-rotifers fed-larvae. DHA levels in polar lipids decreased in larvae, particularly when fed with HD-Pav rotifers. In all larvae, arachidonic acid (ARA) levels increased in the polar and neutral lipids, regardless of treatments. In both lipid fractions, the levels of ARA were quite stable in time, but still higher in larvae fed with Com-rotifers. Bacterial load was lower in larvae fed with Pav-rotifers. Denaturing gradient gel electrophoresis (DGGE) bacterial profiles of larvae and rotifers were all similar. This study shows the potential of usingHDsystems to produce rotifers, but highlights the necessity of adjusting the nutritional composition of rotifers prior to being fed to larvae.

Harvest control rules (HCRs) have been implemented for many fisheries worldwide. However, in most instances, those HCRs are not based on the explicit feedbacks between stock properties and economic considerations. This paper develops a bio-economic model that evaluates the HCR adopted in 2004 by the Joint Norwegian–Russian Fishery Commission to manage the world's largest cod stock, Northeast Arctic cod (NEA). The model considered here is biologically and economically detailed, and is the first to compare the performance of the stock's current HCR with that of alternative HCRs derived with optimality criteria. In particular, HCRs are optimized for economic objectives including fleet profits, economic welfare, and total yield and the emerging properties are analyzed. The performance of these optimal HCRs was compared with the currently used HCR. This paper show that the current HCR does in fact comes very close to maximizing profits. Furthermore, the results reveal that the HCR that maximizes profits is the most precautionary one among the considered HCRs. Finally, the HCR that maximizes yield leads to un-precautionary low levels of biomass. In these ways, the implementation of the HCR for NEA cod can be viewed as a success story that may provide valuable lessons for other fisheries., Highlights ► We examine harvest control rules (HCRs) for Northeast Arctic cod, the world's largest cod stock. ► The bio-economic model is detailed and estimated with long-term data. ► The currently used HCR is compared with alternatives optimizing profit, welfare or yield. ► The currently used HCR comes very close to the one that maximizes profits. ► Maximizing yield leads to un-precautionary low levels of biomass.

Little is known about how quickly natural populations adapt to changes in their environment and how temporal and spatial variation in selection pressures interact to shape patterns of genetic diversity. We here address these issues with a series of genome scans in four overfished populations of Atlantic cod (Gadus morhua) studied over an 80-year period. Screening of >1000 gene-associated single-nucleotide polymorphisms (SNPs) identified 77 loci that showed highly elevated levels of differentiation, likely as an effect of directional selection, in either time, space or both. Exploratory analysis suggested that temporal allele frequency shifts at certain loci may correlate with local temperature variation and with life history changes suggested to be fisheries induced. Interestingly, however, largely nonoverlapping sets of loci were temporal outliers in the different populations and outliers from the 1928 to 1960 period showed almost complete stability during later decades. The contrasting microevolutionary trajectories among populations resulted in sequential shifts in spatial outliers, with no locus maintaining elevated spatial differentiation throughout the study period. Simulations of migration coupled with observations of temporally stable spatial structure at neutral loci suggest that population replacement or gene flow alone could not explain all the observed allele frequency variation. Thus, the genetic changes are likely to at least partly be driven by highly dynamic temporally and spatially varying selection. These findings have important implications for our understanding of local adaptation and evolutionary potential in high gene flow organisms and underscore the need to carefully consider all dimensions of biocomplexity for evolutionarily sustainable management.

Link to publishers version: 10.1371/journal.pone.0160354 It is well established that increasing mesh number in the circumference of a diamond-mesh trawl codend can reduce size selection for round fish, whereas selection for flat fish species is unaffected. This effect has also been documented in Mediterranean trawl fisheries. In contrast, no information is available with regard to the effect of increasing mesh number in the circumference of square-mesh codends on the size selection of round fish and flat fish species. A field study was devised to bridge this gap and formulate proposals aimed at improving trawl fishery management. Size selection data were collected for a round fish species, red mullet (Mullus barbatus), and two flat fish species, Mediterranean scaldfish (Arnoglossus laterna) and solenette (Buglossidium luteum). Fishing trials were conducted in the Adriatic Sea (Central Mediterranean) using three square-mesh codends that differed only in mesh number around the circumference. Results demonstrated that increasing the number of meshes from 107 to 213 reduced the 50% retention length (L50) for red mullet by 2.5 cm but did not affect size selection for the two flat fish species. In some fisheries, regulatory provisions regarding the number of meshes in the circumference should therefore be carefully considered both for diamond- and square-mesh codends.

International audience; Soft sediments in coastal shallow waters constitute nursery habitats for juveniles of several flatfishes. The quality of a nursery is defined by its capacity to optimize the growth and the survival of juvenile fish. The influence of biotic factors, such as food availability, is poorly studied at the scale of a nursery ground. Whether food availability limits juvenile survival is still uncertain. A spatial approach is used to understand the influence of food availability on the distribution of juvenile fish of various benthic and demersal species in the Bay of Vilaine (France), a productive nursery ground. We quantified the spatial overlap between benthic macro-invertebrates and their predators (juvenile fish) to assess if the latter were spatially covering the most productive areas of the Bay. Three scenarios describing the shapes of the predator prey spatial relationship were tested to quantify the strength of the relationship and consequently the importance of food availability in determining fish distribution. Our results underline that both.food availability and fish densities vary greatly over the nursery ground. When considering small organisational levels (e.g., a single fish species), the predator prey spatial relationship was not dear, likely because of additional environmental effects not identified here; but at larger organisational level (the whole juvenile fish community), a strong overlap between the fish predators and their prey was identified. The evidence that fish concentrate in sectors with high food availability suggests that either food is the limiting factor in that nursery or/and fish display behavioural responses by optimising their energetic expenditures associated with foraging. Further investigations are needed to test the two hypotheses and to assess the impact of benthic and demersal juvenile fish in the food web of coastal nurseries. (C) 2015 Elsevier B.V. All rights reserved.

At the end of May, 17 scientists involved in an EU COST Action on Conservation Physiology of Marine Fishes met in Oristano, Sardinia, to discuss how physiology can be better used in modelling tools to aid in management of marine ecosystems. Current modelling approaches incorporate physiology to different extents, ranging from no explicit consideration to detailed physiological mechanisms, and across scales from a single fish to global fishery resources. Biologists from different sub-disciplines are collaborating to rise to the challenge of projecting future changes in distribution and productivity, assessing risks for local populations, or predicting and mitigating the spread of invasive species.

This paper introduces a non-destructive method for monitoring headspace ammonium as an indicator for changes in the freshness status of packed fish. Electrodes in an aqueous phase in the package monitor changes in the concentration of ammonia produced in/on the packed fish and released in the headspace. The outputs of an ammonium ion-selective electrode (NH 4 + -ISE) were compared with the volatile amines content of the fish fillets. The method was tested in triplicate with fresh cod fillets stored between −0.5 and 1.9 °C. Changes in the ammonia content of the fish could be monitored in the aqueous phase with the NH 4 + -ISE. The changes in the NH 4 + -ISE signal correlated with the content of volatile amines (TVB-N) in the cod fillets. This non-destructive method might be the basis for the development of an intelligent packaging for monitoring freshness of packed fish.

Brunel, T., Piet, G. J., van Hal, R., and Röckmann, C. 2010. Performance of harvest control rules in a variable environment. – ICES Journal of Marine Science, 67: 1051–1062. Population dynamic models used for fisheries management assume that stocks are isolated entities, ignoring the influence of environmental factors on stock productivity. An operating model parameterized for North Sea cod, plaice, and herring is developed, in which the link between recruitment and environment is assumed to be known and described by generalized additive models. This tool is used to compare the performance of harvest control rules (HCRs) when recruitment is independent of the environment or when recruitment is affected by an environment varying according to different scenarios. The first HCR exploited the stock with a fixed fishing mortality (F) corresponding to maximum sustainable yield, and in the second HCR, F was set equal to the precautionary approach F (i.e. Fpa), but reduced from Fpa when stock biomass fell below Bpa. The performance of the HCRs altered only slightly in a randomly varying environment compared with a constant one. For a detrimental change in the environment, however, no HCR could prevent a massive decrease in stock size. The performance of the HCRs was also influenced by the stock characteristics, such as recruitment variability or the shape of the stock–recruitment relationship. The performance of “environmental” HCRs (eHCRs), in which F varies depending on environmental conditions, was compared with that of conventional HCRs. The gain in using the eHCR was small, except for a detrimental change in the environment, where the eHCR performed markedly better than a conventional HCR. The benefits of using the eHCR were the greatest for the stock with the strongest environment–recruitment relationship.

The population dynamics of the Eastern Baltic cod (Gadus morhua callarias L.), unlike many other stocks, shows a strong dependency on environmental conditions. To test the implications of different management policies on the stock and the fishery in a system of global environmental change, we apply a spatially disaggregated, discrete time, age-structured model of the Eastern Baltic cod stock in 50 year simulation analyses. The simulation provides an analysis of stock, yield, and revenue development under various management policies and environmental scenarios. The policy analysis, focusing on different regulations of fishing mortality, is embedded into three environmental scenarios, assuming low, medium, or high climate and environmental change. The environmental assumptions are based on simulation results from a coupled atmosphere-ocean regional climate model, which project salinity in the Baltic Sea to decrease by 7-47% in the period 2071-2100 relative to the reference period 1961-1990. Our simulation results show that a significant reduction in fishing mortality is necessary for achieving high long-term economic yields. Moreover, under the presented environmental scenarios, a stock collapse cannot be prevented. It can, however, be postponed by the establishment of a marine reserve in ICES subdivision 25.

The gut microbiota of fish larvae evolves fast towards a complex community. Both host and environment affect the development of the gut microbiota; however, the relative importance of both is poorly understood. Determining specific changes in gut microbial populations in response to a change in an environmental factor is very complicated. Interactions between factors are difficult to separate and any response could be masked due to high inter-individual variation even for individuals that share a common environment. In this study we characterized and quantified the spatio-temporal variation in the gut microbiota of tilapia larvae, reared in recirculating aquaculture systems (RAS) or active suspension tanks (AS). Our results showed that variation in gut microbiota between replicate tanks was not significantly higher than within tank variation, suggesting that there is no tank effect on water and gut microbiota. However, when individuals were reared in replicate RAS, gut microbiota differed significantly. The highest variation was observed between individuals reared in different types of system (RAS vs. AS). Our data suggest that under experimental conditions in which the roles of deterministic and stochastic factors have not been precisely determined, compositional replication of the microbial communities of an ecosystem is not predictable.

Detrimental host-pathogen interactions are a normal phenomenon in aquaculture animal production, and have been counteracted by prophylactic use of antibiotics. Especially, the youngest life stages of cultivated aquatic animals are susceptible to pathogen invasion, resulting in disease and mortality. To establish a more sustainable aquatic food production, there is a need for new microbial management strategies that focus on 'join them' and not the traditional 'beat them' approaches. We argue that ecological theory could serve as a foundation for developing sustainable microbial management methods that prevent pathogenic disease in larviculture. Management of the water microbiota in aquaculture systems according to ecological selection principles has been shown to decrease opportunistic pathogen pressure and to result in an improved performance of the cultured animals. We hypothesize that manipulation of the biodiversity of the gut microbiota can increase the host's resistance against pathogenic invasion and infection. However, substantial barriers need to be overcome before active management of the intestinal microbiota can effectively be applied in larviculture.

Fish stocks experiencing high fishing mortality show a tendency to mature earlier and at a smaller size, which may have a genetic component and therefore long-lasting economic and biological effects. To date, the economic effects of such ecoevolutionary dynamics have not been empirically investigated. Using 70 y of data, we develop a bioeconomic model for Northeast Arctic cod to compare the economic yield in a model in which life-history traits can vary only through phenotypic plasticity with a model in which, in addition, genetic changes can occur. We find that evolutionary changes toward faster growth and earlier maturation occur consistently even if a stock is optimally managed. However, if a stock is managed optimally, the evolutionary changes actually increase economic yield because faster growth and earlier maturation raise the stock’s productivity. The optimal fishing mortality is almost identical for the evolutionary and nonevolutionary model and substantially lower than what it has been historically. Therefore, the costs of ignoring evolution under optimal management regimes are negligible. However, if fishing mortality is as high as it has been historically, evolutionary changes may result in economic losses, but only if the fishery is selecting for medium-sized individuals. Because evolution facilitates growth, the fish are younger and still immature when they are susceptible to getting caught, which outweighs the increase in productivity due to fish spawning at an earlier age.

Spontaneous fluctuations in oxygen consumption (R) and ventilation frequency (VF) of stoplight parrotfish Sparisoma viride were measured simultaneously in small now-through respirometers to establish the relationship between these variables. Respiration of 11 fish of varying size (14 to 1052 g) and life phase (juveniles, initial and terminal phase adults) was measured at naturally fluctuating temperatures. A large proportion of the variance in respiration could be explained by the equation: R = 0.00035 x VF1.369 x W-1.118 (r(2) = 93.1%, n = 380, p

It has been hypothesised that, at non-limiting water oxygen conditions, voluntary feed intake (FI) in fish is limited by the maximal physiological capacity of oxygen use (i.e. an ‘oxystatic control of FI in fish’). This implies that fish will adjust FI when fed diets differing in oxygen demand, resulting in identical oxygen consumption. Therefore, FI, digestible energy (DE) intake, energy balance and oxygen consumption were monitored at non-limiting water oxygen conditions in Nile tilapia fed diets with contrasting macronutrient composition. Diets were formulated in a 2 × 2 factorial design in order to create contrasts in oxygen demand: two ratios of digestible protein (DP):DE (‘high’v.‘low’); and a contrast in the type of non-protein energy source (‘starch’v.‘fat’). Triplicate groups of tilapia were fed each diet twice daily to satiation for 48 d. FI (g DM/kg0·8per d) was significantly lower (9·5 %) in tilapia fed the starch diets relative to the fat diets. The DP:DE ratio affected DE intakes (P R20·81,P

Groups orcod, Gadus moriiuu L, presented with two feeding patches with a food abundance ratio of 2 : I, distributed themselves between the patches in a ratio of 2.5 : 1. This is slightly higher than the 2 : I ratio predicted by the ideal free distribution theory. Large differences were observed in competitive ability between individual fish. A strong correlation was found between feeding success of individuals and time spent in a feeding patch. The more successful competitors caught about 2.5 times as many food items in the rich patch as in the poor patch. The less successful competitors caught an equal amount of food in both patches. All competitors, however, spent significantly more time in the rich patch. These results suggest that hunting success is the most important factor in assessingpatchquality. However, it isnot theonlyparameter which cod use in deciding whcrc to feed.

Historically colder regions of the North Atlantic had fisheries dominated by only a few fish species; principally cod and capelin. Possible population dynamic mechanisms that lead to such dominance are investigated by considering how a charmingly simple published multispecies model of the North Sea would react if the system operated at a lower temperature. The existing model equations were modified to describe temperature effects on growth, fecundity and recruitment and the model was rerun based on typical temperatures for the North Sea and a colder system. The results suggest that total fish biomass in the colder system increases but the community is more vulnerable to a given rate of fishing mortality. In the colder system, within species density dependence is reduced but relative predation rates are higher. Consequently, intermediate-sized species are vulnerable to relatively high levels of predation throughout their life history and tend to be excluded, leading to a system dominated by small and large species. The model helps to explain how temperature may govern coexistence and competitive exclusion in fish communities and accounts for the observed dominance of small and large species in Boreal/Arctic ecosystems.

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.

This study adds a cost analysis of the Eastern Baltic cod fishery to the existing model presented in Roeckmann et al. [2007a]. As cost data on this international fishery do not exist, data from Denmark are extrapolated to the whole international fishery. Additionally, unit and total variable costs are simulated, and the sensitivity to a set of different cost-stock and cost-output elasticities is tested. The study supports preliminary conclusions that a temporary marine reserve policy, which focuses on protecting the Eastern Baltic cod spawning stock in the International Council for the Exploration of the Sea (ICES) subdivision 25, is a valuable fisheries management tool to (i) rebuild the overexploited Eastern Baltic cod stock and (ii) increase operating profits. The negative effects of climate change can be postponed for at least 20 years-depending on the assumed rate of future climate change. Including costs in the economic analysis does not change the ranking of management policies as proposed in the previous study where costs were neglected.

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.

A spatially disaggregated, discrete time, age-structured model for the Eastern Baltic cod (Gadus morhua callarias L.) stock was constructed, in order to test the implications of the establishment of a marine reserve in the Baltic Sea. Functional relationships for recruitment and predation mortality were developed by multiple regression analyses. The resultant model output compares well with observed data from the fishery. The model was then applied to simulate stock development over a 50 year time period using different management policies and a variety of environmental conditions. The investigated management policies reduce fishing mortality and range from a moratorium on the Eastern Baltic cod fishery via the establishment of a permanent or a seasonal marine reserve in ICES subdivision 25 to a fishing as usual scenario. The environmental conditions incorporated were based on the size of the reproductive volume (RV) and comprise a best case and a worst case of reproductive conditions, and two more realistic scenarios, where we assumed that a historic series of RV-sizes reoccurs over the simulation period. Our results show a strong dependence of stock dynamics on the environmental conditions. Under prevailing low RV, our model projects stock extinction by the year 2020, if fishing continues as usual. Under the restrictive scenarios, where fishing mortality is reduced either directly or by implementation of a marine reserve, the stock benefits from an increase in stock size and an improved age structure. A seasonal closure of subdivision (SD) 25 as opposed to a closure of the entire Baltic Sea appears to be sufficient to prevent the Eastern Baltic cod stock from falling below safe biological limits. Crown Copyright © 2006.

The population dynamics of the Eastern Baltic cod (Gadus morhua callarias L.), unlike many other stocks, shows a strong dependency on environmental conditions. To test the implications of different management policies on the stock and the fishery in a system of global environmental change, we apply a spatially disaggregated, discrete time, age-structured model of the Eastern Baltic cod stock in 50 year simulation analyses. The simulation provides an analysis of stock, yield, and revenue development under various management policies and environmental scenarios. The policy analysis, focusing on different regulations of fishing mortality, is embedded into three environmental scenarios, assuming low, medium, or high climate and environmental change. The environmental assumptions are based on simulation results from a coupled atmosphere-ocean regional climate model, which project salinity in the Baltic Sea to decrease by 7-47% in the period 2071-2100 relative to the reference period 1961-1990. Our simulation results show that a significant reduction in fishing mortality is necessary for achieving high long-term economic yields. Moreover, under the presented environmental scenarios, a stock collapse cannot be prevented. It can, however, be postponed by the establishment of a marine reserve in ICES subdivision 25.

Historically, cod has been one of the most important fish stocks in the North Atlantic. Recent stock collapses have been attributed to overfishing, and in February 2000 the European Commission established a closed area in the Irish Sea as part of a general recovery plan. The recovery plan was further revised and implemented between 2001 and 2005. However, the recovery plan has not provided the expected benefit, and the stock is still thought to be below the safe limit of Blim = 6000 t. We use stochastic simulations to investigate possible stock trajectories over a seven-year period from 1999 to 2005 under various scenarios of fishing mortality. Comparing the results of simulations with reality, it is clear that more drastic action is required if the stock is to recover in the medium term. The recovery plan was not explicitly designed to deal with uncertainty in the system, and this, we believe, resulted in the failure to meet the recovery plan objectives.