Juvenile Cod Research Articles

Genetic analysis of vibriosis and viral nervous necrosis resistance in Atlantic cod (Gadus morhua L.) using a cure model1

The aim of this study was to investigate whether observed time-until-death of Atlantic cod (Gadus morhua L.) juveniles in separate challenge tests with Vibrio anguillarum (causes vibriosis) and nodavirus [causes viral nervous necrosis (VNN)] are due to differences in susceptibility (whether at risk or not) or increased endurance (individual hazard, given that the animal is susceptible) using a cure mixture (CURE) model with Gibbs sampling. Observed time-until-death, prepared as sequential binary records, were analyzed with the CURE model and results were compared with cross-sectional threshold (SIMPLE) and an ordinary longitudinal survival score (NAÏVE) model (i.e., assuming that all animals are susceptible). Overall mortality at the end of the test was 86 and 71% for vibriosis and VNN, respectively. But the CURE model estimated 92 and 82% of the population to be susceptible to vibriosis and VNN, respectively. Hence, a substantial fraction among the survivors were considered to be susceptible but with high endurance. The underlying heritability of susceptibility was moderate for vibriosis (0.33) and extremely high for VNN (0.91), somewhat greater compared with classical SIMPLE model (0.19 and 0.76 for vibriosis and VNN, respectively), analyzing end survival as a cross-sectional binary trait. Estimates of the underlying heritability were low for single test-day scores of both endurance (0.02 and 0.15 for vibriosis and VNN, respectively) in the CURE model and for the NAÏVE model (0.02 and 0.18 for vibriosis and VNN, respectively). Based on the CURE model, the genetic correlation between susceptibility and endurance was low to moderately positive and significantly different from unity (P < 0.01) for both vibriosis (0.13) and VNN (0.47). Estimated breeding values from the SIMPLE and NAÏVE models showed moderate to high correlations (0.41 to 0.96) with EBV for susceptibility and endurance in the CURE model. The analyses indicate that susceptibility and endurance are apparently distinct genetic traits. Still, the genetic variation estimated in the SIMPLE and NAÏVE models seems to a large extent to be controlled by susceptibility and an efficient genetic selection for reduced susceptibility to vibriosis and VNN is therefore likely feasible even when using classical (noncure) models. Earlier termination of the challenge test or back truncation of survival data is not recommended as this likely shifts the focus of selection towards endurance rather than susceptibility.

Towards cod without spawning: artificial continuous light in submerged sea-cages maintains growth and delays sexual maturation for farmed Atlantic cod Gadus morhua

Sexual maturation and spawning in sea-cages is a major production and environ- mental challenge for the Atlantic cod farming industry. Altered photoperiod has arrested sexual maturation in cod kept in indoor or covered experimental tanks, but only delayed or caused asyn- chronous maturation in cod kept in sea-cages. To arrest the onset of puberty more efficiently, we reduced the relative differences in light intensity between night and day by lowering 4 sub- mersible sea-cages, each stocked with 400 juvenile cod, to 20 m depth for 14 mo. Two cages were continuously illuminated (LL), and 2 cages served as controls with natural light (NL) conditions. Cod grew well in both treatments and showed very low mortality. Final weights did not differ, although the pattern of growth differed through time. Continuous light did not arrest maturation, but delayed it by 3 to 6 mo; gonadosomatic indices reached 21.2% for NL females during spawn- ing in February to April, but reached only 10.2% for the LL females over a protracted spawning period from May to September. The liver index for NL fish decreased during spawning by 6% (females) and 15% (males). In contrast, the liver index in the LL fish decreased relatively little (females = 5%, males = 3%), suggesting these fish invested less energy in maturation. Submer- gence combined with sufficient artificial lighting will both delay and reduce the extent of matura- tion, which can improve production strategies through better feed conversion ratios and reduce the risks associated with 'escape through spawning'. Deeper submergence, combined with shading and stronger lighting, may even arrest maturation.

Influence of photoperiod on expression of DNA (cytosine-5) methyltransferases in Atlantic cod

Photoperiod manipulation during early juvenile stages can influence growth in Atlantic cod. In the present study, one group of cod juveniles were reared under natural photoperiod conditions for Bodø (67° N, 14° E), whereas their counterparts were kept under continuous illumination. The mean weight of juvenile cod reared under continuous illumination was found to be 13% greater than those kept under natural photoperiod after 120days of light treatment. The molecular basis of this phenotypic plasticity is currently unknown but it is likely that DNA (cytosine-5)-methyltransferases (dnmts) are involved, since these genes play a crucial role in epigenetic regulation of gene expression. Phylogenetic analysis of Atlantic cod dnmt1, dnmt2 and dnmt3a revealed that within each group, the phylogeny follows the taxonomic relationship between the various species and comparative mapping of dnmt paralogues showed that these genes lie within regions of conserved synteny amongst teleosts. Of the three dnmt paralogues, dnmt3a had the highest expression in fast muscle of adult cod. In addition, dnmt1 and dnmt2 were differentially expressed between tissues but with prominent expression in gonads. Dnmt1 and dnmt3a transcript levels showed a significant increase in fast muscle of juvenile cod from the continuous light group at several time points. Remarkably, dnmt1 and dnmt3a transcript levels were 2-fold higher at 120days, by which point photoperiod conditions between the two light groups had become identical. Our data revealed that photoperiod can have an extended effect on expression of dnmt genes, which may be involved in the epigenetic regulation of muscle growth by photoperiod in Atlantic cod.

Effect of food deprivation on distribution of larval and early juvenile cod in experimental vertical temperature and light gradients

How and when food limits the growth and survival of larval fish are essential ques- tions which link oceanography and fisheries science, and theory suggests that food availability may affect survival through behavioural responses. We tested the effect of food deprivation on the vertical distribution of larval cod Gadus morhua in light and temperature gradients during larval and early juvenile stages. Groups of fed (ad lib) and unfed (16 h) larval cod at 16, 30 and 37 d post hatch (dph) and early juvenile cod at 44 dph were repeatedly observed in experimental columns with temperature gradients (110 × 15 cm, 6 to 8°C) for a period of 3 h while the directionality of the light was manipulated. At earlier stages (16 and 30 dph), larvae were distributed mainly in the upper part of the column regardless of the water stratification. The presence of a thermocline reduced the likelihood of cod being observed in the lower part of the column at 37 and 44 dph, but this response was not affected by feeding history. Unfed larvae distributed deeper in the column than fed larvae, but this effect was small (on average, 4.3 cm difference in a 115 cm column). Video analysis of individual swimming behaviour at 44 dph indicated that, while unfed cod would not stay longer or swim deeper into the columns than fed cod, there was an increased number of fish swimming up and down in the column, suggesting an increased motivation to sample new environments. We conclude that short-term food deprivation (16 h) has a small but significant effect on larval vertical distribution, most likely as a consequence of increased prey-search behav- iour, but that temperature and ontogeny were the key factors in determining distribution patterns.

Genomic analysis of the host response to nervous necrosis virus in Atlantic cod (Gadus morhua) brain

Genome sequencing combined with transcriptome profiling promotes exploration of defence against pathogens and discovery of immune genes. Based on sequences from the recently released genome of Atlantic cod, a genome-wide oligonucleotide microarray (ACIQ-1) was designed and used for analyses of gene expression in the brain during infection with nervous necrosis virus (NNV). A challenge experiment with NNV was performed with Atlantic cod juveniles and brain samples from virus infected and uninfected fish were used for microarray analysis. Expression of virus induced genes increased at 5 days post challenge and persisted at stable level to the last sampling at 25 days post challenge. A large fraction of the up-regulated genes (546 features) were known or expected to have immune functions and most of these have not previously been characterized in Atlantic cod. Transcriptomic changes induced by the virus involved strong activation of genes associated with interferon and tumour necrosis factor related responses and acute inflammation. Up-regulation of genes involved in adaptive immunity suggested a rapid recruitment of B and T lymphocytes to the NNV infected brain. QPCR analyses of 15 candidate genes of innate immunity showed rapid induction by poly(I:C) in Atlantic cod larvae cells suggesting an antiviral role. Earliest and greatest expression changes after poly I:C stimulation was observed for interferon regulatory factors IRF4 and IRF7. Comparative studies between teleost species provided new knowledge about the evolution of innate antiviral immunity in fish. A number of genes is present or responds to viruses only in fish. Innate immunity of Atlantic cod is characterized by selective expansion of several medium-sized multigene families with ribose binding domains. An interesting finding was the high representation of three large gene families among the early antiviral genes, including tripartite motif proteins (TRIM) and proteins with PRY-SPRY and NACHT domains. The latter two with respectively 52 and 114 members in Atlantic cod have gone through expansions in different groups of fish. These proteins most likely have ligand binding properties and their propagation could be linked to the loss of MHC class II in the Atlantic cod genome.

Stable N and C isotopes in the organic matrix of fish otoliths: validation of a new approach for studying spatial and temporal changes in the trophic structure of aquatic ecosystems

Stable isotope studies of long-term ecosystem change are often hampered by lack of archived tissue samples. Here, we provide a reliable method for extracting the organic matrix from fish otoliths, demonstrate differences in isotope values between the soluble and insoluble organic fractions, and provide the trophic enrichment factors (ε). The analyses were performed on otoliths from wild-caught adult Atlantic cod (Gadus morhua) and otoliths from a 6-month diet-switch experiment with juvenile cod. Acid hydrolysis of otolith powder followed by ultrafiltration enabled us to purify and separate the soluble and insoluble organic material. There was a significant 1.72‰ and 0.15‰ difference in δ15N and δ13C, respectively, between the soluble and insoluble organic fraction of the otolith. This emphasizes the need for separation of the two fractions before analysis. The diet-specific trophic enrichment (εotolith-diet) in the soluble fraction varied between −0.20‰ and 0.31‰ for nitrogen and between 0.09‰ and 0.23‰ for carbon. In conclusion, the organic matrix of otoliths provides an attractive archive of organic material for ecological stable isotope analysis that has the potential to provide important insights into historic changes in the trophic structure of aquatic ecosystems.

Post-escape dispersal of juvenile Atlantic cod Gadus morhua from Norwegian fish farms and their potential for recapture

AEI Aquaculture Environment Interactions Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections AEI 3:107-116 (2013) - DOI: https://doi.org/10.3354/aei00051 Post-escape dispersal of juvenile Atlantic cod Gadus morhua from Norwegian fish farms and their potential for recapture R. M. Serra-Llinares1,2,*, R. Nilsen1,2, I. Uglem3, P. Arechavala-Lopez4, P. A. Bjørn1,2, C. Noble1 1Nofima, Muninbakken 9-13, 9291 Tromsø, Norway 2Institute of Marine Research, PO Box 6404, 9294 Tromsø, Norway 3Norwegian Institute of Nature Research, Tungasletta 2, 7485 Trondheim, Norway 4Department of Marine Science and Applied Biology, University of Alicante, PO Box 99, 03080 Alicante, Spain *Email: rosa.maria.serra.llinares@imr.no ABSTRACT: The escape of fish from sea-based aquaculture facilities is regarded as a potential environmental threat and is one of the biggest challenges the industry has faced in recent years. In addition to preventing escapes by improving farming operations and procedures, effective and operational routines are needed for recapturing fish should an escape occur. This study investigated the post-escape dispersal of juvenile Atlantic cod Gadus morhua L. from commercial sea cages in coastal Norway using acoustic telemetry. It also assessed the efficacy of different techniques (e.g. cod pots, fyke nets, and gill nets) for recapturing escaped fish. Results suggest that escaped juvenile cod can experience a high size-related predation pressure immediately after escape due to high numbers of large fish aggregating around the cages. Consequently, the escapees rapidly disperse from the farms and seek shelter in littoral areas. Once in shallower waters, they stay relatively close (<10 km) to the farm during the first 2 mo following an escape. This suggests a high potential for recapture if the effort is focused on the littoral area and not in the immediate vicinity of the farm. No escapees were caught using live recapture methods, and a low number of recaptures (<5%) were achieved with gill nets deployed in littoral areas. Moreover, high by-catch rates and a subsequent high workload suggest this method may be inappropriate for the recapture of escapees. KEY WORDS: Atlantic cod · Fish escape · Aquaculture · Telemetry · Recapture Full text in pdf format PreviousNextCite this article as: Serra-Llinares RM, Nilsen R, Uglem I, Arechavala-Lopez P, Bjørn PA, Noble C (2013) Post-escape dispersal of juvenile Atlantic cod Gadus morhua from Norwegian fish farms and their potential for recapture. Aquacult Environ Interact 3:107-116. https://doi.org/10.3354/aei00051 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AEI Vol. 3, No. 2. Online publication date: January 30, 2013 Print ISSN: 1869-215X; Online ISSN: 1869-7534 Copyright © 2013 Inter-Research.

Effects of salinity and temperature on growth, plasma ions, cortisol and immune parameters of juvenile Atlantic cod (Gadus morhua)

The effects of salinity and temperature on growth, plasma ions, cortisol and immune parameters were investigated in two experiments. In the first experiment small, medium and large juvenile Atlantic cod Gadus morhua (initial average weights of 1.9, 8 and 83g, respectively) were reared at four constant salinities (6 to 32‰) for a short period (19–57days, depending on size), then returned to seawater (32‰) without acclimation and reared for another period (20–391days). The highest growth rates were found at 10‰ in all size-classes. After the fish were returned to seawater the growth rates were inversely related to the salinity change in all size-classes and long-term rearing of the medium-sized fish revealed that abrupt salinity increase from 6 and 10‰ to seawater may permanently reduce the growth capacity of juvenile cod. In another long-term experiment, there was no significant difference in the growth rate of cod (initial average weight 3.4g) reared at either 13.5 or 32‰ for 187days. Rearing at 13.5‰ neither enhanced growth rate at 6.3°C nor at 10°C in larger juveniles (>245g) compared with rearing in seawater. The study shows that rearing salinity and abrupt salinity changes have limited or no effects on stress and immune-related parameters, and there are no indications of ion regulatory disturbances at salinities as low as 6‰. This shows that the Atlantic cod is an extremely euryhaline marine teleost species, and indicates that commercial Atlantic cod aquaculture can just as well be carried out in locations with low as with high environmental salinity.

Effects of small-scale habitat fragmentation on predator–prey interactions in a temperate sea grass system

During the last decades, fragmentation has become an important issue in ecological research. Habitat fragmentation operates on spatial scales ranging over several magnitudes from patches to landscapes. We focus on small-scale fragmentation effects relevant to animal foraging decision making that could ultimately generate distribution patterns. In a controlled experimental environment, we tested small-scale fragmentation effects in artificial sea grass on the feeding behaviour of juvenile cod (Gadus morhua). Moreover, we examined the influence of fragmentation on the distribution of one of the juvenile cod’s main prey resources, the grass shrimp (Palaemon elegans), in association with three levels of risk provided by cod (no cod, cod chemical cues and actively foraging cod). Time spent by cod within sea grass was lower in fragmented landscapes, but total shrimp consumption was not affected. Shrimp utilised vegetation to a greater extent in fragmented treatments in combination with active predation. We suggest that shrimp choose between sand and vegetation habitats to minimize risk of predation according to cod habitat-specific foraging capacities, while cod aim to maximize prey-dependent foraging rates, generating a habitat-choice game between predator and prey. Moreover, aggregating behaviour in grass shrimp was only found in treatments with active predation. Hence, we argue that both aggregation and vegetation use are anti-predator defence strategies applied by shrimp. We therefore stress the importance of considering small-scale behavioural mechanisms when evaluating consequences from habitat fragmentation on trophic processes in coastal environments.

Prey selectivity by juvenile Atlantic cod Gadus morhua in three coastal habitat types

Coastal habitats are heavily subjected to eutrophication and commercial fisheries, and such alterations can affect organism interaction strengths and potentially influence trophic dynamics. A key species inhabiting coastal environments in temperate waters is the Atlantic cod Gadus morhua, which utilises coastal areas for food and shelter. We used an experimental mechanistic approach to assay prey selectivity by juvenile cod when foraging on grass shrimp Palaemon elegans and brown shrimp Crangon crangon, under light and dark conditions, in 3 of the most abundant habitat types in temperate coastal environments-sand, eelgrass (artificial vegetation mimicking Zostera marina), and bladderwrack Fucus vesiculosus. Using functional response relationships from single-prey experiments, we calculated the energetically best foraging strategy for cod in the 3 habitats, i.e. feeding selectively on either of the shrimp species or on a combination of both. These predictions were tested in experiments where the cod predator was offered both prey species. Cod selected both prey species in accordance with our predictions in eelgrass and in bladderwrack under light conditions, but a lower than predicted consumption of grass shrimp was found in sand and in bladderwrack under dark conditions. Cod decreasingly selected grass shrimp with increasing habitat complexity, i.e. the highest selectivity was in sand and the lowest selectivity was in bladderwrack. As the 2 shrimp species have different trophic roles, cod selective predation may have effects on lower trophic levels. We provide a quantitative prediction of cod selective predation in habitat types that undergo degradation, and suggest that such predation can influence the trophic consequences from environmental change. (Less)

Disentangling genetic change from phenotypic response in reproductive parameters of Flemish Cap cod Gadus morhua

Flemish Cap cod Gadus morhua, as with many other fish populations subjected to high fishing rates, has experienced variation in size and age at maturation. Following intense fishing activity and concurrent with a substantial decline in population biomass, cohorts in the early 1990s exhibited a reduction in the length and age at which there is a 50% chance of being mature (L50 and A50 respectively) with values lower than ever estimated before. A further decline in A50 (not in L50) was found after the cod fishing moratoria, in late 1990s and early 2000s cohorts. At the same time, the water temperature showed substantial changes and the abundance of the main prey species of juvenile cod experienced large variation. Shrimp replaced hyperiids as the main prey item since the late 1990s, which was followed by large changes in feeding habits, fish condition and mean size at age. The analysis of maturation reaction norms and countertrend variations in growth rate, water temperature and condition support the hypothesis of genetic change as the main explanation for the decline in size and age at maturation of the cohorts of the late 1980s and early 1990s. However the change in A50 in the late 1990s and early 2000s cohorts is suggested to be caused mainly by a phenotypic response as a consequence of variation in feeding opportunities, water temperature and density-dependent changes in growth rate. If food availability decreases and there is a decrease in water temperature, a decrease in the reproductive potential and survivorship may be expected.

Effect of different feeding regimes on growth in juvenile Atlantic cod, Gadus morhua L.

Different feeding regimes (fasting/re-feeding and reduced feeding) were tested in a 131day trial with 1400 juvenile (mean±SE, 132g±1) Atlantic cod (Gadus morhua) in order to see if a compensatory growth response could be elicited, and if hyperplastic growth was stimulated during the compensatory growth phase. The Atlantic cod was fed in compliance with the following five feeding (F) fasting (S) schedules: the control group (Cont) was fed 100% according to a commercial growth table every day, the second group was fed 50% for two weeks and 100% for four weeks (S50), the third group was fed 100% every secondday and 0% the following day (ALT), the fourth group was fasted one week and fed 100% two weeks (S1W), and the last group was starved two weeks and fed 100% four weeks (S2W). At the termination of the experiment, the body mass of the fish in the control, ALT and S50 groups were significantly higher than the fish in S1W and S2W groups. The shorter fasting treatment groups (ALT and S50) consumed significantly less feed (42.9 and 37.5% less feed, respectively) compared to control whereas final weight did not differ between these three groups. Growth rate was highest in the S50 group (0.78%day−1), and lowest in the longer fasting period groups (S1W, 0.59%day−1 and S2W, 0.57%day−1). White muscle fibre diameter, white muscle fibre density and the percentage of small fibres below 25μm was measured in the control and S2W group and did not differ at any sample point during the experiment. There were minor differences between the groups in percentage distribution of fibre diameter classes. The results demonstrate that feeding costs can be drastically reduced without compromising biomass growth by using feeding on alternate days or by periodically fasting and re-feeding juvenile cod.

Thermal effects on swimming activity and habitat choice in juvenile Pacific cod (Gadus macrocephalus)

The behavioral responses of fishes to temperature variation have received less attention than physiological responses, despite their direct implications for predator–prey dynamics in aquatic ecosystems. In this paper, we describe the temperature dependence of swimming performance and behavioral characteristics of juvenile Pacific cod (Gadus macrocephalus; 75–125 mm total length). Maximum swimming speeds increased with temperature and body size. Routine swimming speeds of Pacific cod in small groups of similarly sized fish (N = 6) increased with body size and were 34 % faster at 9 °C than at 2 °C. The response to temperature was opposite that previously described for juvenile walleye pollock (Theragra chalcogramma), reflecting species-specific differences in behavioral responses. In a separate experiment, we demonstrated the effect of temperature on habitat selection of juvenile Pacific cod: Use of an artificial eelgrass patch in a 5-m-long laboratory tank was significantly greater at 9 °C than at 2 °C. These results illustrate that temperature affects a range of behavioral traits that play important roles in determining the frequency and outcomes of predator–prey interactions.

Rapid growth of farmed cod in sea cages in the Faroe Islands

The observed growth of cod in a farming trial in the Faroe Islands was compared to the predictions of a known cod-growth model for ambient sea temperatures in Norway, Iceland and the Faroe Islands. The growth was also compared to optimal growth rates (denoted as “optimal growth”), as predicted by the model. According to the model, the Faroese temperatures are most favorable for cod growth and closest to the optimal growth. Assuming a starting weight of 30g on 15 May, the model predicts growth to 4kg (round weight) in 667, 708, 778 and 896days for optimal, Faroese, Norwegian and Icelandic temperatures, respectively. The farming trial used semi-intensively produced juvenile cod from broodstock collected on the Faroe Bank. The on-growing was made in three commercial sea cages: Cage 1 with natural light, and Cages 2 and 3 with artificial underwater light to inhibit sexual maturation. At 836days post hatching, the mean weight of about 30 sampled fish from Cages 1, 2 and 3 was 2.95, 4.06, and 4.16kg, respectively. By determining the gonad weight of the sampled fish, the artificial light is shown to successfully delay sexual maturation in Cages 2 and 3 until 4kg body weight. The best economical feed conversion ratio and yield per stocked juvenile — estimated for Cage 3 — were about 1.2 and 4.0, respectively. Total accumulated mortality was about 20%. The realized growth data indicate that cod farming with rapid growth: 4kg in about 28months post hatching with controlled sexual maturation is possible in the near-optimal sea temperatures around the Faroe Islands. The stable sea temperature and the relatively low and stable ambient daylight during summer in Árnafjørður (62°15′N), Faroe Islands, which is further damped by the sea water above the underwater lights, are suggested to have assisted the control of the sexual maturation.

Foraging performance of juvenile Atlantic cod Gadus morhua and profitability of coastal habitats

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 456:245-253 (2012) - DOI: https://doi.org/10.3354/meps09705 Foraging performance of juvenile Atlantic cod Gadus morhua and profitability of coastal habitats Anders Persson*, Peter Ljungberg, Magnus Andersson, Elin Götzman, P. Anders Nilsson Department of Biology, Aquatic Ecology, Lund University, Ecology Building, 223 62 Lund, Sweden *Email: anders.persson@biol.lu.se ABSTRACT: Overfishing and eutrophication affects coastal communities worldwide, leading to dwindling fish stocks and deteriorated habitats. Hence, attempts to rebuild overfished stocks to past fish productivities need to account for functional relations between habitat types and fish performance. Here we quantified resource availability, foraging performance, and anti-predator behavior of juvenile Atlantic cod Gadus morhua (hereafter ‘cod’) to assess the costs and benefits associated with different coastal habitats. In the laboratory, cod foraged more efficiently in sand habitats compared to the structurally more complex habitats of eelgrass Zostera marina and the canopy forming bladderwrack Fucus vesiculosus. Presence of chemical cues from a cannibal reduced cod consumption rates in all habitats, but most strongly in the sand habitat. Field observations in the 3 habitats showed highest resource density in the bladderwrack habitat and lowest in the sand habitat, irrespective of season. Habitat profitability, calculated by combining data from field estimates of prey density and experimental quantifications of foraging performance, revealed the bladderwrack habitat to be the most profitable, independent of season. The difference in profitability between the complex habitats was relatively small, suggesting that cod in the field contributed to drive habitat profitability towards equalization. The results strengthen the view that the ongoing loss of seagrass and macroalgae habitats may have significant ramifications for juvenile cod performance, which ultimately may lower the productivity of entire stocks. Consequently, future and ongoing rebuilding of commercial fish stocks should revise the expectations of stock productivity (and hence harvesting intensity) accordingly. KEY WORDS: Cod · Habitat choice · Macroinvertebrates · Functional response · Seagrass · Bladderwrack · Fucus vesicolosus · Zostera marina Full text in pdf format PreviousNextCite this article as: Persson A, Ljungberg P, Andersson M, Götzman E, Nilsson PA (2012) Foraging performance of juvenile Atlantic cod Gadus morhua and profitability of coastal habitats. Mar Ecol Prog Ser 456:245-253. https://doi.org/10.3354/meps09705 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 456. Online publication date: June 07, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

No differences in egg buoyancy and anti-freeze protein production in genetically divergent subpopulations of Gulf of Maine Atlantic Cod (Gadus morhua)

Locally-adapted subpopulations of Atlantic cod are found in both the northwest and northeast Atlantic and exhibit differences in morphology, behavior and physiological characteristics. We conducted experiments to determine if demonstrable differences were evident in egg buoyancy and antifreeze glycoprotein production between captive populations of genetically divergent winter and spring-spawning cod from the Gulf of Maine. Fertilized eggs (<24h post-spawning) were collected from both populations on 3 dates and transferred to 3 controlled-temperature rooms (5, 10, and 12°C). Egg buoyancy was determined in triplicate samples, at each temperature, in seawater ranging from 28 to 35ppt with 0.5ppt increments. No significant differences in mean neutral buoyancy (∼1.024g/mL) were found between stocks or treatment temperatures. Antifreeze glycoprotein (AFGP) production was examined in captive-bred cod juveniles (13–24cm) from both populations held at 0°C for periods ranging from 20 to 35 days. AFGP were first produced on day 30, although no differences were found in AFGP production in either population. Our results do not support the hypothesis that physiological differences in egg buoyancy and anti-freeze protein product exist between these two cod populations. The similarity in expression of these traits may reflect the high level of nutrition that both broodstock populations received, and the common juvenile size during cold-water exposure.

Predators and distance between habitat patches modify gap crossing behaviour of juvenile Atlantic cod (Gadus morhua, L. 1758)

When habitat refugia is fragmented and limited, movement of potential prey animals between patches of protective habitat may be driven by biotic factors including foraging opportunities, risk of predation mortality, as well as density effects. However, few studies have examined these factors in marine landscapes. We examined gap crossing behaviour of juvenile Atlantic cod, Gadus morhua L. 1758, a species with high mortality throughout its juvenile stage. We examined the effect of inter-patch distance, fish length, group size, and the presence of a predator on gap crossing behaviour by juvenile cod in a 12×3m experimental arena. Habitat patches consisting of artificial eelgrass were positioned within the arena with inter-patch distances of 3.0m and 7.5m. We observed a 37% reduction in frequency of gap crossing movements at larger distances compared to small ones. At the greater inter-patch distance, fish delayed departing a patch, depending on average group size. Juvenile fish released into a patch with a nearby predator moved across gaps 75% less frequently than fish originating in a predator-free patch. We also found that the presence of a predator delayed fish departure from a patch, again depending on group size. Our study demonstrates a suite of complex behavioural mechanisms that juvenile cod may exhibit to reduce predation risk as they navigate a landscape of fragmented habitat patches.

Output of organic material from land-based juvenile Atlantic cod (Gadus morhua) tanks

A mass balance experiment was conducted in which dry mass, dissolved organic carbon, total nitrogen, lipid classes, and fatty acids were quantified in the inflow and outflow of land-based juvenile Atlantic cod (Gadus morhua) tanks. The inflow contained 89.9 ± 15.4 g/day of dry mass that increased to 187 ± 39 g/day in the outflow. Correction for the inflow indicated 96.8 ± 33.8 g/day of excess feed and faeces left the tanks amounting to a feed to waste conversion ratio of 25 %. The amount of feed in the waste was estimated to be 3 %. The breakdown indicator free fatty acid (43.1 ± 9.5 % total lipid), the markers of zooplankton and subsequent indicators of the feed, 20:1ω9 and 22:1ω11, and the essential fatty acid docosahexaenoic acid (DHA) were all significantly higher (p < 0.020) in the outflow compared to the inflow. The output from the entire aquaculture facility was calculated from the experimental tanks and gave 1,060 ± 370 g/day dry mass and 1.5 ± 0.4 g/day DHA, which could support 0.44 tonnes of mussels assuming an average weight of 5.31 g/mussel. Additionally, the output data were calculated per kilogram of fish and used to estimate potential output for operational Atlantic cod farms. An 1,880 tonne farm would produce 3,170 ± 870 kg/day dry mass and 4.7 ± 1.7 kg/day of DHA, which could theoretically support 1,400 tonnes of mussels. Natural mussels and those grown in co-culturing systems could assimilate such valuable compounds that would otherwise be lost.

Optimal stocking density of juvenile Atlantic cod (Gadus morhua L.) reared in a land-based farm

The effects of stocking density on the productivity in a juvenile cod farm were studied in a flow-through system for fish weighing initially 44g. The fish densities increased throughout the experiment from 4 to 21, 11 to 54 and 22 to 95kg/m3. Growth rates were negatively affected at densities above 50kg/m3 and at the end of the experiment the mean weights were 242, 221 and 205g at low, medium and high densities, respectively. The average cumulative mortalities increased with density and were 1.8, 4.2 and 7.3% of initial number of fish at low, medium and high densities, respectively. Antibiotics were administered twice at high, once at medium and never at low density. The average feed conversion ratios were 0.81, 0.91 and 1.02 at low, medium and high densities, respectively. The average biomass increase was 16.8, 42.7 and 72.8kg/m3 at low, medium and high densities. A simple economic analysis indicates a parabolic relationship between profit and density with optimal density at the peak of the curve. Given reasonable assumptions about production costs in a land-based farm, the break-even density is 35kg/m3 and the optimal final density is 95kg/m3. Changes in fixed costs, such as financial costs and salaries, have no effects on the optimal final density whereas changes in variable costs, such as feed and juvenile costs, directly affect the optimal final density.

Photoperiod Influences Growth and mll (Mixed-Lineage Leukaemia) Expression in Atlantic Cod

Photoperiod is associated to phenotypic plasticity of somatic growth in several teleost species. However, the molecular mechanisms underlying this phenomenon are currently unknown but it is likely that epigenetic regulation by methyltransferases is involved. The MLL (mixed-lineage leukaemia) family comprises histone methyltransferases that play a critical role in regulating gene expression during early development in mammals. So far, these genes have received scant attention in teleost fish. In the present study, the mean weight of Atlantic cod juveniles reared under continuous illumination was found to be 13% greater than those kept under natural photoperiod conditions for 120 days. We newly determined cDNA sequences of five mll (mll1, mll2, mll3a, mll4b and mll5) and two setd1 (setd1a and setd1ba) paralogues from Atlantic cod. Phylogenetic analysis revealed that the cod genes clustered within the appropriate mll clade and comparative mapping of mll paralogues showed that these genes lie within a region of conserved synteny among teleosts. All mll and setd1 genes were highly expressed in gonads and fast muscle of adult cod, albeit at different levels, and they were differentially regulated with photoperiod in muscle of juvenile fish. Following only one day of exposure to constant light, mll1, mll4b and setd1a were up to 57% lower in these fish compared to the natural photoperiod group. In addition, mRNA expression of myogenic regulatory factors (myog and myf-5) and pax7 in fast muscle was also affected by different photoperiod conditions. Notably, myog was significantly elevated in the continuous illumination group throughout the time course of the experiment. The absence of a day/night cycle is associated with a generalised decrease in mll expression concomitant with an increase in myog transcript levels in fast muscle of Atlantic cod, which may be involved in the observed epigenetic regulation of growth by photoperiod in this species.