Atlantic Cod Gadus Morhua Research Articles

Year-round genetic monitoring of mixed-stock fishery of Atlantic cod (Gadus morhua); implications for management

Abstract Mixed-stock fisheries pose a challenge for fisheries management recommendations and management, as mixed stocks can lead to unintentional over- or under-exploitation of stocks, with both short- and long-term biological and economic consequences. This study demonstrates practical application and implementation of year-round high-resolution genetic stock monitoring in management of mixed Atlantic cod (Gadus morhua) stocks in western Greenland. Using a panel of 96 high resolution gene-associated SNPs, we identified the stock of origin for four cod stocks in the two major inshore mixed-stock fisheries. Contribution of individual stocks was quantified to identify if fjord sections, times of year, and fish size, would allow a “clean fishery” with a minimal contribution of the depleted West Greenland Offshore (WGO) population. High level of mixing was identified with 50% of inshore catches originating from the inshore stock, 20% from the East Greenland/Iceland offshore stock, and 30% from the WGO stock. Although some spatiotemporal variation of stock proportions was identified, a practice allowing exclusively fishing a specific stock was not possible. Accordingly, we demonstrate the need to allocate catches at the biological stock level and show that application of high-resolution genetics is a reliable and necessary tool.

High site-fidelity and low mortality of juvenile Atlantic cod (Gadus morhua) in subarctic coastal habitat during their first winter

Abstract In subarctic Newfoundland, age-0 juvenile Atlantic cod (Gadus morhua) settle into coastal habitats in several summer and fall pulses, yielding a broad length-frequency distribution prior to their first winter. The first winter is often associated with physiological and metabolic stress and has been considered a critical survival period determining cohort strength. We evaluated size-structured overwinter mortality and movement using mark–recapture and condition metrics by marking 226 cod in two batches one week apart, in October 2016. We estimated fall and overwinter mortality, and documented movement of fish recaptured in May 2017 using Cormack–Jolly–Seber models. We recaptured 30 marked juveniles. High fall mortality characterized late settling cohorts relative to earlier settling cohorts (16.6%∙d−1 vs. 4.5–7.7% ∙d−1). Overwinter mortality was unexpectedly low (0.0052 and 0.0022% ∙d−1). Individual condition (Fulton’s K) of juvenile cod remained high throughout winter across all size groups. We expected higher mortality of juvenile cod and broad dispersal of juveniles over winter (32 weeks). In contrast, our results indicated low mortality and high site-fidelity in their first winter. This study indicates the period leading up to winter is important for survival, suggesting winter is not a survival bottleneck and may even provide a refuge compared to the rest of the year.

Absence of zoonotic parasites in salmonid aquaculture in Denmark: Causes and consequences

A parasitological investigation of Danish farmed freshwater (180 fish) and marine rainbow trout (170 fish), Oncorhynchus mykiss , was performed with an emphasis on the occurrence of zoonotic parasites. Reliability of the parasitological methods applied was assessed by examining fish from a stock of wild Atlantic cod Gadus morhua caught in the Sound (Øresund). While wild fish carried a number of zoonotic parasites, we found that farmed fish, both in the fresh and marine environment, were free of these parasites. Non-zoonotic trout parasites found were eye-flukes Diplostomum paracaudum, D. pseudospathaceum and Eubothrium crassum in maricultured rainbow trout. The acanthocephalan Echinorhynchus truttae and Diplostomum sp. were found in freshwater trout. In contrast, wild cod were infected with five different nematode species ( Anisakis simplex , Contracaecum osculatum , Pseudoterranova decipiens , Hysterothylacium aduncum , Cucullanus cirratus ), three trematode species ( Lepidapedon elongatum , Cryptocotyle lingua , Hemiurus communis ), one copepod species ( Lernaeocera branchialis ) and one myxozoan ( Myxobolus aeglefini ). The risk factors associated with aquaculture production in an infectious aquatic environment are analyzed and reasons for absence of zoonotic parasites in aquacultured trout are pinpointed. Based on the analysis recommendations for management practices securing parasite freedom in aquacultured fish are given. • Salmonid aquaculture in Denmark is free from zoonotic parasites. • Fish parasitological methods are validated by examination of wild fish. • Risk factors associated with aquaculture production are evaluated. • Recommendations for prevention of zoonotic parasite introduction are presented.

Understanding Recreational Angler Diversity and Its Potential Implications on Promoting Responsible Fishing Practices in a Multispecies Gulf of Maine Fishery

AbstractRecent work in the Gulf of Maine multispecies recreational fishery has established responsible fishing practices that anglers can use to reduce bycatch and the discard mortality of three key groundfish species. However, anglers represent a diverse stakeholder group whose backgrounds and experiences may influence how they receive, support, and adopt responsible fishing practices that aim to sustain catch‐and‐release angling opportunities. We therefore surveyed Gulf of Maine recreational anglers who target groundfish, including Atlantic Cod Gadus morhua, Haddock Melanogrammus aeglefinus, and Cusk Brosme brosme, to assess whether differences among anglers influenced how likely anglers were to voluntarily adopt or consider adopting responsible fishing practices and which information channels they used to obtain such information. By broadly sampling Gulf of Maine recreational fishing license holders via an online survey, we collected responses from 306 respondents who targeted groundfish in some capacity; several topics, including fishing activity and experience, responsible fishing practices, information channels, and sociodemographics, were addressed in the survey. A latent class cluster analysis found that respondents who participate in this regional fishery can be broadly categorized into three distinct classes (Striped Bass [Morone saxatilis] Enthusiasts, All‐rounders, and Offshore Groundfishers) from their primary fishing mode and target species. Despite the presence of these latent classes, class membership did not affect how likely respondents were to voluntarily adopt or consider adopting species‐specific fishing practices from previous scientific investigations. However, class membership was observed to influence how respondents used information channels to receive angling information, with Offshore Groundfishers relying more often on captains and crew than the other classes. Therefore, to promote responsible fishing practices alongside regulations, we recommend that fishery managers use a mixed outreach program to effectively communicate and engage with this portion of the community until more directed studies can be conducted.

Seasonal Trends in Groundfish Discards on Georges Bank: Implications for Adaptive Quota Management

AbstractFor fisheries to be sustainable, management should account for all major sources of fishing mortality. On the Canadian side of Georges Bank, landings and discards of transboundary stocks of Atlantic Cod Gadus morhua, Haddock Melanogrammus aeglefinus, and Yellowtail Flounder Limanda ferruginea are monitored against quotas that are shared by the Canadian groundfish and scallop (sea scallop Placopecten magellanicus) fisheries to limit fishing mortality. The shared quota allocations are managed using an adaptive quota management system; quota can be redistributed between the fisheries during the season to maximize fishing opportunities, while still respecting the overall catch limits. However, the redistribution relies on estimates of the total end‐of‐year discards of Atlantic Cod, Haddock, and Yellowtail Flounder from the scallop fishery. Here, we evaluated and compared two approaches for projecting end‐of‐year discards within season: an empirical method based on scallop fishery landings and a seasonal modeling approach. Seasonal trends in discards were identified, with discards of all three groundfish species highest in April or May. The seasonal models out‐performed the empirical landings‐based projection method, and our results demonstrate that accounting for seasonal patterns in discards better informs risk‐based fishery management decisions.

Using molecular and stable isotope markers to identify the main predators of Nephrops norvegicus in Mediterranean deep-water ecosystems

To obtain a better understanding of the functioning of ecosystems and how they respond to disturbance, it is necessary to identify the relevant biotic interactions and specific trophic roles. Predation is one of the most important biotic interactions that can also define the spatial patterns of other species. Many predators are considered key species for the functioning and maintenance of ecosystems, as they play an important ecological role that can influence the dynamics at lower trophic levels. The Norway lobster Nephrops norvegicus is one of the most valued European fishing stocks. However, its value and capture have declined over the last decade. In the Atlantic Ocean, Atlantic cod Gadus morhua is the main predator of N. norvegicus. However, this species is not present in the Mediterranean Sea, and little is known about which species might prey on N. norvegicus in this area. Here, we combine 2 methodologies—genetic identification of stomach contents and stable isotope analyses—to identify, for the first time, the main predators of N. norvegicus in the Mediterranean Sea. Moreover, we have created the Predation Index, which determines the most influential predator affecting N. norvegicus population dynamics. Our results reveal that the major predators are the cephalopods Sepietta spp. and Abralia veranyi, which probably affect the early stages of N. norvegicus, followed by the elasmobranch Scyliorhinus canicula and the bony fishes Merluccius merluccius, Trigla lyra, and Conger conger. To evaluate possible fluctuations in the N. norvegicus population, we consider the assessment of these predator populations crucial.

Attraction of cod Gadus morhua from coastal spawning grounds to salmon farms

Wild fish aggregate at aquaculture net-pens, but the underlying mechanisms are not fully understood. This study examined how salmon farms attract coastal Atlantic cod Gadus morhua from their inshore spawning grounds. Acoustic receivers were deployed at 5 known cod spawning grounds and 6 salmon Salmo salar farms located at varying distances from these grounds in a mid-Norway study site. Cod were caught at each spawning ground annually from 2017-2019, fitted with acoustic transmitters and released (n = 535). A total of 289 tagged cod (54%) were detected at the salmon farms, with more cod detected at farms closest to the focal spawning grounds and at operational farms. The latter result is likely linked to the availability of feeding opportunities at farm locations. Those cod that were detected by the receivers spent less time at farms farther from their release locations. For the farm-associated cod, 70% were detected for <1 wk at the farms. However, 48 cod spent >1 mo close to the farms, with 1 individual staying 720 d underneath the farm. A total of 135 cod visited 2 or more farms, with farms in proximity more connected in terms of inter-farm movement. Some of the cod utilizing these local spawning grounds likely have considerable dietary input from salmon feed.

Susceptibility and Pathology in Juvenile Atlantic Cod Gadus morhua to a Marine Viral Haemorrhagic Septicaemia Virus Isolated from Diseased Rainbow Trout Oncorhynchus mykiss.

Simple SummaryVHSV (viral haemorrhagic septicaemia virus) is a globally important group of viruses, infecting a wide range of fish species, in both marine and freshwater. New initiatives are now being taken to re-establish Atlantic cod as an aquaculture species. Any susceptibility to diseases would be relevant for this industry. Atlantic cod was experimentally infected with a strain of VHSV originating from a disease outbreak of farmed rainbow trout in Norway. The fish infected by injection did develop the disease, as verified by investigations of tissue samples. However, no transmission of the virus to cohabitant fish (i.e., individuals sharing the tank with infected fish) was found. This indicates that, despite the fact that the virus is capable of causing disease in Atlantic cod, the degree and ability to infect the fish is limited.The first known outbreak caused by a viral haemorrhagic septicaemia virus (VHSV) strain of genotype III in rainbow trout occurred in 2007 at a marine farm in Storfjorden, Norway. The source of the virus is unknown, and cod and other marine fish around the farms are suspected as a possible reservoir. The main objective of this study was to test the susceptibility of juvenile Atlantic cod to the VHSV isolate from Storfjorden. As the pathology of VHS in cod is sparsely described, an additional aim of the study was to give a histopathological description of the disease. Two separate challenge experiments were carried out, using both intra peritoneal (ip) injection and cohabitation as challenge methods. Mortality in the ip injection experiment leveled at approximately 50% three weeks post challenge. Both immunohistochemical and rRT-PCR analysis of organs sampled from diseased and surviving fish confirmed VHSV infection. No VHSV was detected in the cohabitants. The results indicate that Atlantic cod has a low natural susceptibility to this VHSV genotype III strain. One of the most extensive pathological changes was degeneration of cardiac myocytes. Immunohistochemistry confirmed that the lesions were related to VHSV. In some fish, the hematopoietic tissue of spleen and kidney showed degeneration and immunostaining, classical signs of VHS, as described in rainbow trout. Positive immunostaining of the capillaries of the gills, suggests this organ as a useful alternative when screening for VHSV.

Размерный состав и продукционные характеристики сеголеток трески и сайды в губе Зеленецкая (Восточный Мурман) в 2018 году

In this article presented data on the growth of the atlantic cod Gadus morhua Linnaeus, 1758 and pollock Pollachius virens (Linnaeus, 1758) juveniles in Zelenetskaya Bay in 2018. The daily specific growth rate and production rate were calculated. The length of atlantic cod juveniles in July–September varied from 3.5 to 7.9 cm, and pollock – from 2.5 to 9.4 cm. The daily specific weight growth rate (qw) of atlantic cod juveniles in Zelenetskaya Bay from July to August was 0.41, and from August to September – 1.08; for pollock juveniles, these indicators are 1.32 and 0.94, respectively. The daily specific production (C) of atlantic cod juveniles in Zelenetskaya Bay over the summer period varied from 0.015 to 0.036, and of pollock juveniles – from 0.011 to 0.042.

Parasite load of Atlantic cod Gadus morhua in the Baltic Sea assessed by the liver category method, and associations with infection density and critical condition

AbstractDuring the 2010s, Atlantic cod Gadus morhua L. in the eastern Baltic Sea experienced increasing infection loads of the parasitic nematode Contracaecum osculatum (Rudolphi) in their livers. Starting in 2021, a mandatory part of the routine sampling protocol on Baltic monitoring surveys is to assign a liver category to individual cod livers, based on the number of nematodes visible on the liver surface, to follow spatiotemporal changes in nematode infection loads. The validity of the liver category method has never been evaluated. Based on data from 642 cod livers, the method was verified and found to be a good predictor of the total number of nematodes. Moreover, the probability of cod being in a critical condition increased with the parasite load. In addition to their direct applicability to Baltic cod, the present findings may inspire others working with disease in fish stocks to include parasite monitoring.

Pilot study to investigate the effect of long-term exposure to high pCO2 on adult cod (Gadus morhua) otolith morphology and calcium carbonate deposition

To date the study of ocean acidification on fish otolith formation has been mainly focused on larval and juvenile stages. In the present pilot study, wild-captured adult Atlantic cod (Gadus morhua) were exposed to two different levels of pCO2, 422µatm (ambient, low pCO2) or 1091µatm (high pCO2), for a period of 30 weeks (from mid-October to early April 2014–2015) in order to study the effects on otolith size, shape and CaCO3 crystallization amongst other biological parameters. We found that otoliths from cod exposed to high pCO2 were slightly smaller (− 3.4% in length; − 3.3% in perimeter), rounder (− 2.9% circularity and + 4% roundness) but heavier (+ 5%) than the low pCO2 group. Interestingly, there were different effects in males and females; for instance, male cods exposed to high pCO2 exhibited significant changes in circularity (− 3%) and roundness (+ 4%) compared to the low pCO2 males, but without significant changes on otolith dimensions, while females exposed to high pCO2 had smaller otoliths as shown for length (− 5.6%), width (− 2%), perimeter (− 3.5%) and area (− 4.8%). Furthermore, while the majority of the otoliths analysed showed normal aragonite deposition, 10% of fish exposed to 1091µatm of pCO2 had an abnormal accretion of calcite, suggesting a shift on calcium carbonate polymorph crystallization in some individuals under high pCO2 conditions. Our preliminary results indicate that high levels of pCO2 in adult Atlantic cod might affect otolith growth in a gender-specific way. Our findings reveal that otoliths from adult cod are affected by ocean acidification, and we believe that the present study will prompt further research into this currently under-explored area.

Morphological and molecular identification of Cryptocotyle lingua metacercariae isolated from Atlantic cod (Gadus morhua) from Danish seas and whiting (Merlangius merlangus) from the English Channel

Trematode larvae (metacercariae) causing black spot disease occur frequently in gills, fins, skin and the superficial muscle layers of marine fish. Species within the genus Cryptocotyle Lühe, 1899 are frequently associated with this disease. Descriptions of the metacercarial stage are relatively limited and none has hitherto been reported from fish from the English Channel. The present study reports the morphological and molecular identifications of encysted black spot-inducing parasites from whiting (Merlangius merlangus) and Atlantic cod (Gadus morhua) caught respectively from the north coast of France (English Channel) and from Danish sea waters. Metacercariae were characterised morphologically based on microscopic observations and molecularly using Sanger sequencing of fragments of the mitochondrial cox1 gene and rDNA ITS region. Morphological data were compared with available data in the literature. Phylogenetic trees including reference sequences were built to confirm morphological and molecular identifications. This survey constitutes the first description of C. lingua metacercariae in the English Channel ecosystems.

A millennium of trophic stability in Atlantic cod (Gadus morhua): transition to a lower and converging trophic niche in modern times

Stable isotope analyses of zooarchaeological material can be used to examine ecological variability in exploited species at centennial to millennial scales. Climate change is a notable driver of marine ecosystem change, although historical fishing is also likely to have impacted past marine systems. Fishing removes the oldest and largest individuals and may thereby result in shorter trophic pathways and reduced niche width of predatory fish species. In the current study we examine the trophic niche of Atlantic cod, haddock and Atlantic wolffish, in the last millennium using δ13C and δ15N values of bone collagen. We report a lower trophic level of Atlantic cod and haddock but higher level of wolffish in present times, following centuries at consistent and higher trophic levels of Atlantic cod. This results in a concurrent converging trophic niche of the demersal fish. We suggest that the current data set provides a valuable historical baseline facilitating interpretation of current variability in the trophic ecology of northern demersal fish.

Transborder Gene Flow between Canada and the USA and Fine‐Scale Population Structure of Atlantic Cod in the Broader Gulf of Maine Region

AbstractFishery managers have struggled for decades to rebuild stocks of Atlantic Cod Gadus morhua in the northwestern Atlantic Ocean, and the resolution of current fine‐scale population structure will surely help those efforts. Using double digest restriction‐site associated DNA sequencing, we analyzed 248 adult and 215 juvenile Atlantic Cod collected from areas bordering the Canada–USA boundary in 2017–2018. Chromosomal inversions from linkage group (LG) 2 and LG 7 contribute the largest amount of variance to the data set. A southwesterly decrease in the proportion of LG 1 inversion haplotypes may indicate gene flow from areas north of 45°N. Samples from both the northern Gulf of Maine and Bay of Fundy were genetically distinct from all other samples, suggesting that local spawning groups may still be active. Genetic similarities between Atlantic Cod from Browns Bank and eastern Georges Bank indicate that transborder gene flow is ongoing. These results will help to better define the stocks in order to guide the implementation of management strategies in the USA and Canada, which should consider rebuilding exhausted and genetically isolated populations.

Sea temperature effects on depth use and habitat selection in a marine fish community.

Understanding the responses of aquatic animals to temperature variability is essential to predict impacts of future climate change and to inform conservation and management. Most ectotherms such as fish are expected to adjust their behaviour to avoid extreme temperatures and minimize acute changes in body temperature. In coastal Skagerrak, Norway, sea surface temperature (SST) ranges seasonally from 0 to over 20°C, representing a challenge to the fish community which includes cold-, cool- and warm-water affinity species. By acoustically tracking 111 individuals of Atlantic cod Gadus morhua, pollack Pollachius pollachius and ballan wrasse Labrus bergylta in 2015-2018, we examined how coexisting species within a fish community adjusted their behaviour (i.e. vertical distribution in the water column and habitat selection) to cope with the thermal variation. Mixed-effect models showed that thermal preference was a main driver of behaviour and habitat use of the fish community in a southern Norwegian fjord. Cod used colder waters, compared with pollack and ballan wrasse. Increases in SST during summer were associated with the use of deeper, colder waters by cod, especially by larger individuals, and conversely with the occupancy of shallower areas by pollack and ballan wrasse. During winter, when SST dropped and the thermal stratification reversed, pollack and ballan wrasse moved to deeper, relatively warmer areas, while cod selected shallower, colder habitats. Although habitat selection was affected by temperature, species-specific habitat selection was observed even when temperature was similar throughout habitats. This study shows how cohabiting fish species respond to thermal heterogeneity, suggesting that (a) temperature regulates the access to the different depths and habitats and (b) behavioural plasticity may be an important factor for coping with temperature variability and potentially for adaptation to climate change.

551 Fish Skin Compared to Cadaver Skin as a Temporary Coverage and Wound Bed Preparation for Full Thickness Burns: An Early Feasibility Trial.

Abstract Introduction Full-thickness thermal burns may require staged procedures with temporary coverage to ensure the wound bed is optimized for autografting. A potential dermal substitute for thermal injury is fish skin grafts. These are made in Iceland from freeze dried, sterilized, decellularized skin of North Atlantic cod (Gadus morhua). The gentle processing of the fish skin removes the risk of viral disease transmission to humans and retains its naturally occurring Omega3 fatty acids which are known for their pain and inflammation modulating effects. Fish skin graft has a shelf life of 3 years at room temperature, whereas cadaver skin, the current standard of care at many institutions, has to be cryopreserved, is donor derived, and contains only trace amounts of Omega3. While fish skin graft has been cleared by the FDA as a medical device for use in acute, surgical, chronic wounds, and partial thickness burns, it has not yet been indicated for use in full-thickness burns. The purpose of this clinical trial is to assess the safety and efficacy of decellularized, freeze dried fish skin as an alternative to cadaveric skin (standard of care) for temporary coverage in the setting of a full-thickness burn requiring staged grafting. Methods Patients with full thickness burns to their extremities and/or chest were randomized to have two adjacent areas (70–140 cm2 each) covered with either fish skin or cadaver skin for one week following excision. Patients then received a split thickness skin graft (STSG). Healing was observed by blinded assessors for each area weekly for three weeks, with a scarring assessment completed at 3 months post STSG. Background pain was measured by a visual analogue scale (VAS) for pain and wounds were photographed at each time point. Results Five patients are included in this initial analysis and all completed the 3-month post STSG follow up. TBSA ranged from 8–37%. The fish skin treated area was assessed to be 90% healed at the same timeframe or earlier than cadaver treated areas in all subjects. There was a trend towards a reduction in pain in the fish skin covered areas. One patient was unable to give a pain score due to heavy sedation. Adverse events were reported in all five patients, but none were associated with the use of the investigational product. Graft failures were observed in two cadaver skin covered areas and one partial failure in a fish skin covered area. Conclusions Results from this trial so far indicate that the fish skin is safe and non-inferior to cadaver skin as an early coverage for full thickness burns. Enrollment in this study is still ongoing, and patients will be followed up for 12 months.

Reducing bycatch impacts in recreational fisheries: Case study examining terminal tackle in the multispecies Gulf of Maine groundfish fishery

AbstractTerminal tackle regulations can be a valuable tool for fisheries management, especially in multispecies fisheries where bycatch and discards are common issues. In the Gulf of Maine, recreational anglers frequently discard critically depleted Atlantic cod Gadus morhua L. as bycatch when targeting the abundant haddock Melanogrammus aeglefinus (L.) stock. The present study investigated species catch composition, catch rates and animal welfare across various terminal tackle setups, and aspects of capture and handling with the goal of reducing bycatch and discard mortality. Overall, 2558 cod and 4266 haddock were captured with six terminal tackle setups in the western Gulf of Maine from April to October 2018. Along with angler experience and capture depth, lure type primarily influenced species catch composition and catch‐per‐unit‐effort; hook types additionally influenced hook‐removal times and physical injuries to fish. Results indicate that using baited hooks can both promote haddock catch and reduce cod bycatch, with specific hook types promoting increased survival.

An evaluation of temporal changes in the trophic structure of Gulf of Maine ecosystem

The Gulf of Maine (GOM) ecosystem has experienced significant changes in the last few decades with a great increase in the American lobster Homarus americanus population and substantial reduction in some groundfish populations like Atlantic cod Gadus morhua. This study evaluated and compared the energy flows and trophic structures of the GOM ecosystem in the mid-2000s and mid-2010s. Following an early study, two mass-balance ecosystem models, each of which include 25 functional groups, were updated for the mid-2000s and mid-2010s using Ecopath. Fishing efforts were used as forcing drivers to simulate the variation of the GOM ecosystem in the mid-2000s to mid-2010s. Similar to the GOM ecosystem from the 1980s to 1990s found in a previous study, dominant species in both the mid-2000s and mid-2010s ecosystems were transferred from top predators to crustacean species in the last decade. However, food web structure of the GOM ecosystem became more complex, stable, and flexible to perturbations, indicating that the GOM ecosystem is moving toward a more mature stage. Simulation results indicated that fishing activities were considered to be the main driver to the variations dynamics of most species in the GOM ecosystem, while some species like Atlantic herring Clupea harengus might also suffer from changes in environmental conditions. This study provides a comprehensive overview on the changes in the GOM ecosystem in the last two decades.

Predator–prey overlap in three dimensions: cod benefit from capelin coming near the seafloor

Spatial overlap between predator and prey is a prerequisite for predation, but the degree of overlap is not necessarily proportional to prey consumption. This is because many of the behavioural processes that precede ingestion are non‐linear and depend on local prey densities. In aquatic environments, predators and prey distribute not only across a surface, but also vertically in the water column, adding another dimension to the interaction. Integrating and simplifying behavioural processes across space and time can lead to systematic biases in our inference about interaction strength. To recognise situations when this may occur, we must first understand processes underlying variation in prey consumption by individuals. Here we analysed the diet of a major predator in the Barents Sea, the Atlantic cod Gadus morhua, aiming to understand drivers of variation in cod's feeding on its main prey capelin Mallotus villosus. Cod and capelin only partly share habitats, as cod mainly reside near the seafloor and capelin inhabit the free water masses. We used data on stomach contents from ~2000 cod individuals and their surrounding environment collected over 12 years, testing hypotheses on biological and physical drivers of variation in cod's consumption of capelin, using generalized additive models. Specifically, effects of capelin abundance, capelin depth distribution, bottom depth and cod abundance on capelin consumption were evaluated at a resolution scale of 2 km. We found no indication of food competition as cod abundance had no effect on capelin consumption. Capelin abundance had small effects on consumption, while capelin depth distribution was important. Cod fed more intensively on capelin when capelin came close to the seafloor, especially at shallow banks and bank edges. Spatial overlap as an indicator for interaction strength needs to be evaluated in three dimensions instead of the conventional two when species are partly separated in the water column.

Personality‐dependent inter‐ and intraspecific foraging competition in the invasive round goby, Neogobius melanostomus

This study examines the impact of boldness on foraging competition of the highly invasive round goby Neogobius melanostomus Pallas 1815. Individual risk tolerance, or boldness, was measured as the time to resume movement after a simulated predation strike. Fish that resumed movement faster were categorized as "bold," fish that took more time to resume movement were categorized as "shy" and those that fell in between these two categories were determined to have "intermediate" boldness. Competitive impacts of boldness in N. melanostomus were determined in a laboratory foraging experiment in which interspecific (juvenile Atlantic cod Gadus morhua Linnaeus 1758) and intraspecific (intermediate N. melanostomus) individuals were exposed to either bold or shy N. melanostomus competitors. G. morhua consumed fewer prey when competing with bold N. melanostomus than when competing with shy N. melanostomus, whereas intermediately bold N. melanostomus foraging was not affected by competitor boldness. Bold and shy N. melanostomus consumed similar amounts of prey, and the number of interactions between paired fish did not vary depending on the personality of N. melanostomus individuals. Therefore, intraspecific foraging competition was not found to be personality dependent. This study provides evidence that individual differences in boldness can mediate competitive interactions in N. melanostomus; nonetheless, results also show that competition is also governed by other mechanisms that require further study.