Large Cod Research Articles

Changes in fishing behaviour of two fleets under fully documented catch quota management: Same rules, different outcomes

A Dutch pilot study of fully documented fisheries provided the opportunity to observe actual changes in fishing behaviour under catch quota management (CQM). Interviews with fishers in the pilot study aided in interpreting the results and giving insight in the decision making process and reasoning of fishers. The CQM pilot study entailed a fleet of small and large demersal vessels. For these vessels, all cod catches were counted against quota, including catches of individuals below minimum landings size. To obtain reliable catch data all vessels were equipped with electronic monitoring (EM) systems. These systems recorded videos of all fishing and processing activities on board. In return, fishers received a 30% quota bonus for cod and were compensated with more flexibility on effort regulations. It was hypothesized that vessels in the CQM will (i) increase their landings by 30% according to their quota bonus, (ii) increase the use of gear with large mesh size, and (iii) change effort towards fishing locations with high catch rates of large cod and avoid areas with high catch rates of undersized cod. The results showed that CQM had no effect on fishing behaviour of the small vessels. In contrast, large vessels significantly increased their cod landings (216%) and avoided undersized cod. This difference in response of different fleets suggested that implementation of CQM, for instance in the context of the European Common Fisheries Policy, should consider fleet characteristics. It seemed that larger vessels in this study more easily adapted their behaviour to new management regimes and that the quota bonus opened up new fishing strategies, that were not envisaged during the implementation.

Correlation between microbiota and growth in Mangrove Killifish (Kryptolebias marmoratus) and Atlantic cod (Gadus morhua).

The vertebrate gut is host to large communities of bacteria, and one of the beneficial contributions of this commensal gut microbiota is the increased nutritional gain from feed components that the host cannot degrade on its own. Fish larvae of similar age and under the same rearing conditions often diverge with regards to growth. The underlying reasons for this could be differences in genetic background, feeding behavior or digestive capacity. Both feeding behavior and digestion can be influenced by differences in the microbiota. To investigate possible correlations between the size of fish larvae and their gut microbiota, we analyzed the microbiota small and large genetically homogenous killifish and genetically heterogeneous cod larvae by Bray-Curtis Similarity measures of 16S DNA DGGE patterns. A significant difference in richness (p = 0.037) was observed in the gut microbiota of small and large killifish, but the overall gut microbiota was not found to be significantly different (p = 0.13), indicating strong genetic host selection on microbiota composition at the time of sampling. The microbiota of small and large cod larvae was significantly different with regards to evenness and diversity (p = 0.0001), and a strong correlation between microbiota and growth was observed.

Seasonal variation in cod feeding and growth in a changing sea

Seasonal variation in feeding and growth of 3- to 9-year-old Atlantic cod (Gadus morhua) was studied using data from joint Norwegian–Russian surveys in January–March (winter survey) and September–August (ecosystem survey) in the Barents Sea. The study encompassed the warmest period on record, with large cod stock and both low (2004–2007) and high (2008–2013) abundance of Barents Sea cod’s main prey, capelin (Mallotus villosus). Feeding on capelin was most important in winter. Energy acquisition (kJ·day−1) was higher during fall, but in years with high capelin abundance the seasonal difference was smaller. There was no difference in energy acquisition in years with high and low capelin abundance, underpinning recent findings on decoupling between capelin abundance and cod demography. Compensatory feeding on alternative prey takes place in fall, but not in winter. These findings were consistent across age groups. The results found for energy acquisition was mirrored in growth; in years with low capelin abundance, there was a seasonal difference in growth, whereas in years with high capelin abundance, the difference was absent.

Long-term shift in coastal fish communities before and after the collapse of Atlantic cod (Gadus morhua)

Abstract The collapse of the Northwest Atlantic cod (Gadus morhua) stocks in the early 1990s led to widespread ecological changes offshore. Changes in coastal fish communities are less known, largely due to the lack of historical records and long-term, standardized research surveys in coastal ecosystems. We aimed to overcome this with a unique dataset known as the Fleming survey. From 1959 to 1964, a systematic beach-seine survey was conducted to examine juvenile cod abundance in 84 bays (42 of which were consistently sampled and therefore analysed) along the east coast of Newfoundland. In addition to cod, all other fish collected in the seines were recorded. These surveys were repeated from 1992 to 1996 after the cod collapse, and document a substantial reduction in a dominant inshore species—juvenile Atlantic cod. We show that total fish abundance declined significantly with the decrease of cod, whereas Shannon diversity and species evenness significantly increased. Species richness increased in some regions but decreased in others. We also found significant changes in the composition of the fish community likely due to a combination of release from predation from fewer large cod feeding in inshore areas as well as a release from competition from fewer juvenile cod occupying the coastal habitat. Region and the presence of vegetated habitats also significantly influenced the fish community. This study shows a strong reorganization of coastal fish communities after a large-scale fisheries collapse with implications for ecosystem-based and cross-ecosystem management.

Species diversity affects ecosystem structure and mass flows in fjords

Latitude and temperature have been hypothesized to influence species richness, diversity and trophic control (top-down vs. bottom-up) in marine ecosystems. Ecosystem structures and mass flows of two moderately exploited fjord ecosystems with different temperatures and species diversity were compared. The Ullsfjord and Sørfjord systems (69°–70° N) are located between the relatively simple, low-diversity arctic Barents Sea in the north (71°–80° N) and the more species-rich and temperate North Sea (51°–62° N) in the south. Ullsfjord is the outer part of the fjord system and is deeper, warmer and more diverse than Sørfjord. Ecopath mass-balance models containing 40 ecological groups were developed for Ullsfjord and Sørfjord for the time period 1993–96. To obtain input data, abundance and diet of top-predators, fish, pelagic and benthic invertebrates were investigated. In the more diverse Ullsfjord system, large krill and pelagic shrimps were abundant and lower trophic level groups (TL<3) had the highest keystoneness, suggesting importance of bottom-up control. In contrast, large cod had the highest keystoneness and a large top-down effect as predator on small fishes and the larger crustacean groups in Sørfjord. For the diverse benthic invertebrates, the warmer and faster system (Ullsfjord) had higher mortality rates, shorter life spans and lower biomass (6.3 vs. 9.2 g C m−2) than the colder system (Sørfjord), but production (3.2 vs. 3.6 g C m−2 year−1) was similar in the two systems suggesting bottom-up control of benthic invertebrates.

Behavioral responses of Atlantic cod to sea temperature changes.

Understanding responses of marine species to temperature variability is essential to predict impacts of future climate change in the oceans. Most ectotherms are expected to adjust their behavior to avoid extreme temperatures and minimize acute changes in body temperature. However, measuring such behavioral plasticity in the wild is challenging. Combining 4 years of telemetry-derived behavioral data on juvenile and adult (30–80 cm) Atlantic cod (Gadus morhua), and in situ ocean temperature measurements, we found a significant effect of sea temperature on cod depth use and activity level in coastal Skagerrak. During summer, cod were found in deeper waters when sea surface temperature increased. Further, this effect of temperature was stronger on larger cod. Diel vertical migration, which consists in a nighttime rise to shallow feeding habitats, was stronger among smaller cod. As surface temperature increased beyond ∼15°C, their vertical migration was limited to deeper waters. In addition to larger diel vertical migrations, smaller cod were more active and travelled larger distances compared to larger specimens. Cold temperatures during winter tended, however, to reduce the magnitude of diel vertical migrations, as well as the activity level and distance moved by those smaller individuals. Our findings suggest that future and ongoing rises in sea surface temperature may increasingly deprive cod in this region from shallow feeding areas during summer, which may be detrimental for local populations of the species.

Arctic Cod (&lt;i&gt;Boreogadus saida&lt;/i&gt;) as Prey: Fish Length-Energetics Relationships in the Beaufort Sea and Hudson Bay

Although Arctic cod (Boreogadus saida) is widely recognized as an important trophic link to top predators in Arctic marine ecosystems, the challenges of conducting fieldwork in the Arctic make this species difficult to study. We establish some basic relationships to improve prey energetics modeling when only in-field parameters (e.g., fork length) can be measured. We investigated the intraspecific relationships among energy density, fork length, mass, and water content for Arctic cod captured by Black Guillemots and Thick-billed Murres at two sites (Western Beaufort and Hudson Bay). Dry energy density was similar between sites (21.6 – 22.2 kJ g-1) and increased with fork length (Dry EDkJ/g = 0.028 (± 0.01) • Fork Lengthmm + 18.12 (± 1.33). Even though fish lost some water as they were transported to the nest by avian predators, wet energy density also increased with fork length. We suggest that environmental conditions had a similar effect on growth at these two locations although they occur in very different oceanographic regimes. Arctic cod, especially large cod, is one of the most energy-rich prey species in the Arctic. Our results highlight the importance of this valuable prey to Arctic ecosystems and the utility of using seabirds opportunistically as samplers of the marine environment.

Challenges and Successes in the Management of a Shared Fish Stock – The Case of the Russian–Norwegian Barents Sea Cod Fishery

This article discusses the Russian–Norwegian fishery cooperation in the Barents Sea. The focus is on the common management of the shared cod resource, the Northeast Arctic cod stock. The article emphasizes the historical development of the cooperation in periods of changing political environment and different challenges for the common management agreement. The political changes define two distinct periods of bilateral cooperation and joint management. The first period is from 1975, when the Soviet Union and Norway established the Joint Soviet–Norwegian Fisheries Commission, to the end of the Soviet Union in 1991. The second period is from 1992 when the Russian Federation took over the obligations of fisheries cooperation with Norway from the Soviet Union. Although significant political, institutional and economic changes took place during these periods the biological facts remained, as the growth patterns and distribution of the exploited ecosystem did not change. The Russian exclusive economic zone (EEZ) is the main feeding area for juveniles and small cod, while the Norwegian EEZ includes spawning areas and the main feeding grounds for the mature and larger cod. The trust and cooperative spirit that developed through the Joint Fisheries Commission, both among researchers and managers of the two countries, has been crucial for the development of successful annual agreements and their outcomes. The Russian–Norwegian management of the Northeast Arctic cod stock has succeeded in protecting and maintaining the cod stock in the interest of both countries.

Spatial analysis of North Sea cod recruitment: concurrent effects of changes in spawning stock biomass, temperature and herring abundance

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 480:263-275 (2013) - DOI: https://doi.org/10.3354/meps10315 Spatial analysis of North Sea cod recruitment: concurrent effects of changes in spawning stock biomass, temperature and herring abundance Dag Ø. Hjermann1,*, Jonathan A. D. Fisher2, Tristan Rouyer1, Kenneth T. Frank3, Nils C. Stenseth1,4 1Centre for Ecological and Evolutionary Synthesis (CEES), Department of Biology, University of Oslo, PO Box 1066 Blindern, 0316 Oslo, Norway 2Centre for Fisheries Ecosystems Research, Fisheries and Marine Institute of Memorial University of Newfoundland, PO Box 4920 St. John’s, Newfoundland A1C 5R3, Canada 3Fisheries & Oceans Canada, Bedford Institute of Oceanography, Ocean Sciences Division, Dartmouth, Nova Scotia B2Y 4A2, Canada 4Institute of Marine Research, Department of Coastal Zone Studies, Flødevigen Research Station, 4817 His, Norway *Email: d.o.hjermann@bio.uio.no ABSTRACT: The decline of the North Sea cod Gadus morhua has been attributed to both overfishing and ocean warming. However, another hypothesis is that overfishing of piscivorous cod has caused an increased abundance of formerly predatory-controlled pelagic fish including herring Clupea harengus, which in turn has suppressed the recruitment of cod through predation on the early life stages of cod. Here we analyze 40 yr of trawl survey data in order to explore how the abundance of young herring affects cod recruitment, and how cod biomass affects the abundance of herring. In both cases we also take into account the effects of spawner biomass (of cod and herring, respectively) and sea temperature. We take a novel, explicitly spatial, approach by analyzing these effects on a local (185 × 167 km) scale. Our results indicate large spatial variability in ecological mechanisms. In the German Bight, high cod recruitment is associated with low herring abundance, low temperatures and high overall cod spawner biomass. This area used to contain a large portion of the cod recruits, and there is a strong correlation between the fraction of cod recruits found in this area and overall recruitment. In this area, herring recruitment is also negatively associated with the abundance of large cod. Thus, for this part of the North Sea, our findings are consistent with a reversal of dominance between cod and herring; however, herring may affect the cod by competition rather than by predation. KEY WORDS: Interspecific interaction · Ecosystem dynamics · Cultivation effect · Predator-prey reversal · Spatial heterogeneity · Warming Full text in pdf format Supplementary material PreviousNextCite this article as: Hjermann DØ, Fisher JAD, Rouyer T, Frank KT, Stenseth NC (2013) Spatial analysis of North Sea cod recruitment: concurrent effects of changes in spawning stock biomass, temperature and herring abundance. Mar Ecol Prog Ser 480:263-275. https://doi.org/10.3354/meps10315 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 480. Online publication date: April 22, 2013 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2013 Inter-Research.

Tracking cod diet preference over a century in the northern Gulf of Maine: historic data and modern analysis

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 474:263-276 (2013) - DOI: https://doi.org/10.3354/meps10068 Tracking cod diet preference over a century in the northern Gulf of Maine: historic data and modern analysis Theodore V. Willis1,*, Karen A. Wilson1, Karen E. Alexander2, William B. Leavenworth2 1Department of Environmental Science, University of Southern Maine, 37 College Ave., Gorham, Maine 04038, USA 2Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, New Hampshire 03824, USA *Email: theowillis06@aim.com ABSTRACT: Limited information is available regarding long-term compensation in the food webs of exploited ecosystems. We compared the results of 3 studies that assessed the diets of Atlantic cod from Passamaquoddy Bay, Maine, USA, spanning 100 yr, using diet summary statistics to compare historical and modern datasets. We found that 1965 cod consumed fewer invertebrates than cod caught in 1896, but that cod caught in the period 2005 to 2008 ate diet items more similar to the 1896 fish. Large cod caught in the summer of 1965 primarily ate fish, particularly Atlantic herring, whereas few fish were found in the diets of 2005 cod. Documented changes in diet may reflect changes in technology and resource exploitation that altered the prey field available to cod over time. In the 1890s, the herring and finfish fisheries were widespread and landings were large, but other components of the food web, particularly invertebrates, were comparatively unexploited. Impacts on the benthos of Passamaquoddy Bay increased by the 1960s with the widespread use of heavy dredge and trawl gear. By 2005, most fisheries had abandoned Passamaquoddy Bay, and cod and herring were reduced in numbers. Ecosystem resilience can be seen in the return of modern cod to the 1896 diet, but compensatory dynamics in the food web appear to have favored benthic, then pelagic, and finally a ‘balance’ of diet constituents, but with fewer commercially important finfish in the ecosystem overall. When and where the data exist, 18th and 19th century accounts of fishery activity can be used to put modern trends in perspective. KEY WORDS: Historical ecology · Cod · Clupeid · Diet · Frequency of occurrence · Passamaquoddy Bay · Trophic position Full text in pdf format PreviousNextCite this article as: Willis TV, Wilson KA, Alexander KE, Leavenworth WB (2013) Tracking cod diet preference over a century in the northern Gulf of Maine: historic data and modern analysis. Mar Ecol Prog Ser 474:263-276. https://doi.org/10.3354/meps10068 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 474. Online publication date: January 31, 2013 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2013 Inter-Research.

Big fish and great auks: Exploitation of birds and fish on the Isle of Portland, Dorset, during the Romano-British period

The paper considers the assemblage of bird and fish bones from a Romano-British settlement on the Isle of Portland, on the southern coast of England. Compared with contemporary sites, the assemblage includes an unusually large number of fish bones from a wide range of marine species, including large cod, other Gadidae, several species of seabream, scad and bass. The bird assemblage includes bones of a butchered great auk. This provides the first evidence that this extinct species was nesting off the shores of central southern England and being exploited for food in this period. Other seabirds identified included razorbill, great northern diver and gannet. The species represented are discussed in relation to other Romano-British sites, particularly the Roman town of Dorchester, situated 15 km away. Many of the species have been discovered on only a few contemporary sites and the presence of the seabream in particular indicates that seawater temperatures may have been warmer than until very recently. Possible cultural changes in diet and food procurement in the Roman period are also considered.

Winter foraging by a top predator, the grey seal Halichoerus grypus, in relation to the distribution of prey

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 462:273-286 (2012) - DOI: https://doi.org/10.3354/meps09844 Winter foraging by a top predator, the grey seal Halichoerus grypus, in relation to the distribution of prey V. Harvey1, M. O. Hammill1,*, D. P. Swain2, G. A. Breed3, C. Lydersen4, K. M. Kovacs4 1Maurice Lamontagne Institute, Mont-Joli, Quebec G5H 3Z4, Canada 2Gulf Fisheries Centre, Moncton, New Brunswick E1C 9B6, Canada 3UC Santa Cruz, Long Marine Lab, Santa Cruz, California 95060, USA 4Norwegian Polar Institute, Tromsø 9296, Norway *Email: mike.hammill@dfo-mpo.gc.ca ABSTRACT: Identifying foraging areas of individuals and correlating them with potential food resources allows for a better understanding of predator–prey relationships. Herein, we examine whether grey seal movements were associated with overwintering concentrations of several commercial fish species in the Cabot Strait, Atlantic Canada, using data from satellite transmitters deployed on grey seals (between 1993 and 2005) and winter bottom-trawl survey data (1994 to 1997). The distribution of searching effort by male grey seals varied throughout the winter. In early winter, males concentrated their movements around St. Paul’s Island. In late winter, they were found to the southeast of this area, where females also occurred. The fish community differed between apparent foraging and non-foraging areas. Densities of small plaice, hake and redfish, large herring and cod of all sizes were relatively high in the male grey seal foraging zones; female foraging zones were characterized by higher densities of small plaice and redfish and large cod. Areas where grey seal foraging was not concentrated were characterized by high densities of medium and large redfish as well as large turbot and witch flounder. Diet samples are needed to determine whether grey seals are feeding on the fish groups that distinguish foraging from non-foraging areas or alternatively on other prey that occur in the same areas. KEY WORDS: Marine predator · Foraging areas · Cod · Spatial overlap · First-passage time · Groundfish · Gulf of St. Lawrence Full text in pdf format PreviousNextCite this article as: Harvey V, Hammill MO, Swain DP, Breed GA, Lydersen C, Kovacs KM (2012) Winter foraging by a top predator, the grey seal Halichoerus grypus, in relation to the distribution of prey. Mar Ecol Prog Ser 462:273-286. https://doi.org/10.3354/meps09844 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 462. Online publication date: August 21, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Capture of wild saithe (Pollachius virens) and cod (Gadus morhua) in the vicinity of salmon farms: Three pot types compared

Large numbers of wild members of commercially important fish species tend to congregate around fish farms. This effect is in conflict with the interests of fishermen because wild fish cannot be harvested close to fish farms due to the fishery exclusion zone, which is intended to prevent fishing gear from damaging the cages. We studied the potential for harvesting wild fish around a Norwegian salmon farm using three different types of pots. Our video observations showed that large quantities of wild fish, in particular saithe (Pollachius virens), aggregated in close vicinity of the cages. Pots set underneath salmon cages produced 17 times higher catches of saithe and five times higher catches of cod (Gadus morhua) than pots set at a distance of 100m from the cages. The pots set underneath cages also caught larger cod. Large rigid pots were shown to be more efficient than smaller flexible pots. The stomach content of small cod was dominated by pellets, while large cod were feeding mainly on saithe. We suggest that dense aggregations of saithe and small cod beneath fish cages were associated with the supply of waste feed, whereas larger cod were attracted by the saithe. We conclude that pots have great potential for harvesting gadoids beneath salmon cages, but catches decline dramatically with the distance from cages.

Using tagging experiments to evaluate the potential of closed areas in protecting migratory Atlantic cod (Gadus morhua)

Abstract Schopka, S. A., Solmundsson, J., Ragnarsson, S. A., and Thorsteinsson, V. 2010. Using tagging experiments to evaluate the potential of closed areas in protecting migratory Atlantic cod (Gadus morhua). – ICES Journal of Marine Science, 67: 1024–1035. About 5200 cod (Gadus morhua) were tagged on three fishing grounds and within two marine protected areas (MPAs) northwest of Iceland. Two of the fishing grounds were closed to otter trawling, and the third was open to all fishing gear. Tagged fish were divided into two size classes, below (small) and above (large) a reference length of 55 cm, and the following variables were estimated: (i) proportion of the recaptured small cod that did not reach the reference size, (ii) size increase from tagging to recapture, (iii) time at liberty, and (iv) spatial distribution of recaptures. For the small cod, the results suggest that not only protection status but also the distance to areas of intense fishing mainly account for differences in the variables among tagging areas. No differences were observed for the large cod among tagging areas. The results show that immature cod are relatively stationary, but perform seasonal migrations between feeding and spawning grounds after reaching maturity. The estimated distribution of tagged cod generally changed after standardizing the recapture data with fishing effort (catch per unit area). Our results suggest that area closures on nursery grounds can be useful in protecting immature cod, but the MPAs studied are of little use in protecting highly migratory adults.

The benthic impacts of a large cod farm — Are there indicators for environmental sustainability?

A cod farm in Vidlin Voe, Shetland was at the time of the study the largest cod farm in Europe, and was started in 2003. Run according to organic principles, the farm consisted of 17 ⁎ 32 m diameter cages in the 3 km long sea loch (locally known as voe), which though extremely exposed at the mouth, is sheltered near the cages. Benthic macrofauna and physical/chemical sediment parameters were measured in the summer months of 2004, 2005 and 2006 as biomass increased in the growing cycle. A range of biotic indices showed the benthic environment was enriched at peak biomass of the farm, but by contrast, surficial CHN levels and organic matter were low and uniform. The commonly used sediment indicators redox, total organic carbon and total organic nitrogen did not correlate well with biotic indicators at this site. Predictions from a near-field particle tracking and resuspension model (CODMOD) and a simple nutrient enhancement box model of the voe system (Equilibrium Concentration Enhancement (ECE) model), were used to assess the assimilative capacity of the system.

Modelling the impact of cod ( Gadus morhua L.) farming in the marine environment—CODMOD

The environmental impact of Atlantic cod ( Gadus morhua L.) farming was quantified by assessing the benthic effects of a large cod farm in Shetland, UK and by parameterising a mathematical model based on an existing salmon model (DEPOMOD, [Cromey, C.J., Nickell, T.D., Black, K.D., 2002a. DEPOMOD—Modelling the deposition and biological effects of waste solids from marine cage farms. Aquaculture 214, 211–239.]) to predict solids deposition from farmed cod to the seabed. Benthic physico-chemical and macrofaunal data for three consecutive years provided synoptic data for model validation, allowing model testing throughout the growing cycle, from low biomass to near maximum. A video technique for quantifying the particle size and settling rates of cod faeces was further developed (weighted mean settling velocity for cod faeces was 3.7 cm s − 1 ), and faecal output experiments yielded temporal data on post-prandial faecal output (faecal output was 8.5% of feed input d − 1 by mass), which was lower than faecal output rates that have been used in modelling salmon impacts. Feed settling experiments showed large and significant differences between three proprietary cod feeds. At most benthic stations, there was a good fit between modelled and measured benthic Infaunal Trophic Index (ITI) but it failed to reproduce the moderate to heavy impact observed at an inshore transect. Possible reasons for the underperformance of the model in this area include the hydrographic data available and unreported cage movements in the area of the transect. When these factors were taken into account in the model, performance improved. Surveys of the sea bed surrounding the cod farm indicated that over a cod growing cycle up to near maximum biomass, there is a biological response to organic enrichment of the near-field sediments. This enrichment is not excessive in terms of the biomass present, and is comparable to salmon culture sites of similar scale in similar environments. Provided that species-specific parameters are used for particulate wastes no special considerations for modelling of this relatively new aquaculture species are necessary.

Residency of adult Atlantic cod ( Gadus morhua) in the western Gulf of Maine

A mark and recapture study of cod in the western Gulf of Maine was conducted to study the seasonal movements of fish, particularly as they related to areas closed to commercial fishing. A total of 27,772 cod were tagged, and 1334 (4.8%) were recaptured with sufficiently detailed recapture location and date to be included in the study. Results indicated that the group is resident to the area and sedentary. Although there were a small percentage of fish (2.5%) that traveled long distances (>100 km), most were recaptured near their release location. There was no linear relationship between fish length and linear distances traveled, and linear distance traveled was only weakly related to days-at-large. For all groups of cod tagged and released in particular areas and months, mean distances traveled were small (<65 km), rates of travel were slow (<0.2 km/day), and rates of group dispersion were <50 km 2/day. There were no recognizable spatial or temporal patterns in the mean angles of travel for groups released in various month/area combinations, and the angular deviations associated with the mean angles were quite large. Although there seems to be little pattern in the movement of cod in our study area, temporal and spatial changes in abundance indicate that movements are occurring. The general pattern was a concentration of large cod into one small, inshore area (Area 133) in both the spring and winter, and dispersion from this area in the ensuing months. Monthly percentages of ripe cod in Area 133 provide evidence for two spawning groups; a winter group that spawns from November through January, and a spring group that spawns from April through July. Thus it is likely that the observed spring and winter concentrations of fish in Area 133 were associated with spawning. Fish in the spring group were the largest encountered in the study. The study also provided some evidence of natal homing, i.e. a return to the same spawning grounds year after year, for the spring spawning group. Although there is some exchange of fish between most of the closed areas, the timing of the closures appears to protect the largest aggregations of cod.

Growth model for Atlantic cod ( Gadus morhua): Effects of temperature and body weight on growth rate

Results from several laboratory experiments showed that at each temperature there was a linear relationship between the logarithms of specific growth rate ( G%/day) and body weight ( W g) of Atlantic cod fed to satiation: ln G = α + βln W. Both α and β were found to be a function of temperature ( T °C): α = a + bT + cT 2; β = d + eT; a = − 0.7620, b = 0.3982, c = − 0.0128, d = − 0.1500, e = − 0.0239. The e-parameter was altered by 10% from the laboratory value (to e = − 0.0215) to tune the model in accordance with growth rate of large cod reared in sea cages in Norway. The model predicts that the optimal temperature for growth ( T opt.G) declines with body weight: T opt.G = 15.57–0.8426ln W, i.e. 15.0, 13.0, 11.1 and 9.2 °C for 2, 20, 200 and 2000 g fish, respectively. The predicted growth rates at optimal temperature ( G max) were 7.41, 2.62, 1.02 and 0.44%/day for 2, 20, 200 and 2000 g fish, respectively. Model calculations show that 30 g cod juveniles stocked in sea cages on 15 May have reached 1.6 and 2.1 kg by the end of the second year and 4.6 and 6.3 kg by the end of the third year, in Northwest Iceland and West Norway, respectively.

Prey switching of cod and whiting in the North Sea

ABSTRACT: Predator–prey switching may stabilise predator–prey interactions, promote co-exis-tence of prey that share a common predator, and increase the overall stability of homogeneous sys-tems of interacting species. This study presents an investigation of prey switching of 2 major NorthSea fish predators, cod Gadus morhua and whiting Merlangius merlangus . Relative food compositionderived from analysis of more than 36000 stomachs is compared to the relative density of fish preyreflected by trawl survey catches, and generalised linear models are used to examine how preyswitching is affected by predator length and prey species and length. Possible effects of stomachsample size and predator density are also investigated. Prey preference is a decreasing function ofthe relative density of the prey (negative prey switching), in particular for large cod (40 to 100 cm).This was neither the result of variations in stomach sample size, nor of changes in local predatorabundance. If not counteracted by compensatory changes in spatial overlap or total food intake, neg-ative switching is likely to destabilise the interactions between cod and whiting and their fish prey. KEY WORDS: Prey switching · Piscivorous fishes · North Sea · Ecosystem stability

Vitellogenin in the blood plasma of male cod (Gadus morhua): A sign of oestrogenic endocrine disruption in the open sea?

An ELISA for cod vitellogenin (VTG) has been set up using cod lipovitellin for plate coating and standardisation. The assay has been applied to plasma samples collected from male and female cod caught in three distinct areas around the UK, three areas off the Norwegian coast and also to cod reared initially at an aquaculture site and subsequently maintained at a research station. The aim of the study was to determine whether there were any signs of oestrogenic endocrine disruption in a fish species living offshore. VTG induction was found in male cod caught in the North Sea, the Shetland Box area, in Oslofjord and also in cultivated fish. There was a strong relationship between concentrations of VTG and fish size. There was no evidence that the presence of VTG in the plasma of males is a natural part of their life cycle. On the other hand, the size of fish at which these elevated VTG concentrations appear (ca. 5 kg) is about the size that cod change from feeding primarily on benthic invertebrates to mainly other fish, both benthic and pelagic. The possibility is suggested that large cod pick up oestrogenic endocrine disrupters through the food chain.