Small Cod Research Articles

Murray cod waterhole woes

During bouts of prolonged drought and little to no river flow, waterholes are a haven for freshwater fishes. However, as humans divert more water for their own use and as global temperatures rise, these small bodies of water are increasingly vulnerable to climbing temperatures and low oxygen levels. Fortunately, some fishes are able to change quickly to cope with high temperatures, and occasionally these changes also help them do better with low oxygen levels. But size matters when it comes to dealing with these stressors. Big fish are thought to deal with low oxygen better than small fish, and small fish are thought to deal better with high temperatures, but all are likely to be vulnerable when both stressful circumstances occur simultaneously. To investigate how body size influences a fish's ability to tolerate both high heat and low oxygen, Darren McPhee, with researchers from the University of Queensland and the Queensland Department of Regional Development, Australia, turned to massive Murray cod – which can grow to the size of a giant panda (over 100 kg) – to find out how they deal with the combined threat.Working with fish ranging from 0.2 g to 3 kg, the team transferred the animals to water at temperatures mimicking a hot summer (28°C). After 4 weeks, all of the cod, regardless of size, increased the temperature at which they lose their balance (known as the upper thermal limit), indicating that the cod are able to deal with persistent high temperatures. But when the team tested the fish's abilities to remain upright at high temperatures as the water oxygen levels decreased, their ability to tolerate heat declined. Surprisingly, the fish's body sizes had an unusual impact on their ability to withstand high temperatures when their oxygen supply was restricted. As expected, when the fish had access to well-oxygenated water (100% and 50% oxygen saturation), the smaller animals coped better with high temperatures than the big fish. But this pattern flipped when the oxygen levels in the water were low (30% and 16% oxygen saturation), where the largest fish tolerated the high temperatures better than the smaller fish. Intriguingly, this didn't mean that the big fish were thermal tolerance champions. At the lowest oxygen level (16% oxygen saturation), the mid-sized fish coped best with the high temperatures.The scientists also tested how short-term and persistent exposure to different temperatures affected the cod's ability to survive low oxygen in several different ways. In the short term (less than 24 h), fish exposed to high temperatures breathed faster and lost their balance at an oxygen level twice as high as what they would normally handle. But when exposed to a high heat for 4 weeks, the responses to low oxygen weren't quite as drastic. Again, the largest cod were the least bothered by decreasing oxygen – not changing their breathing frequency and taking longer to lose their balance (unlike the little fish).Although Murray cod of all sizes demonstrated an amazing ability to cope with high temperatures in the lab, McPhee and colleagues also went into the wild to check the water quality of three Queensland waterholes that are much loved by the cod. The researchers found that the fish are living dangerously close to the edge – at the limit of the high temperatures and low oxygen levels that they can endure – especially during the summer. And, as the smallest cod are most vulnerable to the combined stress of high temperature and low oxygen, this slow-growing species is at genuine risk from catastrophic loss if their littlest ones succumb in the face of the deadly duo.

Effects of Marine Residue-Derived Fertilizers on Strawberry Growth, Nutrient Content, Fruit Yield and Quality

An outdoor experiment was performed for six months to evaluate the effects of organic fertilizers obtained from marine residual materials on strawberry plants. Three types of organic fertilizers were used, i.e., cod (Gadus morhua) bone powder, common ling (Molva molva) bone powder, and pellets obtained by mixing small cod bone powder and rockweed (Ascophyllum nodosum) residues. A tabletop system for strawberry cultivation was designed, in which two bare-root strawberry plants of cultivar ‘Albion’ were planted in a peat substrate in each pot. Five treatments were applied, i.e., cod bone powder (F1), common ling bone powder (F2), small cod bone powder and rockweed residue pellets (FA), chemical fertilizer (E), and a control (C). The number of leaves and their nutrient content, fruit yield and quality characteristics of the strawberries grown using the organic fertilizers were similar or better than those corresponding to treatments E and C. Organic fertilizers derived from the residues of fish and macroalgae could be a promising alternative to chemical fertilizers in strawberry production.

The impact of electronic monitoring on fleet wide discarding of small cod in Scottish demersal fisheries

Abstract In Europe, uptake of electronic monitoring (EM) has been hindered by a lack of potential cost savings, given low existing observer coverage, and resistance from industry to greater enforcement of the landing obligation. To assess why certain vessels volunteer for EM and what their subsequent changes in behaviour tell us about those of the wider fleet over time, this study investigates effects of EM on discarding of North Sea cod by Scottish demersal trawlers. As discard data were limited, weight of small cod (grade 5) landed per trip was selected as an indicator of discarding and modelled to describe discard patterns from 2006 to 2016 encompassing the EM-verified cod quota management scheme (2010–2016). Findings show that EM leads to reduced instances of discarding of small cod by participants. However, evidence suggests non-participant vessels were driven to greater levels of discarding due to quota acquisition by participants. Vessels volunteering for EM trials were not predisposed towards greater compliance. Landings data do not support the trial objective of incentivising avoidance of small cod being met. The influence of participants on non-participants has implications for how vessels are selected, and management of behavioural adaptions required to ensure participants remain representative of the wider fleet.

Support for the hypothesis that growth of eastern Baltic cod is affected by mild hypoxia. A comment on Svedäng et al. (2022)

Abstract Stock biomass and size composition of eastern Baltic cod (EBC) has declined, with low recent stock productivity attributed to loss of major spawning areas, reduced growth rates, and possibly increased natural mortality. Svedäng et al. present valuable new data for evaluating the causes of these changes. However, contrary to their own conclusions, those new data actually provide both direct and indirect support for the hypothesis that the reduced growth rate observed may be due to a decline in ambient oxygen for small cod in the Bornholm Basin area (SD25). Their long time series of reproductive volume (RV) extends our knowledge of historic changes in the limits to spawning imposed by salinity and oxygen. However, the causes and consequences of such spawning area changes are well established. Therefore, this Comment only addresses the contentious debate about causes of decline in growth.

Food and initial size influence overwinter survival and condition of a juvenile marine fish (age-0 Atlantic cod)

In subarctic Newfoundland, age-0 Atlantic cod (Gadus morhua) settle into coastal habitats in several summer–autumn pulses, resulting in broad length-frequency distributions before winter. Low winter temperatures and potential decreases in food availability pose challenges for young-of-year fish. To examine how size variation affects overwintering success under contrasting food scenarios, we conducted 114-day laboratory feeding trials at ambient overwinter sea temperatures, using demersal age-0 cod collected from Newman Sound, Newfoundland. We reared two size classes of juvenile cod under four daily ration levels (starvation, low, medium, high). We used Fulton’s K condition factor to interpret effects of food availability and fish size on survival over winter. We showed that small amounts of consumed food (<1% BW·day−1) maximized winter growth and condition potential of juvenile cod in Newfoundland waters. With no food, survival of small cod dropped below 80% on Day 47, whereas survival of large juveniles remained >80% until Day 74. Therefore, we expect higher survival of earlier settlers and increased size-selective mortality in age-0 cod during either unproductive or protracted winters, when food abundance is often low.

Reduced growth in Baltic Sea cod may be due to mild hypoxia

Abstract Daily food consumption by small Baltic Sea cod has declined in recent decades, resulting in reduced growth and biomass. Declining oxygen may cause lower production of benthic prey for small cod, but an alternative explanation presented here is that the mildly hypoxic conditions that small cod experience reduces their rate of digestion and hence food consumption. Better information about the distribution and behaviour of small cod and their metabolic response to low oxygen levels is required.

Baltic cod endohelminths reflect recent ecological changes

We suggest helminthological investigations of cod as a supplement to traditional biological and hydrographical methods for elucidation of ecological changes in the Baltic Sea. It is under discussion if oxygen deficit or seal abundance should explain the present critical situation of Baltic cod. A comparative investigation of endoparasitic helminths in Baltic cod (Gadus morhua), captured in the same marine habitat with an interval of 35 years (1983/2018) recorded 11 species of helminths comprising trematodes (Hemiurus luehei, Podocotyle atomon, Lepidapedon elongatum), nematodes (Contracaecum osculatum, Hysterothylacium aduncum, Capillaria gracilis, Cucullanus cirratus), cestodes (Bothriocephalus sp.) and acanthocephalans (Echinorhynchus gadi, Pomphorhynchus laevis, Corynosoma semerme). Significant prevalence and intensity increases were recorded for third-stage larvae of the nematode C. osculatum (liver location) and larvae of C. semerme (encapsulated in viscera). Both parasite species use grey seal as their final host, indicating the recent expansion of the Baltic seal population. A lower E. gadi intensity and an increased prevalence of L. elongatum of small cod (31-40 cm body length) suggest a lowered intake of amphipods (intermediate host) and elevated ingestion of polychaetes, respectively, but no significant changes were seen for other helminths.

Feeding and growth of Atlantic cod (Gadus morhua L.) in the eastern Baltic Sea under environmental change

AbstractFive decades of stomach content data allowed insight into the development of consumption, diet composition, and resulting somatic growth of Gadus morhua (Atlantic cod) in the eastern Baltic Sea. We show a recent reversal in feeding level over body length. Present feeding levels of small cod indicate severe growth limitation and increased starvation-related mortality. For young cod, the low growth rate and the high mortality rate are manifested through a reduction in size-at-age. The low feeding levels are likely the result of a decrease in benthic prey abundance due to increased hypoxic areas, while decreasing abundances of pelagic species in the area of cod distribution have prevented a compensatory shift in diet. Our study emphasizes that environmental forcing and the decline in pelagic prey caused changes in consumption and growth rates of small cod. The food reduction is amplified by stunted growth leading to high densities of cod of smaller size competing for the scarce resources. The average growth rate is negative, and only individuals with feeding levels well above average will survive, though growing slowly. These results suggest that the relation between consumption rate, somatic growth and predatorprey population densities is strongly environmentally mediated.

Effect of a quality-improving cod end on size selectivity and catch patterns of cod in bottom trawl fishery

A new cod-end concept developed and tested exhibited significantly improved quality of caught cod (Gadus morhua) compared with that of the conventional cod end used in the Barents Sea bottom trawl fishery. However, the design of the new quality-improving cod end raised concerns about its size selectivity and the possibility that higher retention probability could negatively impact the catch pattern by increasing the proportion of undersized cod. Therefore, the goal of this study was to quantify and compare the size selectivity and catch pattern for cod when deploying, respectively, the conventional and new quality-improving cod end in the Barents Sea bottom trawl fishery. The new quality-improving cod end had significantly lower relative size selectivity than the conventional cod end, but no significant difference in the catch patterns was detected in the trawl. Further, estimation of the total size selectivity in the trawl revealed that the increased retention of small cod when using the quality-improving cod end was minor. Hence, despite the reduced selectivity, the quality-improving cod end can be used with low risk of retaining small cod.

Causes of different impact of Major Baltic Inflows on cod reproduction in the Gotland Basin of the Baltic Sea

The impact of the Major Baltic Inflows (MBIs) of 1976, 1993, and 2003 on cod reproduction in the Gotland Basin of the Baltic Sea is considered based on calculations of the eastern Baltic cod egg abundance and offspring survival indices. The reproductive success (1976–1977) was defined by the elevation of the 11 psu isohaline and deepening of the 2 mL/L isooxygene surface, which provided the formation of a water layer suitable for cod spawning at a depth of 90–130 m. Prerequisites of the appearance of the reproductive layer in 1976–1977 were a previously high salinity level and penetrating two successive MBIs in one year. The cause of the weak MBI impact in 1993 was the previous salinity minimum; in 2003 it was rapid oxygen depletion due to an increase in temperature. As a result of their weaker influence, small and fragmented cod spawning biotopes appeared in the Gotland Basin.

Host size-dependent anisakid infection in Baltic cod Gadus morhua associated with differential food preferences.

A significant increase in the infection level of Baltic cod Gadus morhua with the anisakid nematode larvae Contracaecum osculatum and Pseudoterranova decipiens has been recorded during recent years due to the expanding local population of grey seals Halichoerus grypus, which act as final hosts for these parasites. Here, we report from an investigation of 368 cod (total length [TL] 6-49 cm; caught in ICES Subdivision 25) that the infection level of juvenile cod (TL 6-30 cm) with larvae of C. osculatum and P. decipiens is absent or very low, whereas it increases drastically in larger cod (TL 31-48 cm). A third nematode Hysterothylacium aduncum was rarely found. The study indicates that the prey animals for large cod act as transport hosts for the parasite larvae. Analyses of stomach contents of cod caught in the same area (2007-2014) showed that small benthic organisms (including polychaetes Harmothoë sarsi) are preferred food items by small cod, the isopod Saduria entomon is taken by all size classes, and sprat Sprattus sprattus are common prey items for cod larger than 30 cm. Parasitological investigations (microscopic and molecular analyses) of H. sarsi (100 specimens) and S. entomon (40 specimens) did not reveal infection in these invertebrates, but 11.6% of sprat (265 specimens examined) was shown to be infected with 1-8 C. osculatum third stage larvae per fish. Analyses of sprat stomach contents confirmed that copepods and cladocerans are the main food items of sprat. These observations suggest that the C. osculatum life cycle in the Baltic Sea includes grey seals as final hosts, sprat as the first transport host and cod as second transport host. It may be speculated that sprat obtain infection by feeding on copepods and/or cladocerans, which could serve as the first intermediate hosts. One cannot exclude the possibility that the size-dependent C. osculatum infection of cod may contribute (indirectly or directly) to the differential mortality of larger cod (>38 cm) compared to smaller cod (<30 cm) recently recorded in the Baltic cod population.

Marine ecosystem connectivity mediated by migrant-resident interactions and the concomitant cross-system flux of lipids.

Accumulating research argues that migrants influence the functioning and productivity of local habitats and ecosystems along migration routes and potentially drive cross‐system energy fluxes of considerable magnitude, yet empirical documentation of local ecological effects and descriptions of the underlying mechanisms are surprisingly rare. In this study, we discovered migrant–resident interactions and substantial cross‐system lipid transportation in the transition zone between the Baltic Sea and the North Sea where a resident cod population (predators) was found to interact with a herring population (prey) on a seasonal basis. We traced the lipids, using fatty acid trophic markers (FATM), from the herring feeding grounds in the North Sea to the cod livers in the Western Baltic Sea. Time series analysis of population dynamics indicated that population‐level production of cod is positively affected by the herring subsidies. However, the underlying mechanisms were more complicated than anticipated. During the herring season, large cod received most of its dietary lipids from the herring, whereas smaller cod were prevented from accessing the lipid pool due to a mismatch in predator–prey size ratio. Furthermore, while the herring were extremely rich in bulk energy, they were surprisingly poor in a specific functional fatty acid. Hence, our study was the first to illustrate how the magnitude cross‐system fluxes of subsidies in migrant–resident systems are potentially constrained by the size structure of the resident predator population and the nutritional quality of the migrants.

Estimating and mitigating the discard mortality of Atlantic cod (Gadus morhua) in the Gulf of Maine recreational rod-and-reel fishery

Abstract In recent years, the recreational contribution to the total catch of Atlantic cod (Gadus morhua) in the Gulf of Maine (GOM) has increased with recreational discards outnumbering recreational landings by 2:1. However, the discard mortality (DM) rate of cod released in the recreational fishery remains poorly understood, thus contributing to the uncertainty in stock assessments and fishery management plans. The current study examined the capture-related factors most detrimental to cod DM in the GOM recreational rod-and-reel fishery. Atlantic cod (n = 640; 26.0–72.0 cm) were angled from June–October 2013 on southern Jeffreys Ledge in the western GOM using fishing gear representative of the local recreational fishery. A subset (n = 136) was also tagged with pressure-sensing acoustic transmitters before being released into an acoustic receiver array (n = 31) deployed to monitor survival up to 94 days. To properly model DM up to the fishery-wide level, all cod were visually assessed for capture-related injuries according to a four-level injury score index. Mean tackle-specific DM rates of 15.4 and 21.2% were estimated for bait- and jig-captured cod, respectively, with an overall 16.5% mean DM rate for the 2013 GOM recreational cod fishery. Twenty-nine cod tagged with acoustic transmitters were identified as dead, where the majority (∼90%) died within 16 h post-capture. Upon evaluation with a specifically adapted parametric survival analysis, greater incidence of mortality was attributed to the capture and handling process (rather than release) for moderately and severely injured cod. Based on the capture-related factors associated with the highest injury rates, we recommend minimizing fight and handling times, avoiding areas with small cod, educating inexperienced anglers, and favouring bait over jigs to mitigate mortality. Results will continue to inform the development of fishery management plans and enhance survival through dissemination of “best practice” techniques to fishery stakeholders.

Pulse trawl fishing: characteristics of the electrical stimulation and the effect on behaviour and injuries of Atlantic cod (Gadus morhua)

Abstract In the North Sea flatfish fishery, electric pulse trawls have been introduced to replace the conventional mechanical method. Pulse trawls reduce the fuel consumption, reduce adverse impact on the ecosystem but cause injuries in gadoids. We describe the design and electrical properties of pulse trawls currently in use and study the behavioural response and injuries in cod exposed to electrical pulses under controlled conditions. Pulse trawls operate at an average power of 0.7 kW m−1 beam length and a duty cycle of ∼2%. The electric field is heterogeneous with highest field strength occurring close to the conductors. Cod were exposed to three different pulse types for a range of field strengths, frequencies, and duty cycles. Two size classes were tested representing cod that escape through the meshes (11–17 cm) and market-sized cod that are retained in the net (34–56 cm). Cod exposed to a field strength of ≥37 V m−1 responded by moderate-to-strong muscular contractions. Some of the large cod (n = 260) developed haemorrhages and fractures in the spine, and haemal and neural arches in the tail part of the body. The probability of injuries increased with field strength and decreased when frequency was increased from 100 to 180 Hz. None of the small cod (n = 132) were injured and all survived. The field strength at the lateral boundaries of the trawl was too low to inflict injuries in cod.

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.

The effect of temperature and body size on metabolic scope of activity in juvenile Atlantic cod Gadus morhua L.

Changes in ambient temperature affect the physiology and metabolism and thus the distribution of fish. In this study we used intermittent flow respirometry to determine the effect of temperature (2, 5, 10, 15 and 20°C) and wet body mass (BM) (~30–460g) on standard metabolic rate (SMR, mgO2h−1), maximum metabolic rate (MMR, mgO2h−1) and metabolic scope (MS, mgO2h−1) of juvenile Atlantic cod. SMR increased with BM irrespectively of temperature, resulting in an average scaling exponent of 0.87 (0.82–0.92). Q10 values were 1.8–2.1 at temperatures between 5 and 15°C but higher (2.6–4.3) between 2 and 5°C and lower (1.6–1.4) between 15 and 20°C in 200 and 450g cod. MMR increased with temperature in the smallest cod (50g) but in the larger cod MMR plateaued between 10, 15 and 20°C. This resulted in a negative correlation between the optimal temperature for MS (Topt) and BM, Topt being respectively 14.5, 11.8 and 10.9°C in a 50, 200 and 450g cod. Irrespective of BM cold water temperatures resulted in a reduction (30–35%) of MS whereas the reduction of MS at warm temperatures was only evident for larger fish (200 and 450g), caused by plateauing of MMR at 10°C and above. Warm temperatures thus seem favourable for smaller (50g) juvenile cod, but not for larger conspecifics (200 and 450g).

Food Composition and Prey Selection of Cod, &lt;I&gt;Gadus Morhua&lt;/I&gt; (Actinopterygii: Gadiformes: Gadidae), in the Southern Baltic Sea

Background. At the end of 1980s, ecological regime shifts occurred in the Baltic. Since, the Baltic fish assemblage has been dominated by sprats, whereas before cod was the dominant fish species. The majority of previous food studies refer to the period before the shift. This raises the question, how changes in the Baltic ecosystem have affected the diet of cod. The aim of the presently reported study was to identify the differences in the food composition of cod depending on the area, depth, season, and the cod size and compare it with the findings from the 1970s and 1980s. Materials and methods. Food composition and selection of prey size for Baltic cod was evaluated by examining the stomach contents of 556 adult cod, collected in 2006 and 2007. Fish stomachs were sampled from research catches within the Polish Exclusive Economic Zone. Results. The main component of the cod diet were the clupeids, constituting more than 67% of the total weight of the food recovered from the specimens studied. Other fishes in the diet represented Gobiidae, Ammodytidae, as well as young cod. The dominant invertebrate in the diet was the isopod Saduria entomon which accounted for 13% of the diet by weight. The weight proportion of larger fish, such as herring, Clupea harengus (L.), and cod, increased with the cod size, while the proportion of small- and medium-sized fish (sprats, gobies) decreased. The food composition of cod varied seasonally. The weight proportion of European sprat, Sprattus sprattus (L.), was much higher in winter (55%) than in fall (27%), while the proportion of cod in the diet was higher in the fall (10%) than in winter (4%). The proportion of sprat in the diet increased with depth, while that of Ammodytidae and Gobiidae showed a decreasing trend. At depths greater than 40 m, the proportion of invertebrates in cod stomachs decreased. This study demonstrated a significant area effect for sprat; year effect for sprat and Saduria entomon; a seasonal effect for young cod and S. entomon; and a depth effect for sprat, young cod, and S. entomon. The occurrences of herring and small cod increased in the stomach as the predator increased, while no significant relation was found for sprat and S. entomon. The data collected do not indicate that cod select a specific size of sprat or herring, though cod were found to use size selection for S. entomon and juvenile cod. Conclusion. The food composition of cod has undergone certain changes compared to results of Załachowski (1977, 1985), which covered the 1970s and 1980s. Currently, the proportion of clupeids in the food is approximately 67% (in weight), while during the 1977–1982 it was in the range of 25%–50% depending on cod length. In 2006 and 2007 sprat was the main cod diet component rather than herring.

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 assessment of predator–prey relationships in the North Sea: the influence of abiotic habitat properties on the spatial overlap between 0‐group cod and grey gurnard

AbstractThe understanding of spatio‐temporal dynamics of marine ecosystems is crucial for ecosystem‐based fisheries management and climate change impact assessments. We quantified temporal changes in the distribution of 0‐group cod (Gadus morhua) and grey gurnard (Eutriglia gurnardus), a primary predator of 0‐group cod, with the help of regression kriging and assessed the temporal dynamics of the related spatial predator–prey overlap of these two species at different spatial scales. We analysed the robustness of relationships among abiotic habitat properties (temperature, salinity and depth) and abundance. Small cod was mainly found in low salinity areas of the Skagerrak but larger year classes were able to expand their distribution area towards the central and northern North Sea. In contrast, grey gurnard was mainly found in waters with salinities above 33 and temperatures above 14°C. This species has expanded its high density areas in the central North Sea northward over the last two decades. Recruitment success of cod was negatively correlated to a Moran's I cross‐correlation index, a proxy for the degree of spatial overlap between both species. Strong cod year classes overlapped less with grey gurnard at the large and medium spatial scale. In general, the relationships between abiotic habitat properties and abundance showed an increased inter‐annual variability, which was likely caused by underlying factors not taken into account in the distribution models. Thus assemblage modeling approaches combining the strength of different model types should be considered in the future to predict potential distribution patterns under climate change scenarios.

Marine ash-products influence growth and feed utilization when Atlantic cod Gadus morhua L. are fed plant-based diets

Atlantic cod Gadus morhua L. were fed high plant protein diets added to either shrimp-shell meal or crab-shell meal. The aims were to investigate if diluting dietary energy would reduce the liver index (HSI) and if marine ash would add value to plant protein-based diets. Two control diets were used: a high plant protein control diet (PP) with no marine ash addition, and a fishmeal-based diet (FM) with no marine ash addition. All diets were evaluated in small cod (initial weight 79 ± 15g) and in market-size cod (initial weight 1579 ± 20 g). Addition of crab-shell meal up to 20% and shrimp-shell meal up to 10% did not influence liver size in either small or market-size cod. An addition of up to 20% crab-shell meal and 10% shrimp-shell meal improved growth compared to the PP control diet, and stimulated increased feed intake. However, 10% shrimp-shell meal and 20% crab-shell meal diets resulted in a similar intake of energy and protein as the control groups. Increasing shrimp-shell meal to 20% resulted in reductions in feed intake, fat digestibility and growth, and in altered gut histology. All diets, except the 20% shrimp added diet, resulted in normal ranges of plasma nutrients and blood hematological values, showing good fish health with or without the marine ash addition.