0-group Cod Research Articles

Liver condition of 0 and 1-group cod (Gadus morhua) in the Barents Sea

Liver condition is hypothesized to be a better condition index for juvenile cod than other somatic condition indices, since cod store most of their excess energy in their liver. It is suggested that population changes in liver condition can be used as a proxy for overwinter mortality. Here, we have collected 0- and 1-group cod from the same cohort in the Barents Sea before and at the end of the winter season. We have taken measurements for somatic- and liver-based condition indices, as well as enumerating nematode endoparasites found in each individual cod. The average wet weight of sampled cod increased by 85% (approx. 0.4% day−1) from 6.0 to 10.1 g from September to February, while the average liver wet weight decreased by 15%. The relative water content of the liver also increased significantly over the winter, resulting in a 42% reduction in average dry liver weight from 0.116 to 0.067 g over the 4–5-month period between samplings. About 30% of the 1-group cod surviving the winter had less than 20% of the liver energy content of same-sized 0-group cod. The liver condition was also 5–10% lower in cod with high nematode endoparasite load than the cod without nematode endoparasites. The significance of the loss of liver energy content is discussed in relation to the marked seasonality combined with climate change and the challenges this imposes on the survival of 0-group cod in high-latitude environments during their first winter.

Habitat associations of juvenile Atlantic cod (Gadus morhua L.) and sympatric demersal fish communities within shallow inshore nursery grounds

Resolving the relationship between demersal fish and sublittoral biotic habitats and substrates is a key element in the protection of important nursery grounds for strengthening fish recruitment. In Norway, coastal Atlantic cod (Gadus morhua L.) is a commercially and culturally important demersal fish for Norwegian coastal communities, and in recent decades the stock has declined to such an extent that a plan to rebuild the stock to biologically safe limits has been implemented. Yet, little is known about the specific biotic and abiotic habitat associations of the early-life stages of coastal cod, which is important for the management and protection of the species. The same shallow, sublittoral zones are inhabited by juveniles and adults of other commercial demersal gadoids and wrasses. This study presents novel findings on associations between juvenile coastal cod and other demersal fish species with seafloor substrates and biological habitats, inferred from five years of extensive fyke net surveys. Newly settled 0-group cod were typically associated with eel grass and red algae biotic habitats on sand and shell sand substrates. However, there was an ontogenic habitat shift amongst one year old (1-group) individuals that became more ubiquitous with their biotic habitat or substrate type choices. The juvenile gadoids saithe (Pollachius virens) and pollack (Pollachius pollachius), were most associated with hard bottom dominated sites with saithe being more abundant at exposed sites, compared to pollack. Goldsinny wrasse (Ctenolabrus rupestris) and corkwing wrasse (Symphodus melops) appeared to favor sugar kelp forests and red algae. Overall, the study identifies the specific sublittoral biotic habitats and substrates important to different early life stages of juvenile coastal cod and other commercially important demersal fish, providing critical information needed for identifying candidate coastal habitats for protection.

The Record-Warm Barents Sea and 0-Group Fish Response to Abnormal Conditions

The Barents Sea is a nursery area for many fish stocks, and this whole region is presently subject to rapid climatic change from a cold period in the 1980s to a record warm period in the latest decade, with a peak in 2016. The present study focuses exclusively on year 2016, which was characterized by record warm air and seawater and an exceptionally large horizontal coverage of Atlantic waters. Earlier studies have suggested that environmental conditions during the first year of life are the most critical for year class strength and development of fish stocks. We focus on 8 fish species (age 0) and document spatial distributions of their abundances and lengths as well as ambient environmental conditions. Data for most of the Barents Sea obtained from the ecosystem survey were used to explore if the record-warm conditions in 2016 limited 0-group fish distributions, abundances and size. Abundances and lengths for the 8 species were related to physical conditions (seawater temperature and salinity) and biological features (mesozooplankton biomass and jellyfish biomass). In 2016, 0-group capelin, haddock, herring and long rough dab were more abundant and all species except long rough dab were larger than the long term mean (1980-2015). Larger individuals and higher abundances were observed mainly in the areas covered by relatively warm water masses apparently holding a sufficient amount of plankton. Most of the 0-group fishes were distributed within their thermal habitats, but with some geographical shift most likely reflecting a shift in the distribution of water masses. A significantly lower abundance of polar cod was observed in 2016, with very few individuals registered within the traditional core area in the south eastern Barents Sea. The increased temperature and low plankton biomass may have limited polar cod distribution and abundance there. A spatial analysis showed that biomass of C. capillata was positively related to abundances of 0-group herring, capelin and cod, indicating that they were inhabiting similar water masses. The high abundances of capelin, haddock, herring and long rough dab, and generally large individuals of most species, may suggest suitable living and feeding conditions in 2016

Predation mortality from ambush and cruising predators on newly-settled 0-group gadoids

We investigated predation from an ambush predator (shorthorn sculpin) and two cruising predators (Atlantic cod and saithe) on bottom-settled 0-group cod and saithe juveniles in a fjord in northern Norway. Stomach contents of potential fish predators caught by gill net in shallow waters (depth of 3–10 m) during September in 2013–2015 were analysed. The frequency of occurrence (FO) of 0-group gadoid prey was highest for shorthorn sculpin (FO = 10%) and cod (FO = 9%), while it was lower for saithe (FO = 0.8%). Each predatory fish species had different alternative prey to 0-group gadoids: shrimps for cod; sandeels for saithe; and Brachura/Anomura for shorthorn sculpin. Amongst the fish predators, shorthorn sculpin had the highest catch rates in gill nets. The mean number of 0-group cod and saithe in shorthorn sculpin stomachs increased with increasing abundance of prey as measured by beach seine hauls.Predation mortality from shorthorn sculpin was estimated based on abundance of shorthorn sculpins, number of 0-group cod and saithe juveniles in predator stomachs, prey recognition times and abundance of 0-group cod and saithe. The average predation mortality rate from shorthorn sculpin on 0-group gadoid juveniles varied from 0.40 to 1.14 month−1 between years and the pooled estimate from 2013 to 2015 was 0.70 month−1 (95%CI: 0.30, 1.73). This is comparable to mortality rates from other studies on newly-settled 0-group Atlantic cod, other gadoid species and plaice. Based on gill-net catches and stomach data from this study and literature values for gill-net catching area, calculated predation mortalities from the cruising cod and saithe were less than 47% of the sculpin predation mortality rates. This show that predation mortality from the ambush sculpin predator was much higher than from the cruising Atlantic cod and saithe.

Synergism between cruising cod and ambush sculpin predators on 0-group gadoids is modified by daylight cycle and presence of aggressive wolffish

Shorthorn sculpin (Myoxocephalus scorpius) and large cod (300–400 g body weight) are important predators on newly settled 0-group cod (Gadus morhua) and saithe (Pollachius virens) in Porsangerfjorden in northern Norway. The Atlantic wolffish (Anarhichas lupus) used to be abundant but is now very rare in the fjord. Video recorded experiments showed that interactions between shorthorn sculpins, wolffish and large cod affected predation rates on small 0-group cod and saithe in tanks with a gravel, cobble and seaweed (Fucus serratus) substrate. We found that sculpins were unable to catch 0-group specimens when they were the only predatory species in the experimental tank and the lights were left on 24 h a day (to simulate the midnight sun), and that they did not even attempt to catch them. The addition of a large cod to the tank increased sculpin attack rates and predation success significantly.The “cod effect” was due to 0-group specimens avoiding attacks by escaping into the cobbles or vegetation, were they could be more easily reached by the sculpins. When tanks were supplemented with a wolffish in addition to the cod, both sculpin and cod predation success was lowered by approximately 50% in the 24-h light regime. When the experimental setup was changed to a day/night light cycle, however, the wolffish presence had no effect. Also, sculpins in tanks without cod and wolffish went from consuming zero to 29% of the 0-group specimens in the tank when the light regime went from 24-h light to day/night. Sculpins and cod were most effective predators on 0-group cod and saithe at dusk/night/dawn, and the presence of a cruising predator like cod increased sculpin predation success and probably vice versa.The experimental data indicate that rebuilding wolffish stocks in the fjord system may make the bottom substrate more protective for 0-group specimens of cod and saithe, by limiting the foraging volume of an effective bottom-dwelling 0-group predator like the sculpin. Further field and lab studies must be conducted before the reported findings optionally can be turned into a management advice.

Ontogenetic development of otolith shape during settlement of juvenile Barents Sea cod (Gadus morhua)

Abstract This study documents how settlement of juvenile Atlantic cod (Gadus morhua) in the Barents Sea affects otolith growth and morphology. A simple method to objectively discriminate between age 0 and age 1 cod sampled in late summer was demonstrated by using only two otolith morphometric descriptors: area and perimeter. In the pre-settled 0-group cod, otolith lobe formation clearly increased with fish size, resulting in high otolith crenulation. This trend was disrupted during settlement, resulting in noticeably less crenulated otoliths of the settled 1-group cod sampled in winter. Combined observations of otolith shape, fish size, and body condition suggest that environmental factors associated with settlement during autumn, particularly reduced food intake, directly affect lobe formation leading to less crenulated otoliths. Comparably reduced body condition and otolith crenulation of 0-group cod in bottom trawls (vs. pelagic trawls) may indicate early settlement or vertical exploratory behaviour in the Barents Sea Ecosystem Survey (in August–September) and, thus, an underrepresentation of 0-group cod from pelagic trawling.

Seabird-fish interactions: the fall and rise of a common guillemot Uria aalge population

A major challenge in population ecology is the prediction of population responses to environmental variance. Food availability has long been hypothesized to play a major role in regulating seabird populations. In general, seabirds feed on small pelagic fish and/or young age classes of larger predatory fish. Here we used a logistic population model to predict the temporal variation in the population size of common guillemots Uria aalge in a colony in NE Norway (Hornoya) between 1987 and 2011 in relation to the variation in abundance (acoustic and trawl surveys) of important fish prey species in the Barents Sea. The fish species considered, all of which have been described in the diet of common guillemot chicks and adults on Hornoya, were capelin Mallotus villosus (all age classes), 1-group herring Clupea harengus and 0-group cod Gadus morhua. The guillemot population collapsed by more than 80% during the winter 1986/1987, when the abundance indices of all fish prey species were very low, but has since steadily in - creased. The annual variation in population growth rate after the population collapse could best be explained by the variation in abundance of 0-group cod (unlagged), and the 0-group cod and capelin 6 and 4 yr earlier, respectively (equalling the age of maturation of guillemots). We also present a numerical ocean model to identify mechanisms affecting spatio-temporal prey availabil- ity of 0-group cod around the colony during the breeding season. These results undermine earlier focus on the capelin stock as the main cause of the population crash in common guillemots.

Settling-depth vs. genotype and size vs. genotype correlations at the Pan I locus in 0-group Atlantic cod Gadus morhua

We sampled 0-group juvenile Atlantic cod Gadus morhua L. within fjords and off- shore in northern Norway from 1994 to 2008 using different gears for the 3 sampling depths: shore seine (0�3 m), pelagic trawl (various depths), and bottom trawl (>80 m). Frequencies of alleles at the Pan I locus (4218 fish analysed) showed highly significant differences among samples col- lected in the different habitats. The Pan I A allele showed a mean frequency of ~80% in the shore seine samples, 12% in the bottom trawl samples, and between 5 and 57% in the pelagic samples. These differences are thought to reflect the co-occurrence of different populations of cod in the area with different settling regimes. Shallow-water settlers are thought to represent the stationary Norwegian coastal cod (NCC), the deep-water settlers represent the migratory Arcto-Norwegian cod (ANC), and the non-settled individuals represent a variable mixture of the 2 populations. For designated samples, we analyzed 16 microsatellites (non-neutral and neutral) that supported a genetic divergence between shallow-water and deep-water settlers. Correlations between length and Pan I genotypes within selected samples showed that the Pan I BB homozygotes (typical of ANC) were significantly longer than the Pan I AA homozygotes (typical of NCC). This could reflect differences in spawning time and growth conditions between ANC and NCC, just as well as one genotype being superior to the others in terms of growth performance. Finally, we argue that the Pan I difference between 0-group NCC and ANC is not caused by contemporary selection but reflects adaptation on an ecological post-glacial time scale.

Thermal habitat for 0-group fish in the Barents Sea; how climate variability impacts their density, length, and geographic distribution

AbstractEriksen, E., Ingvaldsen, R., Stiansen, J. E., and Johansen, G. O. 2012. Thermal habitat for 0-group fish in the Barents Sea; how climate variability impacts their density, length, and geographic distribution. – ICES Journal of Marine Science, 69: 870–879. Between 1980 and 2008, the climate changed from cold to warm conditions in the Barents Sea. Warmer conditions have been indicated as favourable for cod, haddock, and herring recruitment and unfavourable for capelin, although during recent years these relationships have weakened and need to be revised. Extensive survey observations of temperature and 0-group cod, haddock, herring, and capelin are explored, a bounded temperature range occupied by ∼80–90% of 0-group fish (core thermal habitat, CTH) defined for each species, and an analysis made of how climate variability influenced density, length, and geographic distribution. Maximum fish densities were at 6.2°C for cod, 7.2°C for haddock, 7.0°C for herring, and 5.1°C for capelin. The area of CTH for cod, haddock, and herring increased from cold to warm years, as did the occupation area of each species. Capelin underwent a northeast distributional shift from cold to warm years. The 0-group fish were observed in a broad range of temperatures of −1 to 10.5°C. However, fish densities showed a dome-shaped distribution with temperature. Mean fish length was 7.1 cm for cod, 8.9 cm for haddock, 7.0 cm for herring, and 4.6 cm for capelin, with length distributions exhibiting a non-uniform response to temperature.

Ecological significance of 0-group fish in the Barents Sea ecosystem

Investigations into the 0-group fish in the Barents Sea have been carried out since 1965, with the goal of estimating the abundance of 0-group fish. 0-group abundance indices have been used in the assessment of the recruitment level and in recruitment variability studies. However, the ecological importance of the 0-group fish in the Barents Sea has been less studied. Although 0-group capelin, herring, cod and haddock are widely distributed in the Barents Sea, the central area seems to be the most important, accounting for approximately 50–80% of the annual biomass. The total biomass of the four most abundant 0-group fish species can be up to 3.3 million tonnes, with an average of 1.3 million tonnes (1993–2009). Wide distribution and high biomass of pelagically distributed 0-group fish make these fishes an important element in the energy transport between different trophic levels and different geographical areas, having a critical impact on the entire Barents Sea ecosystem. In recent years, capelin have shown a pronounced northward shift in biomass distribution, and several successive strong year classes occurred during warm temperature conditions. Cod biomasses were unexpectedly low during warm years and were positively correlated with spawning stock biomass, while the correlation with temperature was not significant. Haddock and herring show, as expected, increasing biomass with increased temperature when the spawning stock is at a sufficiently high level.

The use of otolith chemistry to determine the juvenile source of spawning cod in Icelandic waters

Abstract Thorisson, K., Jónsdóttir, I. G., Marteinsdottir, G., and Campana, S. E. 2011. The use of otolith chemistry to determine the juvenile source of spawning cod in Icelandic waters. – ICES Journal of Marine Science, 68: 98–106. Chemical fingerprinting was used to discriminate spatial groups of juvenile cod (Gadus morhua) and to backtrack spawning cod in Icelandic waters to their area of origin as 0-group juveniles. Juvenile 0-group cod were collected around Iceland in August 1996 and 1997 to establish the spatial distribution of otolith chemistry at the juvenile stage. Spawning cod from the same year classes were sampled in the same areas in April 2002 and April/May 2003. The core, corresponding to the juvenile otolith, was extracted from the adult otolith and its content of Ba, Mn, and Sr compared with the chemistry of whole otoliths of juveniles of the same year class. High Atlantic inflow into the shelf area north of Iceland in 1997 mixed the juveniles from different areas, and the spawners of that year class were not backtraceable to their origin. For the 1996 year class, however, mixed-stock analysis indicated that most of the spawning cod northwest, north, and northeast of Iceland originated from juveniles off the central part of the north coast. Using otolith chemistry to backtrace the origin of spawners appears well suited for areas with limited mixing, but less well suited for areas or years of high current velocity.

Distribution and diet of 0-group cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) in the Barents Sea in relation to food availability and temperature

Distribution of 0-group cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) in August–September 2005 and 2006 was mainly restricted to the Atlantic waters of the western and central areas of the Barents Sea. The main distribution of 0-group fish overlapped largely with areas of high biomass (>7 gm−2 dry weight) of zooplankton. The copepod Calanus finmarchicus and krill Thysanoessa inermis, which are dominant zooplankton species in both Atlantic and boreal waters of the Barents Sea, were the main prey of 0-group cod and haddock. The main distribution, feeding areas and prey of 0-group cod and haddock overlapped, implying that competition for food may occur between the two species. However, though their diet coincided to a certain degree, haddock seems to prefer smaller and less mobile prey, such as Limacina and appendicularians. As 0-group fish increased in size, there seems to be a shift in diet, from small copepods and towards larger prey such as krill and fish. Overall, a largely pelagic feeding behaviour of 0-group cod and haddock was evident from this study.

Effects of predation from pelagic 0-group cod (Gadus morhua) on mortality rates of capelin (Mallotus villosus) larvae in the Barents Sea

Surveys were carried out in 2002 and 2003 to test whether predation from pelagic 0-group cod (Gadus morhua) juveniles affects mortality rates of Barents Sea capelin (Mallotus villosus) larvae. In 2002, capelin larvae were observed in 17% of the cod stomachs and predation was observed at 19 of 50 stations. In 2003, capelin larvae were observed in 8% of the cod stomachs and predation was observed at 19 of 37 stations. The stomach contents and zooplankton samples were dominated by copepods and krill. The number of capelin larvae in cod stomachs increased with increasing capelin larvae abundance and cod length and decreased with increasing stomach content of copepods and prey abundance of krill. The time when capelin larvae could be recognised after ingestion in the cod stomachs was estimated experimentally and depended on predator and prey lengths. The estimated overall predation mortality rate from juvenile cod on capelin larvae was about 1.5% per day for both years and can potentially have a significant effect on the survival of capelin larvae, depending on the overlap in the different years.

Isolation of Atlantic cod (Gadus morhua) nursery areas

This study examines the degree of isolation among 0-group Atlantic cod in a region where a number of resident adult congregations have been found. The location of key nursery areas in Scottish coastal waters was identified from trawl surveys of settled 0-group cod around the Scottish coast. Segregation among four of these key areas; within the Clyde, Shetland Isles, Moray Firth and Buchan was investigated using otolith elemental composition. Isotope concentrations in the otoliths of cod were analysed by solution-based inductively coupled plasma mass spectrometry of the whole otolith. Discriminant analyses showed a clear separation between nursery areas based on the concentrations of three elements (manganese, barium and magnesium). The concentration of these elements was not affected by the age or size of individuals. Classification accuracy exceeded 90% for all four sites. Evidence of temporal persistence in the relative spatial differences in elemental concentrations between two sites for 2 year-classes was found. The results suggest limited exchange between sites during some period of the early life history. The significance of this limited exchange is discussed in relation to recent evidence for adult fidelity and the utility of closed areas for protecting nursery grounds.

Shift in fish assemblage structure due to loss of seagrass Zostera marina habitats in Sweden

The areal extent of Zostera marina in the archipelago of the Swedish Skagerrak has decreased by 60% over two decades. To investigate the effects of Z. marina loss on the local fish assemblages, the fish fauna was compared between existing seagrass beds and sites where seagrass had vanished. A field study was carried out at four shallow locations in the outer archipelago of the coast in June 2004. Within each location two sites were sampled, one with an existing Z. marina bed and another where Z. marina had disappeared. Fish were sampled semi-quantitatively with a beach seine. Samples were taken during both day and night and captured fish were examined to species, enumerated and measured in the field, and released thereafter. The number of fish species was found to be significantly higher in Z. marina habitats compared to areas where seagrass was missing, and density and biomass of fish were generally lower in areas dominated by bare sediment compared to those in the seagrass habitats. Several species and groups of fishes (i.e., gadoids, labrids, syngnathids) were absent or occurred in low densities at sites where Z. marina was missing. For example, juvenile 0-group cod density was reduced by 96% at sites where Z. marina had disappeared. Such a reduction in recruitment of cod is in the same order of magnitude as the combined effect of seal predation and mortality due to by-catches in the eel fyke-net fishery estimated for the archipelago of the Swedish Skagerrak. Hence, the results clearly indicate a shift in the fish assemblage, including a loss of taxa at the family level as a result of degradation in habitat-forming vegetation.

How can the stock recruitment relationship of the Barents Sea capelin (Mallotus villosus) be improved by incorporating biotic and abiotic factors?

We studied whether incorporation of terms for environmental factors (temperature, herring and 0-group cod) into the stock recruitment functions for the Barents Sea capelin (Mallotus villosus) would increase the ability to predict recruitment. We also investigated the effects of alternative estimates of the capelin spawning stock (SSB) and juvenile herring on model predictability. By using nonlinear regression, the modified Beverton–Holt and Ricker stock recruitment models were fitted to time series of capelin spawning stock, capelin recruitment, juvenile herring, 0-group cod and temperature from 1973 to 2000. The modified Beverton–Holt model with a term for juvenile herring fit capelin recruitment data well. Inclusion of terms for temperature and 0-group cod abundance did not significantly improve the model fit. The best model predicts an almost proportional relationship between the spawning stock and the capelin recruitment when the abundance of juvenile herring is high.

How can the stock recruitment relationship of the Barents Sea capelin (Mallotus villosus) be improved by incorporating biotic and abiotic factors?

We studied whether incorporation of terms for environmental factors (temperature, herring and 0-group cod) into the stock recruitment functions for the Barents Sea capelin (Mallotus villosus) would increase the ability to predict recruitment. We also investigated the effects of alternative estimates of the capelin spawning stock (SSB) and juvenile herring on model predictability. By using nonlinear regression, the modified Beverton–Holt and Ricker stock recruitment models were fitted to time series of capelin spawning stock, capelin recruitment, juvenile herring, 0-group cod and temperature from 1973 to 2000. The modified Beverton–Holt model with a term for juvenile herring fit capelin recruitment data well. Inclusion of terms for temperature and 0-group cod abundance did not significantly improve the model fit. The best model predicts an almost proportional relationship between the spawning stock and the capelin recruitment when the abundance of juvenile herring is high.

Indirect climatic forcing of the Barents Sea capelin: a cohort effect

Planktivorous capelin is a key species in the Barents Sea, being of great importance in the exploitation of plankton production in this subarctic region. However, in years with a successful reproduction of the Norwegian spring-spawning herring, large amounts of herring larvae drift into the Barents Sea, where they stay for 2 to 3 yr. The 1 to 2 yr old herring has a pronounced impact on capelin, eating large amounts of capelin larvae. The main fish predator of the Barents Sea, the Arcto- Norwegian cod, also consumes large amounts of post-larval capelin. In this study, we show how tem- perature and the North Atlantic Oscillation (NAO) indirectly influence the population dynamics of capelin by influencing the reproduction of herring and cod. After 1980, when the herring spawning stock had recovered after its collapse in 1969, we found that temperature strongly negatively influ- ences capelin cohorts 2 yr before spawning. Capelin cohorts which have spawned 2 yr after a warm year tend to experience high predation from both young herring as larvae and from 3 to 6 yr old cod during the rest of their life. Other analyses confirm that present sea temperatures or previous NAO conditions have strong positive effects on the abundance of 0-group cod and herring. Thus, the cli- matic regime of the region ultimately determines the balance between capelin and herring, which in turn has pronounced consequences for the species composition and energy flow of the entire ecosys- tem.

Modelling the influences of atmospheric forcing conditions on Baltic cod early life stages: distribution and drift

Retention or dispersion of larvae from the spawning ground has been identified as one of the key processes influencing recruitment success in fish stocks. An exercise combining 3-D hydrodynamic model simulations and field data on spatial distributions of juvenile Baltic cod was utilised to investigate the potential drift of larvae from the centre of main spawning effort in the Bornholm Basin, Baltic Sea. In the simulations cod larvae were represented as Lagrangian drifters. Habitats in which larvae and juvenile cod potentially dwell and where juveniles settle were identified to ascertain the importance of predicting transport. The transport of Baltic cod larvae was investigated by detailed drift model simulations for the years 1986 to 1999. The results yielded a clear dependency on wind-induced drift of larval cod, which is mainly controlled by the local atmospheric conditions over the Baltic Sea. Seasonally averaged distributions of drifters were compared with actual distributions of 0-group cod, as determined from bottom and pelagic trawl surveys conducted in autumn of the years 1993 to 2000 in and around the Bornholm Basin. The results suggest that juveniles caught in different areas can be assigned to different times of the spawning season. Because of seasonal differences in the circulation patterns, the southern coastal environment is on average most important for early and late spawners, whereas larvae hatching in mid-summer were on average transported towards the north or to a higher degree remained in the spawning ground.

Developing Baltic cod recruitment models. II. Incorporation of environmental variability and species interaction

We investigate whether a process-oriented approach based on the results of field, laboratory, and modelling studies can be used to develop a stock–environment–recruitment model for Central Baltic cod (Gadus morhua). Based on exploratory statistical analysis, significant variables influencing survival of early life stages and varying systematically among spawning sites were incorporated into stock–recruitment models, first for major cod spawning sites and then combined for the entire Central Baltic. Variables identified included potential egg production by the spawning stock, abiotic conditions affecting survival of eggs, predation by clupeids on eggs, larval transport, and cannibalism. Results showed that recruitment in the most important spawning area, the Bornholm Basin, during 1976–1995 was related to egg production; however, other factors affecting survival of the eggs (oxygen conditions, predation) were also significant and when incorporated explained 69% of the variation in 0-group recruitment. In other spawning areas, variable hydrographic conditions did not allow for regular successful egg development. Hence, relatively simple models proved sufficient to predict recruitment of 0-group cod in these areas, suggesting that key biotic and abiotic processes can be successfully incorporated into recruitment models.