Drivers of growth for Atlantic cod (Gadus morhua L.) in Icelandic waters – A Bayesian approach to determine spatiotemporal variation and its causes

This study assesses spatiotemporal and sex-specific growth of Atlantic cod Gadus morhua in Icelandic waters. We use a Bayesian approach which lends itself to fitting and comparing nested models such as these. We then compare fitted parameters of these models to potential explanatory variables using a redundancy analysis (RDA) to look for drivers of growth in G. morhua. Results indicate that models that incorporate differences in growth among time, space and sex are the best-fitting models according to deviance information criterion (DIC). Results from RDA indicate that capelin Mallotus villosus recruitment and biomass is highly correlated with deviations in the von Bertalannfy growth parameter k and that L_∞ is correlated with G. morhua landings in the model that uses year to account for time-varying growth and estimated G. morhua recruitment in the model that uses cohort to account for time-varying growth.     

Lethal oxygen levels at different temperatures and the preferred temperature during hypoxia of the Atlantic cod, Gadus morhua L.

The behavioural thermoregulation of Atlantic cod Gadus morhua L. was investigated in a shuttlebox at normoxia and at three levels of hypoxia: 30, 20 and 15% oxygen saturation.
The preferred temperatures at normoxia, 30, 20 and 15% oxygen saturation were 13·9, 13·1. 10·0 and 8·8° C, respectively.
A decrease in metabolism and an increased blood oxygen affinity are among the physiological advantages of selecting a lower temperature during hypoxia. Furthermore the chances of surviving low oxygen saturations are better at low temperatures.
In natural environments, this behaviour may result in habitat shifts of fish living in heterothermal environments with changing oxygen saturations, especially in coastal areas with eutrophication, as for example the Baltic Sea.     

Differences in growth performance between Arcto-Norwegian and Norwegian coastal cod reared under identical conditions

Offspring from seven family groups of Arcto-Norwegian cod (AN) and a genetically marked Norwegian coastal cod (NC) broodstock, were mixed at metamorphosis and raised in the same rearing unit. The fish were transferred subsequently to a net-pen and held under standard farming conditions. In December 1992, 466 cod juveniles were measured, weighed, and tagged individually. Length and weight changes were monitored until the fish matured (January 1994). Genotyping of each individual was performed using enzyme electrophoresis to identify the fish to strain. Prespawning females were examined for organ weights and stage of maturity. There were population specific Differences in growth performance. NC displayed significantly higher specific growth rate (SGR) and daily length increment (DLI) during spring/summer season. The AN had significantly lower hepatosomatic and gonadosomatic indices, and were thinner than the NC, indicating Differences in body form and energy allocation pattern between the two strains. All NC (both sexes) became sexually mature at the age of 2 years while 2% of fish in the AN group were still immature at the end of the experiment.     

The feeding, growth and behaviour of juvenile cod, Gadus morhua L., in cold environments

Most studies on feeding-related activities of fish in cold water (0° C) have not used eurythermal species or examined a broad suite of activities. In this study we report on the feeding, growth, behaviour and conversion-efficiency of 0 + Atlantic cod in response to water temperature and food availability. In one experiment, activity, opercular beat and growth decreased with decreasing temperature over the range 8·3–0·6° C. However, the conversion efficiency increased with decreasing temperature, a result partly explained by the relative change in activity and opercular rate. In another experiment, temperature had a linear effect on growth in cod fed different food rations. Both food availability and temperature were found to influence growth in group 0 + cod. The specific growth rate of cod in this experiment exceeded 2% day^-1. Growth data collected from cod sampled in the field during the cold-water period showed a progressive increase of mean length over the season. Overall, these results demonstrate that 0 + cod have adapted to life in cold-water environments, in contrast to adult cod which mostly inhabit warmer environments.     

Microsatellite variation in cod Gadus morhua throughout its geographic range

Previous genetic studies using neutral markers such as allozymes, mtDNA and minisatellite loci have demonstrated varying amounts of population structure in cod Gadus morhua throughout the Atlantic. Microsatellite loci, which are potentially the most informative of presently available neutral genetic markers, have been applied extensively within western and eastern Atlantic areas but not on a range-wide basis. In the present study, six microsatellite DNA loci were used to screen cod samples from nine locations throughout the geographic range from the Scotian Shelf in the West Atlantic to the Barents and Baltic Seas in the east. Overall F _ST value was 0·03 ( P = < 0·001) across all samples. Statistically significant population differences over all loci combined were evident between more geographically distant samples, using either heterogeneity tests or F _ST analysis, with at least one locus showing significant differences between all samples (prior to Bonferroni correction). A significant correlation was observed between genetic and geographical distance, suggesting a higher level of historical and contemporary gene flow between adjacent populations than more distant populations. Samples from either end of the geographic range (Scotian Shelf and Baltic Sea) were particularly distinct when analysed using the STRUCTURE programme and also showed a high level of self-assignment when individuals of either the Scotian Shelf or Baltic Sea were tested against the entire data set. The present microsatellite study demonstrates a high level of geographic population structure between the western Atlantic, middle and eastern Atlantic and Baltic Sea, and thus, the findings should be useful in devising overall management and conservation strategies for the species.     

The demography of a haemoglobin polymorphism in the Atlantic cod, Gadus morhua L.

Genetic stability was investigated at a polymorphic haemoglobin gene-locus in 13 124 Atlantic cod, Gadus morhua L. A total of 30 year-classes, sampled over 18 years, were variously sampled in 33 named localities. Regional populations showed distinctive allele frequencies, and each major cod fishery showed some degree of genetic imbalance, which was expressed as excessive numbers of homozygotes. This imbalance occurred seasonally in a proportion of the population samples in each of the major cod fisheries. The findings are attributable to migrations of distinctive genetic populations.     

Gene flow and lack of population differentiation in Atlantic cod, Gadus morhua L., from Iceland, and comparison of cod from Norway and Newfoundland

Restriction analysis of mitochondrial DNA was used to study genetic variation and geographic population structure of Atlantic cod from localities around Iceland. Gene phylogenies were constructed and geographic locations superimposed on these. The variation was not localized. Estimated gene flow was large. Thus, Atlantic cod in Iceland belong to a single genetic population. Analyses of published sequence variation of cod from Norway and Newfoundland showed that the extensive continuity of intraspecific phylogeny of cod in Iceland, which is very similar to other marine organisms with a similar life history, extends from Norway to Newfoundland and possibly to larger areas of the Atlantic.     

A nuclear DNA basis for shelf- and bank-scale population structure in northwest Atlantic cod (Gadus morhua): Labrador to Georges Bank

Variation at five microsatellite DNA loci scored in ≈ 1300 individuals provided evidence of genetic structure among 14 cod ( Gadus morhua ) populations spanning the range of the species in the northwest (NW) Atlantic. Using D _A and D _SW measures of genetic distance, as well as F _ST and R _ST measures of population structure, differences are revealed among populations at continental shelf scales (NE Newfoundland Shelf, Grand Banks, Flemish Cap, Scotian Shelf, Georges Bank) where regions are separated by submarine saddles, channels and trenches. However, we also provide evidence of genetic structure at spawning-bank scales consistent with variation in oceanographic features and in the spatiotemporal distribution of spawning, each of which may represent barriers to gene flow among geographically contiguous populations inhabiting a highly advective environment. The differences described are consistent with postdispersal spawning fidelity to natal areas, a behaviour that may be facilitated by topographically induced gyre-like circulations that can act as retention mechanisms. Significant degrees of substructure among neighbouring and contiguous cod populations may be most easily explained by the associated oceanographic features and processes that conceivably form the template for the evolution of the structure. We suggest that bathymetric and hydrodynamic structure represents a rational starting point for developing hypotheses to examine the processes that lead to the genetic structuring of marine fish species.     

A SNP/microsatellite genetic linkage map of the Atlantic cod (Gadus morhua)

A first genetic linkage map of the Atlantic cod ( Gadus morhua ) was produced, based on segregation data from 12 full-sib families of Norwegian origin. The map contained 174 single nucleotide polymorphism markers and 33 microsatellites, distributed on 25 linkage groups and had a length of 1225 cM. A significant difference in recombination rates between sexes was found, the average ratio of female:male recombination rates being 1.78 ± 1.62 (SD).     

A reappraisal of activity metabolism in Atlantic cod (Gadus morhua)

Atlantic cod ( Gadus morhua ) were forced to swim in a swim tunnel respirometer until fatigued; oxygen consumption rate (O₂) was measured during swimming at incremental speeds until the fish was exhausted and during recovery from exhaustion. Maximal oxygen consumption (O_2max) occurred during maximal activity as has been found for other fish species, but at odds with the current paradigm for Atlantic cod. Earlier experiments had drawn the conclusion that O_2max in Atlantic cod occurs during recovery from exhaustive exercise. We found no support for this paradigm in our experiments and we propose that the respiratory physiology of Atlantic cod is not unlike that of other fishes.