Norwegian coastal (NC) and northeast Arctic (NA) Atlantic cod (Gadus morhua) larvae were reared on live zooplankton to investigate temperature- and size-specific growth. Larval and juvenile growth was temperature and size dependent. Growth in length and weight increased with increasing temperature from 4 to 14°C, with a corresponding reduced larval stage duration. Maximum growth rate occurred at a larval size of 0.1-1.0 mg dry weight, followed by a declining trend during the juvenile stage. The temperature optimum of larval cod fed in excess is estimated to be between 14 and 16°C, with a maximum weight-specific growth potential exceeding 25%·day-1. Temperature- and stock-specific growth curves of dry weight at age are well described by a generalized Gompertz model. A stock-specific difference in mean weight at age was observed, with NC growing better than NA. Neither countergradient latitudinal variation in growth capacity of the two larval cod stocks nor temperature adaptation across latitudes was indicated. A stock-specific difference in weight at length was observed in early juveniles, with NC being heavier than NA. Overall, a positive correlation between temperature and condition level was found. No distinct temperature- or stock-specific differences in survival were observed.

Ecological adaptation is of major relevance to speciation and sustainable population management, but the underlying genetic factors are typically hard to study in natural populations due to genetic differentiation caused by natural selection being confounded with genetic drift in subdivided populations. Here, we use whole genome population sequencing of Atlantic and Baltic herring to reveal the underlying genetic architecture at an unprecedented detailed resolution for both adaptation to a new niche environment and timing of reproduction. We identify almost 500 independent loci associated with a recent niche expansion from marine (Atlantic Ocean) to brackish waters (Baltic Sea), and more than 100 independent loci showing genetic differentiation between spring- and autumn-spawning populations irrespective of geographic origin. Our results show that both coding and non-coding changes contribute to adaptation. Haplotype blocks, often spanning multiple genes and maintained by selection, are associated with genetic differentiation.DOI: http://dx.doi.org/10.7554/eLife.12081.001

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Atlantic herring is widespread in North Atlantic and adjacent waters and is one of the most abundant vertebrates on earth. This species is well suited to explore genetic adaptation due to minute genetic differentiation at selectively neutral loci. Here we report hundreds of loci underlying ecological adaptation to different geographic areas and spawning conditions. Four of these represent megabase inversions confirmed by long read sequencing. The genetic architecture underlying ecological adaptation in herring deviates from expectation under a classical infinitesimal model for complex traits because of large shifts in allele frequencies at hundreds of loci under selection.

2004. Temperature-dependent fractionation of stable oxygen isotopes in otoliths of juvenile cod (Gadus morhua L.). e ICES Journal of Marine Science, 61: 243e251.Analysis of stable oxygen isotopes in otoliths is a promising technique for estimating the ambient temperature experienced by fish, but consistent equations relating temperature and fractionation of stable oxygen isotopes in otoliths among different fish species are lacking. Juvenile cod were reared at constant temperatures from 6 to 20(C and the sagittal otoliths were analysed for oxygen isotope values. We determined that temperature-dependent fractionation of oxygen isotopes in the otoliths was close to that reported for inorganic aragonite at low temperatures, but there were deviations from oxygen isotope fractionation equations for otoliths of other species. The linear relationship between oxygen isotope value in the cod otoliths and temperature was determined to be: 1000 Ln a ¼ 16:75ð10 3 TK ÿ1 Þ ÿ 27:09. Temperature estimates with 1(C precision at the 95% probability level require a sample size of R5 otoliths. Only an insignificant amount of the variance in the data was due to variance between left and right otolith, and due to repeated measurements of otolith subsamples. This study confirms that stable isotope values of cod otoliths can give precise and accurate estimates of the ambient temperature experienced by fish.

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This study presents the first intraspecific evaluation of larval growth performance across several different experimental scales, environments, and regions of a marine fish species. Size- and temperature-dependent growth models for larval and early juvenile Atlantic cod (Gadus morhua) are developed based on selected laboratory experiments with cod fed in excess. Observed sizes-at-age of cod from several experiments and stocks are compared with predictions from the models using initial size and ambient temperature history as inputs. Comparisons with results from other laboratory experiments reveal that the model predictions represent relatively high growth rates. Results from enclosure experiments under controlled seminatural conditions generally provide growth rates similar to those predicted from the models. The models therefore produce suitable reference growth predictions against which field-based growth estimates can be compared. These comparisons suggest that surviving cod larvae in the sea typically grow at rates close to their size- and temperature-dependent capacity. This suggests that climatic influences will strongly affect the year-to-year variations in growth of cod during their early life history owing to their markedly temperature-dependent growth potential.

Growth rate of tagged juvenile turbot was significantly influenced by the interaction of temperature and fish size. The results suggest the optimum temperature for growth of juvenile turbot in the size range 25-75 g is between 16 and 19 C. Optimal temperature for growth decreased rapidly with increasing size, and is between 13 and 16 C for 100 g turbot. Although individual growth rates varied highly at all times within the temperature treatments, significant size rank correlations were maintained during the experimental period. The study confirms that turbot exhibit ontogenetic variation in temperature optimum, which might partly explain different spatial distribution of juvenile and adult turbot in ocean waters. 1996 The Fisheries Society of the British Isles