A synthesis of large scale patterns in the planktonic prey of larval and juvenile cod

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Data from 39 published studies of the diet composition of larval and juvenile cod from around the northern North Atlantic were summarised to assess generic patterns in ontogenetic and regional variability in the key prey. The results showed that larvae at the northern edge of the latitudinal range of cod depend primarily on development stages of the copepod Calanus finmarchicus, whilst those at the southern edge depend of Para- and Pseudocalanus species. Juvenile cod preyed on a wider range of taxa than larvae, but euphausids were the main target prey. Analysis of regional variations in the relative abundances of C. finmarchicus and Para/Pseudocalanus spp. in the plankton, as estimated by the Continuous Plankton Recorder (CPR) surveys, showed a similar geographical pattern to the larval cod stomach contents. Comparison of CPR data from the 1960’s and 70’s with data from the 1990’s showed that the boundary between C. finmarchicus and Para/Pseudocalanus spp. dominance has shifted northwards, especially in the northeastern Atlantic, whilst the abundance of euphausids in almost all the cod stock regions has declined. The results are discussed in relation to regional differences in the response of cod stocks to climate variability.