Inverse relationship between cod recruitment in the North Sea and young fish in the continuous plankton recorder survey

Abstract

There is a significant (p<0.01) negative correlation between cod recruitment (age 1 year) and the numbers of young fish taken in the Continuous Plankton Recorder (CPR) Survey in the North Sea. This correlation is analysed with reference to hydrographic, climatic and biological variables in order to determine whether it results from opposite responses to physical forcing, trophic relationships or a combination of these. The results of correlation analyses are consistent with cod recruitment being determined mainly during the planktonic phase of development in the preceding year. Multiple regressions indicate that a combination of the North Atlantic Oscillation index in the preceding year and the abundance of CPR young fish is the pair of variables that provide the best fit to the data. The abundance of CPR young fish is best predicted by the Gulf Stream North Wall Index and the abundance of phytoplankton in the spring of the previous year. Opposite responses to climatic forcing and the regional environmental consequences are identified as of primary importance but interactions, possibly both predation and competition, occur between the planktonic stages of cod and both the planktonic stages of fish and the stocks from which they were derived.