Latitudinal variation in fish recruits in Northwest Europe

In many fish species, ambient temperature is known to govern larval duration and, therefore, interannual variation in temperature can act to induce variation in recruit numbers. In general, interannual variation in water temperature increases with latitude. It may be expected, therefore, that variation in fish recruit numbers will increase with latitude along the coastlines of Northwest Europe. Variation in stock and subsequently in egg numbers is also believed to increase from the geographical centre of a speciesʹ range of distribution towards the borders. It has been hypothesized that the combination of these two sources of variation will yield low variation in recruit numbers near the geographical centre, high variation at the northern edge and intermediate variation near the southern edge of the distribution range. ted the predicted patterns in recruit variation by examining the relationships between latitude, temperature and the variation in recruit numbers in time series of four flatfish (dab, flounder, plaice and sole) and four roundfish species (cod, bib, poor cod and whiting). Data on interannual variation in recruit numbers were obtained for sampling sites ranging from Hinkley Point and the English Channel in south England to the continental coast of the southern North Sea, the Marsdiep tidal inlet, and the Skagerrak in the north. The data were analysed for: (1) correlations between interannual variation and latitude; (2) interannual variation in recruits in relation to distance of the sampling site from the geographical centre of the species distribution; and (3) variation in interannual recruit numbers in response to the combination of these two sources of variation. terns of variation observed did not correspond with the expectations. In plaice and poor cod the sign of the correlation between recruit variation and latitude was opposite to the prediction. All other species examined showed no significant variation in recruit numbers in relation to the variables considered. We neither accept nor reject the hypothesis, however, because the expected relationships between the coefficient of variation in recruits (CVR) and latitude may have been overruled by other sources of variation such as actual water temperatures, sampling methods, average age of the catch, inshore-offshore gradients and possibly northward shifts of species ranges.