The reappearance of spawning Atlantic herring (Clupea harengus) on Aberdeen Bank (North Sea) in 1983 and its relationship to environmental conditions

Evidence is presented in support of the hypothesis that the reappearance of spawning Atlantic herring (Clupea harengus) on Aberdeen Bank in 1983, afte r an absence of 16 years, was related to an increased Atlantic inflow in th e area. Two Atlantic copepod species, Metridia lucens and Candacia armata, showed a simultaneous increase in the years when spawning herring returned to Aberdeen Bank. During the late 1960s, both species declined at the time when the spawning population on Aberdeen Bank disappeared. Earlier work has demonstrated that an increased Atlantic inflow results in a southward disp lacement of plankton concentrations and feeding herring in the northwestern North Sea. I hypothesize that such a southern distribution of the herring stock, caused indirectly by the increased Atlantic inflow, influenced recru it spawners to choose the nearby Aberdeen Bank as their spawning ground in 1983. Fluctuations of the spawning populations in the northern North Sea du ring earlier decades are explained by switches of recruitment between the n orthern and southern population, as a result of variations in latitudinal d istribution of the recruiting year-class.