Modelling spatio-temporal effects of environment on Atlantic herring, Clupea harengus

Trends in mean abundance of North Sea Atlantic herring, Clupea harengus, ov er the period of 1992-1995, were modelled as a function of spatial location and ocean environmental conditions using generalized additive models (GAM) . In all four years, the average herring abundance was found to be highest in latitudes around 60.5 degrees N, and decreased with increasing latitude. The thermocline depth had a significant effect on prespawning herring abun dance both directly, as a main effect, and indirectly, through its interact ive effect with the temperature at 60 m. Average herring abundance was high est in areas having deeper thermocline depths (up to 45 m) and temperatures at 60 m between 9 and 11 degrees C. Prespawning herring abundance was grea ter in areas of cooler surface waters in the south than in the north. Well- mixed waters and transition zones between frontal and stratified areas havi ng sea surface temperatures mainly between 11 and 12 degrees C and to a les ser extent between 13 and 14 degrees C were associated with the highest her ring abundance. Herring appeared to avoid the cold bottom waters in summer. Multiyear GAM analysis revealed consistent environmental preferenda of her ring and affirmed further a significant decrease in herring abundance. As h erring numbers declined, the population aggregated in the most preferred ar eas. The inter-relationships of herring and environmental factors across th e study period, were similar in their structure and significance, suggestin g that preferred areas for location of herring can be reasonably predicted.