Habitat associations of Atlantic herring in the Shetland area: influence of spatial scale and geographic segmentation

This study considers the habitat associations of a pelagic species with a r ange of biotic and abiotic factors at three different spatial scales. Gener alized additive models (GAM) are used to analyse trends in the distribution al abundance of Atlantic herring (Clupea harengus) in relation to thermocli ne and water depth, seabed roughness and hardness, sea surface salinity and temperature, zooplankton abundance and spatial location. Two geographical segments of the population, those east and west of the Shetland Islands (no rthern North Sea, ICES Div IVa), are examined. The differences in the ecolo gical preferences of the species in these two distinct geographical areas a re elucidated and the degree that these environmental relationships might b e modulated by the change of support of the data is also considered. Part o f the observed variability of the pre,spawning distribution of herring was explained by different parameters in these two regions. Notwithstanding thi s, key determinants of the species' spatial aggregation in both areas were zooplankton abundance and the nature of the seabed substrate. The relative importance of the variables examined did not change significantly at differ ent spatial scales of the observation window. The diverse significance of v arious environmental factors on herring distribution was attributed mainly to the interaction of species' dynamics with the different characteristics of the ecosystem, east and west of the Shetland Islands. Results suggest th at the current 2.5 nautical miles as elementary sampling distance unit (ESD U) is a reasonable sampling scheme that combines the need to reduce the dat a volume while maintaining spatial resolution to distinguish the species/en vironment relationships.