North Sea herring (Clupea harengus, L.) distribution in relation to environment : analysis of acoustic survey data (1992-95)
This thesis examines the spatial distribution of prespawning herring (Clupea harengus, L.) in the northern North Sea and its relationship with various environmental factors. Data were collected from 1992 to 1995 during the ICES coordinated herring acoustic surveys. The spatial distribution and organization of the species was studied by means of geostatistics. A significant interannual variability was observed in the spatial distribution and the horizontal dimensions of the herring structures. It was found that herring had narrowed their spatial occupation and had been progressively confined to distinct areas. These areas were stable throughout the four years, had the highest herring abundances throughout the studied period and appeared to be vacated last as the population declines. These were around the Shetland and Orkney islands and in the south part of the studied area. The confinement of the herring population in a smaller region of the surveyed area was believed to be related to the decreasing stock. Herring tended to aggregate mainly in mesoscale clusters of dense schools having a diameter of 79n. miles and occasionally in longer range spatial patterns associated with, or corresponding to, oceanographic features. The number and dimensions of the structures that the population was aggregated in, had gradually decreased. A method for a robust analysis and optimal mapping of the highly aggregated herring population was introduced by combining the robust estimator of the variogram with the cross-validation technique. The method effectively handled the destabilizing effect that the few high value observations had and revealed the underlying spatial dependence (larger scale structural component) by defining better the correlation ranges of herring.