SPATIAL-DISTRIBUTION OF HERRING IN THE ORKNEY SHETLAND AREA (NORTHERNNORTH-SEA) - A GEOSTATISTICAL ANALYSIS

Geostatistical methodology was used to analyse the structure and descr ibe the spatial patterns of North Sea herring (Clupea harengus L.), us ing data from the 1992 ICES (Division IVa) acoustic survey. Three diff erent scales of spatial structures were identified: an unresolved smal l-scale variability, which accounted for 48% of the total variance, an d two structure components, the first being a meso-scale of similar to 9 nmi (nautical miles) (similar to 30%) and the second a large-scale of similar to 17 nmi (22%). Geostatistical analysis permitted the dete rmination of spatial density gradients as well as patch sizes (range f rom 9 to 17 nmi). The use of the truncated data and the robust variogr am on the raw data provided additional information for the structure. The utilization of this information in the variographic analysis resul ted in better estimations. The best unbiased predictor was used to obj ectively map the herring population distribution by kriging. The krigi ng estimates were better with interpolation of a large number of point s. Herring tended to aggregate mainly in mese-scale patches with a dia meter of 9 nmi and to a lesser extent in large-scale patches of 17 nmi diameter, Environmental factors (depth, salinity and temperature) par tly explained the spatial distribution of herring, despite the absence of a trend in the variogram structure. The study demonstrates the exi stence of spatial correlation and an objective way of optimal mapping of the population. Geostatistics provided additional information on he rring spatial organization which is important to understand the behavi our of the species and to study its relationship with the environment.