Sources of variation in the age composition of sandeel landings

The variation of the age composition of the landings of lesser sandeel in t he Danish industrial fishery in the North Sea over the period From 1984-199 3 is analysed by continuation-ratio logits and generalised linear models. T he analysis takes the multinomial characteristics of the age composition in to account and allows both fixed and random effects to be quantified. It al so allows an estimate of over-dispersion, i.e. the extent to which the vari ance exceeds the multinomial variance expected if the samples had been draw n at random from each erratum. The results show that the proportion of 2+ s andeel in the samples is significantly lower in the start and end of the fi shing season. This suggests that the older sandeel are available to the fis hery for a shorter time period that the 1-group. Significant differences ar e found in the age composition between the four laboratories involved in th e age determination. Although the variation between ICES statistical rectan gles is substantial there is a significant difference between the age compo sition in the northern and southern part of the North Sea. However, only on e of the three finer geographical stratifications proposed to improve the a ssessment results in significant differences in age composition between sub -strata. Additional unidentified sources of variation result in a large and significant over-dispersion. (C) 2001 International Council for the Explor ation of thr Sea.