The variation of the age composition of the landings of lesser sandeel in the Danish industrial fishery in the North Sea over the period From 1984-1993 is analysed by continuation-ratio logits and generalised linear models. The analysis takes the multinomial characteristics of the age composition into account and allows both fixed and random effects to be quantified. It also allows an estimate of over-dispersion, i.e. the extent to which the variance exceeds the multinomial variance expected if the samples had been drawn at random from each erratum. The results show that the proportion of 2+ sandeel in the samples is significantly lower in the start and end of the fishing season. This suggests that the older sandeel are available to the fishery for a shorter time period that the 1-group. Significant differences are found in the age composition between the four laboratories involved in the age determination. Although the variation between ICES statistical rectangles is substantial there is a significant difference between the age composition in the northern and southern part of the North Sea. However, only one of the three finer geographical stratifications proposed to improve the assessment 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 Exploration of thr Sea.
- Age composition
- Continuation-ratio logits
- Generalised linear mixed models