Comparing commercial and research survey catch per unit of effort: megrim in the Celtic Sea

Correlation between maps of research Survey and commercial catch data was thought to improve the spatial analysis of fishing mortality and catchability Maps were produced at the spatial scale of the ICES statistical rectangle and commercial cpue (catch per unit of effort) were linearly regressed on research survey cpue, The analysis was performed for two seasons (spring and autumn) over a time interval corresponding to the survey dates. The analysis focused on megrim in the Celtic Sea because of its large spatial extension over the area covered by both research surveys and commercial fishing. Simple mean and variance of commercial cpue were computed using those boats which reported their catch per ICES rectangle in their logbooks and which operated in a period close to the survey dates. Average research survey cpue in the ICES rectangles were estimated by block kriging after modelling the variogram of the survey data. The variance of commercial cpue within rectangles was evidenced to be an important parameter when relating research survey cpue to commercial cpue, It showed strong spatial and seasonal patterns. It was related with fishing strategy in autumn and to a biological cause in spring. Different uses of the variance in the commercial cpue were discussed, in particular for planning research surveys. Selecting those rectangles with low commercial variance allowed fitting a linear regression between commercial and scientific cpue, Differences in the slope was small in autumn for the two years studied but important between autumn and spring. (C) 2003 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.