Sampling trips for measuring discards in commercial fishing based on multilevel modelling of measurements for the North Sea from NE England

This study estimates the effects of variables (fishing, spatial, temporal) on discarding and proposes a scheme for measuring discarding. Data are used from fishing in the English North Sea for cod, haddock and whiting. Analysis of variance of discarding rates (numbers discarded/numbers caught) has provided estimates of components of variance associated with variables. Significant estimates of components of variance have emerged from data spanning 3 years. There is a large component of variance associated with trips (the lowest level at which data are available) reflecting an inherent noisiness in discarding rates. For haddock and whiting, there are significant components of variance associated with years. An effect of years on cod was not found for the years for which data are available. The variables: gear type, port of landing, fishing grounds and quarter, are crossed at a common level. Of these gear contributes the greatest variation in discarding rates. Port and grounds are also important contributors. Quarter has a small but significant effect. Multivariate analysis of variance has produced evidence for strong multilevel correlations in discarding rates between haddock and whiting. Discarding rates vary significantly with size of catch (trip) for cod and for haddock.

Variables are reduced to strata, and analyses of variance for models incorporating strata carried out. An approach to sampling based on stratification with proportional allocation is discussed. This provides efficient sampling and ensures representative sampling in a problem for which there are relatively few observations, and several variables affecting the response variable.