Abstract
Abstract A geostatistical analysis of age 2 North Sea haddock catches from the 1st quarter IBTS (International Bottom Trawl Survey) 1983–1997 is presented. IBTS standard abundance indices are routinely calculated in a way that does not account explicitly for the spatial distribution and night hauls are included in the estimation without any correction for possible daylight effects. In the present study, ordinary kriging was used to correct for sampling irregularities and external drift kriging with a day/night indicator or a cosine function of time of day was applied to account additionally for diurnal differences in the catch rates. Only minor differences between the standard indices and the abundance estimates obtained by ordinary kriging were found. In contrast, the external drift kriging, particularly with time of day, yielded higher estimates of mean abundance for all years with the differences to ordinary kriging being most pronounced for years characterized by a high portion of night hauls and a low mean catch rate at night. This demonstrates that external drift kriging with a day/night indicator but preferably with time of day is capable of compensating successfully for daylight effects and provides a valuable tool for the calculation of survey-based abundance indices.
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