Effect of tow duration, door spread, and swept area on the catch efficiency of a bottom trawl

Trawl surveys are key to estimating changes in fish abundance, distribution and biodiversity. When tow by tow sampling effort in a survey has not been constant, catch rates are often standardized by swept area or haul duration. The nominal tow duration in the North Sea International Bottom Trawl Survey (NS-IBTS) has varied substantially, providing opportunity to test whether the underlying assumption of proportionality between catch and effort is confirmed by data. Here the effects of various effort metrics on indices of biomass are evaluated and compared for the three most common orders of species (gadiformes, pleuronectiformes, and clupeiformes) and two size groups in the North Sea using the NS-IBTS data. Tow duration is found to be a better effort metric than swept area in all cases. Catch is almost proportional to tow duration for gadiformes and pleuronectiformes, although shorter tows have higher catch rates, but for clupeiformes tow duration has no or little effect on expected catch. The relationship between catch and door spread is negatively correlated for clupeiformes, while only weak effects of door spread are found for the other species. Our findings have significant effects on standardized survey indices from NS-IBTS data due to the systematic change in tow-by-tow effort. Similar biases potentially occurs in all surveys where effort varies between hauls.