Subsampling trawl catches from vessels using seawater hoppers: are catch composition estimates biased?

摘要

Subsampling is a commonly used method for estimating the composition of trawl catches. In many fisheries, an increasing number of trawlers use recirculating seawater hoppers to hold catches before extraction by a sorting conveyor belt from which the target species and byproduct are sorted from the bycatch. We tested whether taking subsamples from the sorting conveyor belt at different times during catch-sorting, biased estimates of bycatch composition in Australia's Northern Prawn Fishery. We divided the entire bycatch from three trawls into subsamples (about 10 kg each) numbered chronologically as they were taken from the sorting conveyor belt. Within each catch, we grouped the subsamples into three catch groups: the first, the middle and the last third of the catch (as they were extracted from the seawater hopper). We compared the catch composition estimates from these three catch groups, separately for each catch.