Estimating size selectivity of Calanus spp. in plankton trawls: effect of mesh size for commercial fishery

Abstract

<i>Calanus</i> spp. are marine zooplankton that are considered as a potential source for aquaculture feed, due to their high abundance in the North Atlantic. The diminutive size (< 5 mm) of these zooplankton necessitates the use of fine-meshed trawls to harvest them efficiently. However, the use of fine-meshed trawls leads to high drag and thus energy demand to tow the trawl. Therefore, it is of importance to know the effect of mesh size on the capture efficiency of <i>Calanus</i>, and specifically to quantify which sizes of those that can be harvested effectively depending on mesh size in the trawl. Therefore, experimental trawl fishing targeting <i>Calanus</i> was conducted testing gears differing in mesh size to estimate their size selectivity. We used an image and artificial intelligence-based specially built and trained acquisition method to rapidly measure the size of large numbers of <i>Calanus</i> to prepare the catch data for size selectivity analysis. The results demonstrate a clear size selection process of <i>Calanus</i> in nets with mesh sizes considered for this fishery. Specifically, the size selectivity would result in a loss of 10%, 45% and 99% of <i>Calanus</i> if the trawl has a mesh size of 500, 750 and 1000 μm, respectively.