Codend selectivity in the East China Sea of a trawl net with the legal minimum mesh size

Selectivity curves were obtained for 22 species of fish and squid from stock assessment research data for the East China Sea between 2001 and 2011, conducted using a cover net attached to the codend of a trawl net (Seikai National Fisheries Research Institute SS-RI-type trawl net). The trawl net codend was made of diamond mesh net with a legal minimum mesh opening size of 54 mm (mesh length of 66 mm). A cover net with a mesh opening of 18 mm (or 10.3 mm depending on the research year) was attached to the codend. For each of the 20 fish species and two squid species, we pooled data of hauls where body size for the whole catch was measured without subsampling to obtain the body size compositions of fish caught in both the codend and the cover net. The maximum likelihood method was performed for estimation of parameters in the logistic curve equation representing the codend selection curve. For 18 fish species (excluding Trichiurus japonicus and Muraenesox cinereus), we examined the relationship of the obtained selection parameters [length at 50% retention (l50) and selection range (SR) (= l75–l25)] to fish body shape. We demonstrated that, in fish species with a smaller ratio of body height/width to body size (i.e., more slender body type), there was a tendency for larger l50 and SR. Furthermore, by comparing the l50 of each fish species with reproductive parameters such as minimum length at maturity, we examined the sustainability of the resources based on the minimum mesh size regulation.