The body of chub mackerel Scomber japonicus is covered with small scales and mucus, and it has been pointed out that the stomach contents analysis for chub mackerel caught by purse seine may be biased due to incidental ingestion of them in the net during fishing. In this study, the stomach contents of mackerels caught by purse seine were compared to those caught by pole and line fishing, which are not affected by the incidental ingestion, at the same locations and time. Scales and mucous substances were found in 90 out of 99 (90.9%) stomachs of mackerels caught by purse seine, whereas they were found in a limited fraction (8.3%) of stomachs from mackerels caught by pole and line fishing. The scales found in the stomachs had the characteristics of mackerels. Furthermore, stomach contents index of the mackerels caught by purse seine was significantly higher than that from pole and line fishing when the weights of scales and mucous substances were included, while no difference was found when they were not included in the calculation. It has been concluded that the scales and mucous substances were ingested incidentally during fishing, and they should be excluded from the dietary data.
We investigated the yearly changes in catch size of female snow crabs (Chionoecetes opilio) landed at a fishery cooperative (Echizen-cho Fisheries Cooperative Association), based on data obtained from consignment sales transaction documents. The catch size by brand was investigated immediately after the opening of the season in 2006, 2011, and 2016. This data, together with the results of carapace width composition measurements by brand, conducted by a fisheries research institute, were used to estimate the average carapace width of the catches. While the brand composition shifted towards brands for smaller crabs, the carapace width composition by brand became larger. The estimated average carapace width in 2011 was smaller than that in 2006, but was bigger in 2016. Based on these results, we conclude that the reduced size in brand composition in the market did not correspond to a smaller overall carapace width composition in the catches. The determination of data based on daily commercial transactions and annual measurements of carapace width is a useful method for continuous data generation.