Seasonal changes and influence of the Kuroshio path on catch and body size of mackerel scad Decapterus macarellus in waters around Hachijyo-jima Island in central Japan
2019
Hino, H. (Tokyo Metropolitan Island Area Research and Development Center for Agriculture, Forestry and Fisheries, Tokyo (Japan). Hachijo Branch) | Tomoto, T. | Tanaka, Y.
Seasonal changes and the influence of the Kuroshio path on catch and body size of mackerel scad Decapterus macarellus in waters around Hachijyo-jima Island in central Japan were studied using generalized linear models (GLM) and parameter estimates of a mixture of normal distributions. The results of the selected GLM using Akaike’s information criterion indicated that year, month, and changes in the Kuroshio path influenced catch and body size. The number of fishing boats per day was also found to decrease fishing efficiency (catches per fishing boat per day) probably due to restrictions on catches imposed by the Hachijyo-jima Island Fishermen’s Cooperative Association (FCA). The FCA restriction takes into account shipping capacity to prevent unit price drops. The coefficients of the year of the selected GLM indicate that stock abundance trends and body sizes decreased or increased after 2015, respectively. The selected GLM also indicated seasonal changes, in which catches increased from August through November and decreased from November through December. The results of parameter estimates of the mixture of normal distributions indicate that seasonal changes in catch and body size were probably due to growth and changes in age composition (age 1-3), which were targets for the fishery. Moreover, in the case of the C-type of the Kuroshio path, catches in the same month increased 1.1-1.2 times more than in the non-C-type of the Kuroshio path. Changes in catch by Kuroshio path were considered probable due to recruitment of high-growth fish associated with improvements in feeding environments, including increased nutrient levels, in addition to increases in body weight due to aging.
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