Appropriate Mesh Sizes of Gillnets for Capturing Black Sea Bream Acanthopagrus schlegelii and Fish-Proof Nets to Protect Red Alga Porphyra yezoensis Cultivation

Abstract

Since 2015, a significant decrease in the harvest of the red alga Porphyra yezoensis has been observed within cultivation areas in Chiba Prefecture, Japan. This decrease is primarily attributed to feeding by the black sea bream Acanthopagrus schlegelii. In response to this issue, seven gillnet surveys were conducted between November 4, 2018, and February 27, 2019, in areas managed by the Shinfuttsu Fishery Cooperative. Gillnets with mesh sizes of 92, 112, and 131 mm were used to estimate the mesh selectivity. Additionally, stomach contents from captured black sea breams, categorized by their body size, were measured. Black sea breams with red alga in their stomachs exhibited total lengths ranged from 238 to 496 mm. Based on these findings, using a combination of gillnets with mesh sizes of 100 and 132 mm is recommended for effectively capturing these fish. Furthermore, to protect red alga cultivation nets, it is recommended to utilize fish-proof nets with a mesh size of 53 mm or smaller.