Faculty of Marine Science, Tokyo University of Marine Science and Technology, Minato, Tokyo 108-8477, Japan
In order to determine the effect of ghost fishing by lost gillnet, the relationship between soak time and number of enmeshed animals in an experimentally lost gillnet was examined by diving observation. The gillnet was deployed in a small trough surrounded by artificial reefs at 13 m depth in Tateyama Bay, Chiba Prefecture, Japan for 2000 days from 16 February 2004 to 8 August 2009. During the experimental period, 8 species of crustaceans including Japanese spiny lobster Panulirus japonicus as the main target species, 3 species of gastropod, and 3 species of bony fish were enmeshed in the gillnet. The number of enmeshed animals in the gillnet increased rapidly within 11 days after deployment, and then declined gradually showing fluctuations caused by the decrease in newly enmeshed animals, and dropping off from the gillnet as the dead animals decomposed. The decrease in the number of enmeshed animals was expressed by an exponential equation, and based on this equation, the duration of capture function for the gillnet was calculated to be 182 days. The duration of capture function for the lost gillnet for non-commercial by-catch species such as small crustaceans and gastropods was longer than for commercial species such as Japanese spiny lobster and bony fish.