Discarding of the Japanese mantis shrimp Oratosquilla oratoria by small-scale trawlers in Tokyo bay [Japan].

In Tokyo Bay small-scale trawlers discard considerable percentages of their catch of the Japanese mantis shrimp Oratosquilla oratoria, but no quantitative survey has so far been carried out. We aimed at making clear the actual fishing mortality, performed a monthly survey on discard percentages from April 1989 to April 1990 to discover seasonal changes, and estimated mortality rate after discarding through simultaneous on board experiments. The results revealed that both the discard percentage and the mortality of the discarded individuals are highest in summer, which was considered to be caused mainly by high air temperature. In total, individuals 2.4 times as large as those landed were discarded and died in a year.