A discarding problem of mantis shrimp in Tokyo Bay

In the present study, we studied a small trawlnet fishery in Tokyo Bay which mainly targeted mantis shrimp (Oratosquilla oratoria) and discarded many small mantis shrimp. We surveyed the numbers and body length of discarded and landed mantis shrimp aboard a small trawlnet boat once a month from January 1993 to December 2000. Body length (BL) of the mantis shrimp caught by this boat ranged from 8 to 13 cm, and those less than 10 cm in BL were discarded because they are commercially unmarketable. The number of discarded mantis shrimp markedly increased in summer, reaching a high of more than a million per month. The high numbers of discarded small mantis shrimp in summer are due to 2 main factors: 1) fishermen often use nets with small mesh sizes, in order to catch other species; 2) around May, they begin catching small mantis shrimp that were spawned the year before and are still commercially unmarketable.