Bycatch of jumbo squid Dosidicus gigas in the tuna purse-seine fisheryof the eastern Pacific Ocean and predatory behaviour during capture

摘要

Squid were recorded by observers in 192 (1.9%) of 9928 purse-seine sets in the tuna fishery in and near the Gulf of California, Mexico (north area) and off Peru and Ecuador (south area) during 1997-2002. The squid were identified as jumbo squid (Dosidicus gigas) in 19% of the positive sets, and it was assumed that the squid in the other sets were also D. gigas because it is the only large squid that occurs in great numbers in these areas. The total bycatch recorded during 1997-2002 was estimated to be about 50 tonnes in the north area and 435 tonnes in the south area, and total removals may have been more than 600 tonnes. Five or more tonnes (maximum 100 tonnes) of squid were reported in 22 purse-seine sets, demonstrating that aggregations of hundreds of D. gigas sometimes occur near the surface during the daytime. The probability of squid bycatch varied significantly with year and set type in the north area and with time of day and year in the south area. Squid bycatch was observed most frequently in sets on unassociated tunas, and less often in mammal and floating-object sets in both the north and south areas. The probability of catching squid was lower at midday than in the morning or afternoon in the south area. Three accounts of squid attacks on skipjack (Katsuwonus pelamis) and yellowfin tuna (Thunnus albacares) inside the purse seine, and bite damage to the tuna, are described. This study provides the first account, to the authors' knowledge, of interactions between jumbo squid and tunas, and demonstrates considerable plasticity in the schooling behaviour of D. gigas.