The incidental kill of marine mammals in commercial gillnet fisheries in California waters has been monitored by NMFS since 1990. The Coastal Marine Mammal Program, in cooperation with the California Department of Fish and Game, (CDFG) estimates marine mammal mortality from observed bycatch in the halibut/angel shark set gillnet fishery and the swordfish/thresher shark drift gillnet fishery. Annual estimates of cetacean mortality are presented to the Scientific Committee of the International Whaling Commission (IWC) and published in scientific journals.
The California halibut/angel shark set gillnet and swordfish/thresher shark drift gillnet fisheries are both classified as Category I fisheries under the U.S. Marine Mammal Protection Act (MMPA), meaning that ‘levels of incidental serious injury and mortality of a given marine mammal stock are greater than or equal to 50% of the Potential Biological Removal (PBR) level for that stock’ (NMFS List of Commercial Fisheries, 1996). PBR is defined as ‘the maximum number of animals, not including natural mortalities, that may be removed from a marine mammal stock while allowing that stock to reach or maintain its optimum sustainable population’ (Barlow et al., 1995). Category I fisheries are subject to monitoring by observer programs, which provide data on incidental marine mammal bycatch. NMFS observer programs for both the halibut/angel shark set gillnet and thresher shark/swordfish drift gillnet fisheries were initiated in 1990. Observers are placed on fishing vessels to record catch, bycatch and other gear, environmental variables and collect biological samples from incidentally caught marine mammals. Annual estimates of marine mammal mortality are calculated from the observed number of marine mammals killed, the fraction of the fishery observed, and estimates of overall fishing effort.
The halibut/angel shark set gillnet fishery was observed from 1990-1994 throughout its range (southern and central California), with levels of observer coverage ranging from 2-15% (mean = 9.6%)(Julian and Beeson, 1998). Historically, incidental takes of cetaceans in the set gillnet fishery have been mostly limited to harbor porpoise (Phocoena phocoena) in central California, although two unidentified common dolphins (Delphinus sp.) and one unidentified cetacean have also been reported (Julian and Beeson, 1998). In 1994, area closures restricted set gillnets to waters greater than 5.5 km (3 nmi) from the southern California mainland and greater than 1.85 km (1 nmi) from the Channel Islands. This closure resulted in a marked decrease in fishing effort in this fishery, from approximately 5,500-7,000 fishing days during 1990-93 to 2,000-4,000 days following the closure (Forney et al., 2000; Cameron and Forney, 1999; 2000). In the central California portion of the fishery, depth restrictions in place since 1991 have not allowed fishing inshore of 55 m (30 fm). There was no observer coverage throughout this fishery during 1995-98. In 1999, a NMFS observer program was reinstated in the Monterey Bay portion of the set gillnet fishery, in response to renewed concerns over the incidental take of harbor porpoise. A permanent ban on gill and trammel nets inshore of 110 m (60 fm) from Point Reyes to Point Arguello was implemented by the California Department of Fish and Game in September 2002 because of concerns over the incidental take of common murre (Uria aalge) and California sea otters (Enhydra lutris).
The swordfish/thresher shark drift gillnet fishery has been observed by NMFS every year since 1990. Levels of fishing effort in this fishery have decreased from approximately 5,500 days in 1993 to 1,779 days in 2002 (Julian and Beeson 1998, Carretta et al. 2005 ). Observer coverage levels in this fishery ranged from 4-18% (mean = 13%) during 1990-96 and between 20-25% from 1997-2002. Bycatch in the drift gillnet fishery has included a wide variety of cetacean, pinniped, sea turtle, and seabird species (Julian and Beeson, 1998; Carretta et al. 2005). Initiation of a Take Reduction Plan (TRP) in 1996 followed concerns over incidental take levels that exceeded PBR for some cetacean stocks. The TRP included the use of acoustic pingers all on nets (typically 20 each on the floatline and leadline), net extenders to increase minimum fishing depth to 11 m (6 fm), and mandatory skipper education workshops regarding marine mammals and TRP goals. Barlow and Cameron (2003) report on the effectiveness of acoustic pingers in reducing marine mammal bycatch in this fishery.