PIFSC Sea Turtle Longline Research EA June 1, 2009
3.2.2 Potential Impacts to Cetaceans Potential impacts on cetaceans from interaction with research-related longline fishing gear
Interactions between marine mammals and fishing involve almost all existing fishing gear, including both hooking and entanglement of the marine mammal in the gear, as well as marine mammals removing hooked fish and bait from fishing gear (depredation), damaging gear, and eliminating potential to catch target species on hooks with captured bycatch. Cetaceans that damage gear, eat caught fish, and get caught on fishing gear can frustrate commercial fishers to the point that they have been known to kill the offending animal.
Interactions between marine mammals and longline fishing tend to be rare events and are difficult to predict. Most longline interactions are thought to be the result of toothed cetaceans, such as dolphins being attracted to the gear or boat because of the potential for food, homing in on the vessel or gear based on familiarity with its sounds. Less frequently, marine mammals, including baleen whales, are entangled in the longline gear probably as a result of their swimming paths accidentally encountering gear. Most depredation of catch or bait is believed to occur during hauling rather than setting of the gear (Gilman et al. 2006).
Many strategies have been proposed to reduce cetacean bycatch (hooking and/or entanglement) in fisheries, including changes in fishing gear and methods, regulatory constraints, and creating marine protected areas and implementing area and seasonal closures. Currently, the only known effective method to date to minimize marine mammal bycatch in fisheries involves real-time fleet communication and coordination to assist vessels in avoiding areas and periods with peak cetacean abundance (Gilman et al. 2006).
Between 1992 and 2004, a total of 200 interactions between marine mammals and U.S. Atlantic pelagic longline gear were observed. Of these, there were 10 observed mortalities and 94 observed serious injuries. One hundred of the observed interactions were with pilot whales, 64 were with Risso’s dolphin, and all other species had six or fewer observed interactions (Garrison 2007).
Forney and Kobayashi (2007) reviewed marine mammal interactions in the Hawaii-based deep- set and shallow-set longline fishery and, of 24,542 sets, 67 marine mammal interactions were observed, with an average take of 2.7 cetaceans per 1,000 sets. Although sample sizes were small, cetacean deaths occurred at a similar rate in deep-set and shallow-set longline fisheries. Overall cetacean take rates appeared to be highest in shallow-set swordfish efforts (6.5 cetaceans per 1,000 sets) and lower during tuna or swordfish-style efforts (2.1 and 2.5 cetaceans per 1,000 sets, respectively). Apparent species-specific differences in take rates by set type and EEZ area are likely related to the distribution of the various species, with northern species such as Risso’s dolphin taken primarily in higher-latitude swordfish-targeting sets and tropical species such as false killer whales and short-finned pilot whales taken primarily during lower latitude tuna or swordfish-style sets. These numbers may be an underestimate.
In the tropical Pacific, numerous observations of fishery interactions with false killer whales, pilot whales (Globicephala spp.), and killer whales (Orcinus orca), and at least eight species of