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Estimates of Marine Mammal, Sea Turtle, and Seabird Bycatch from the California Large-Mesh Drift Gillnet Fishery: 1990-2017
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2019
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[PDF-6.45 MB]
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Description:The California large-mesh drift gillnet fishery for swordfish and thresher shark (‘the fishery’) originated in the 1970s as an experimental fishery targeting pelagic sharks. By the mid-1980s, the fishery included approximately 200 vessels and annual effort was approximately 10,000 fishing sets (Hanan et al. 1993, Holts et al. 1998). Effort has steadily declined since then, with current annual effort of about 500-700 sets (Carretta et al. 2017). From 1990-2017, technicians in the NMFS West Coast Regional Office fishery observer program observed 8,956 sets from an estimated 56,358 sets fished, corresponding to 16% observer coverage (Carretta and Barlow 2011, Carretta et al. 2017, Figures 1-2). Prior estimates of marine mammal, sea turtle, and seabird bycatch, based on ratio estimates, are published in Julian and Beeson (1998), Carretta et al. (2004), and in a comparison of ratio and regression tree estimates from 1990-2016 (Carretta et al. 2017, 2018). Here, we present updated estimates of bycatch using both methods, using one additional year of observer data from 2017. Previous bycatch estimates in this fishery were generated with ratio estimators (Julian and Beeson 1998, Carretta et al. 2004, Carretta et al. 2014). For species with large sample sizes (e.g., short-beaked common dolphin, Delphinus delphis and California sea lion, Zalophus californianus), ratio estimates of bycatch are generally unbiased; however, the fishery entangles many species only rarely, including some (e.g., sperm whales, Physeter macrocephalus and short-finned pilot whales, Globicephala macrorhynchus) that are subject to a mandatory drift gillnet Take Reduction Plan (Federal Register 1997).
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