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Research Technical Report

Set Gillnets In The Monterey Bay National Marine Sanctuary: A Review Of Fishing Effort, Distribution, And Bycatch Of Marine Birds And Mammals, 1990-97

Forney, K.A. (1998)

Abstract P-10, California Cooperative Oceanic Fisheries Investigations Conference, November 4-6, 1998, Asilomar, California.


During the 1980's, Monterey Bay was an important area for the California halibut/angel shark set gillnet fishery, which also operates extensively off southern California. Since a 1994 ban on set gillnets within three miles of the southern California coast, effort in this fishery has increased dramatically in the Monterey Bay region, from about 500 sets per year in the early 1990's to nearly 1400 sets in 1997. The distribution of sets has also shifted from areas north of the bay (Santa Cruz to Ano Nuevo Island) to southern areas inside the bay (Seaside to Moss Landing), where high densities of harbor porpoise (Phocoena phocoena) and Common Murre (Uria aalge) are found. In this area, sea otters (Enhydra lutris) are also commonly seen waters deeper than 30 fathoms (56 m), where the nets are set. Recent estimates of incidental marine mammal and seabird mortality in this fishery have been based on California Department of Fish and Game (CDFG) estimates of total fishing effort derived from logbooks and landing receipts, combined with entanglement rates observed during a 1990-94 National Marine Fisheries Service (NMFS) monitoring program. However, the 1990-94 fishing effort was almost entirely north of Santa Cruz, and therefore these historical entanglement rates may not be representative of entanglement rates in the areas currently being fished. Both harbor porpoise and sea otters appear to be declining, and all three species have exhibited increased stranding rates in recent years, coincident with the shift of gillnetting into the bay.

In this study, more representative estimates of mortality have been obtained by including data from a CDFG observer program in 1987-90, when gillnet fishing was mostly in the southern, inner bay region, as it is now. To maintain the greatest degree of comparability to present fishing practices, only nets set in 20 fathoms were included. The Monterey Bay area was divided into 'northern' and ‘southern' strata, and stratum-specific entanglement rates were estimated from the combined CDFG and NMFS data. The resulting estimates of mortality for harbor porpoise in 1990-97 range from 40 (SE=9) to 202 (SE=49) animals per year and are distinctly higher than the previous estimates of 13 (SE=8) to 48 (SE=22) animals per year. These higher levels of mortality may not be sustainable in this population of about 4,000 animals. Sea otter mortality has been considered to be zero since 1990, because no takes were observed in the Monterey Bay area in 1990-94. The revised estimates of sea otter mortality are a total of 3 (SE=0.9) animals during the period 1990-94, and 7 (SE=3), 28 (SE=12) and 41 (SE=18) animals in 1995, 1996 and 1997, respectively. This size of this population is currently estimated to be about 2,000 otters. The recent increase in set gillnet effort within the Monterey Bay National Marine Sanctuary may therefore pose a significant threat to these two marine mammal species. Revised Common Murre mortality estimates for 1990-97 range from 1,741 (SE= 378) to 3,911 (SE=676) per year and are also higher than previous estimates of 284 (SE=82) to 2,333 (SE 292). The impact of this mortality on the Common Murre population is presently not known.

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