Research Technical Report
Managing Marine Mammal Bycatch In Dynamic Habitats: Lessons From A California Gillnet Fishery
Forney, K.A. (1999)
13th Biennial Conference on the Biology of Marine Mammals, Nov 28 - Dec 3, 1999
Bycatch in commercial fisheries poses a significant threat to many marine mammal populations worldwide. Monitoring programs are expensive, logistically difficult, and usually restricted to fisheries with the most severe bycatch. Limited information on bycatch may be derived from anecdotal reports, strandings, or historical data. Although this strategy is often necessitated by a scarcity of funding, it can be dangerously ineffective at detecting changes in mortality patterns, as recently documented in the California halibut gillnet fishery. During the 1980's, central California gillnet fisheries caused significant mortality of several marine species, including harbor porpoise (Phocoena phocoena), and southern sea otter (Enhydra lutris). A series of restrictions on fishing in shallow waters ultimately appeared successful at reducing bycatch, with a 1990-94 observer program reporting no sea otter entanglements and sustainable levels of harbor porpoise mortality. The observer program was discontinued after 1994 because of funding shortfalls and because coast-wide fishing effort had declined. Mortality estimates for 1995-97 were calculated using the 1990-94 entanglement rates and estimates of annual fishing effort. After 1994, however, the fishery underwent significant changes that were not discovered until 1997, including a shift into areas exhibiting greater bycatch during the 1980s. Therefore, 1995-97 mortality was likely underestimated using the 1990-94 entanglement rates. The true level of mortality cannot be assessed retroactively because of critical uncertainties regarding fishing practices and species distributions. However, if various assumptions are made, revised average mortality estimates range from 32 to 147 harbor porpoise (CV=0.2-0.6) and 3 to 29 (CV=0.4-0.5) sea otters. Sea otter mortality- previously assumed to be zero- is of particular concern, because this threatened population appears to be declining. This example illustrates the importance of continuous monitoring of fisheries that are dynamic and have significant potential for marine mammal mortality.