A review of false killer whales in Hawaiian waters: Biology, status, and risk factors

Citation

Baird, R.W. 2009. A review of false killer whales in Hawaiian waters: Biology, status, and risk factors. Report prepared for the U.S. Marine Mammal Commission under Order No. E40475499. 23 December 2009.

Executive summary

Recent management and conservation issues have arisen concerning false killer whales in Hawaiian waters. Two demographically isolated populations have been identified, a small (estimated 123 individuals) island-associated population around the main Hawaiian Islands (hereafter Hawai‘i insular stock) and a larger (estimated 484 individuals) offshore population (hereafter Hawai‘i pelagic stock). Individuals within the Hawai‘i insular stock regularly move among islands and have been documented at distances of 110 km offshore. Less is known of movements/range of individuals from the Hawai‘i pelagic stock; one group has been documented 42 km offshore and individuals likely move beyond the Hawaiian Exclusive Economic Zone. No information is available to assess trends in the Hawai‘i pelagic stock. For the Hawai‘i insular stock, a significant decline in sighting rates from aerial surveys conducted between 1993 and 2003 suggests a large decline in population size. Other available evidence also supports a decline in population size for the insular stock: a reduction in sighting rates from boat-based surveys since the mid-1980s, lower than expected survival based on photo-identification data, and much higher sighting rates and larger group sizes in a 1989 aerial survey compared to boat-based surveys since 2000. False killer whales in Hawai‘i feed primarily on large game fish that are also the target of commercial and recreational fisheries. A number of potential conservation threats have been identified. Individuals from the Hawai‘i insular stock have elevated levels of persistent organic pollutants. Three of nine individuals sampled had levels high enough to potentially influence health. Because of the overlap between false killer whale diet and commercially harvested fish, reduced prey size or abundance could influence false killer whale foraging success or nutritional levels. Significant declines in body size and/or catch per unit effort have been documented for several false killer whale prey species in Hawaiian waters. False killer whales have been documented taking fish off lines in both nearshore and offshore fisheries. Depredation of caught fish may lead to retaliatory shooting by fishermen although, given potential fines and penalties, such shooting is not likely to occur where it may be witnessed; thus there is no information available to assess the potential for this to influence population dynamics. With the overlap in diet with commercially and recreationally harvested fish, the potential for hook ingestion, either from depredation or from free-swimming hooked fish, is relatively high. Based on studies elsewhere, hook ingestion would have a high likelihood of leading to mortality. Bycatch may occur in nearshore kaka line or shortline fisheries that use similar, but shorter gear to offshore longline fisheries, but there is no observer coverage of nearshore fisheries. False killer whales are the most frequently recorded bycaught cetacean in the Hawai‘i-based offshore longline fishery. Rates of serious injury and mortality have exceeded the potential biological removal (PBR) levels since bycatch rates and population levels were first available in 2000. Bycatch rates are underestimated as they do not take into account individuals that are not positively classified as to species or individuals that may break free with gear attached before being documented by observers. A number of research recommendations are presented to help reduce uncertainty and to clarify factors that may be influencing the population trajectories of both the Hawai‘i insular and Hawai‘i pelagic stocks, as well as to provide information that could be used to reduce bycatch rates or otherwise mitigate anthropogenic impacts on these populations.

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