Estimated abundance and trend in aerial counts of beluga whales, Delphinapterus leucas, in Cook Inlet, Alaska, 1994-2012

Aerial surveys of beluga whales, Delphinapterus leucas, have been conducted annually in Cook Inlet, Alaska, from 1993 to 2012. Beluga whales were seen near the coast and within river mouths in all years, with nearly all of the sightings in the northernmost portions of the inlet after 2000. In this paper, we revisit abundance estimates from 1994 to 2012 and present changes in methods and statistical analyses that have occurred since an earlier publication in 2000. Our objectives include a reanalysis of the abundance estimates over the 19-year aerial survey time series (1994–2012), revising the population trajectory through 2012, and estimating population trend rates for endangered Cook Inlet beluga whales. Annual surveys documented a decline of nearly 50% between 1994 and 1998 from an estimated 653 (CV = 0.24) whales to 347 (CV = 0.17) whales. With a very limited hunt (5 whales total taken between 1999 and 2012), the anticipated recovery at a rate of at least 2% per year did not materialize. Instead, from 1999 to 2012, the rate of decline has been -1.60% (SE = 0.75%) per year, with a 97% probability that the growth rate is declining (i.e., less than zero) and a 99.9% probability that the growth rate is less than +2% per year. Introduction In general, beluga whales, Delphinapterus leucas, have strong site fi delity to natal areas and very low dispersal rates (O’Corry-Crowe, 2002). In Alaska, these natal areas occur in the Beaufort Sea, Chukchi Sea, Norton Sound, Bristol Bay, and Cook Inlet. Geographically separated from beluga whale populations in western Alaska, Cook Inlet belugas are genetically distinct (O’Corry-Crowe et al., 1997) and are rarely observed in waters outside the inlet (Laidre et al., 2000). The closest beluga whale population is in Bristol Bay (Lowry et al., 2008), 1,500 km away by sea and separated by the Alaska Peninsula that extends three degrees of latitude south of the southern limit of the Bristol Bay population. If the Cook Inlet population goes extinct, it is highly unlikely that Cook Inlet would be repopulated with beluga whales in the foreseeable future; the result would be a permanent loss of range. Until 1999, Cook Inlet beluga whales were subject to an unregulated subsistence hunt (Mahoney and Shelden, 2000). Following abundance estimates that indicated this stock had declined nearly 50% (from 1994 to 1998), the National Marine Fisheries Service (NMFS) designated the stock as depleted under the U.S. Marine Mammal Protection Act (NOAA, 2000a). During this period of decline, beluga whales taken during the annual hunt ranged in number from 21 to 147 per year (Mahoney and Shelden, 2000), averaging about 70 whales per year killed and struck but not recovered. In 1999, the hunt was suspended and thereafter NMFS co-managed the hunt through a series of agreements with the Native hunting community (NOAA, 2000b). Considering how few whales (only 5) were killed in the subsistence hunt in the 13 years between 1999 and 2012, wildlife managers anticipated that the population would begin to recover. However, the population showed no signs of recovery; accordingly, NMFS determined that the Cook Inlet beluga whale distinct population segment (DPS) was endangered as defi ned by the U.S. Endangered Species Act (ESA) (NOAA, 2008). The abundance estimates noted above were generated from data collected by NMFS personnel from the Alaska Fisheries Science Center’s National Marine Mammal Laboratory and Alaska Regional Offi ce. NMFS conducts annual aerial surveys to study beluga whale distribution and abundance in Cook Inlet, Alaska. These surveys typically occur in June and have been repeated each year since 1993 (Rugh et al., 2000; 2005; Shelden et al., 2013). Abundance estimates from 1994 to 2000 were published in Hobbs et al. (2000a,b). This paper revisits that time period and presents changes in methods and statistical analyses that have occurred since 2000. The objectives of this paper include a reanalysis of the abundance estimates over the 19-year aerial survey time series (1994–2012), revising the population trajectory through 2012, and estimating population trend rates for endangered Cook Inlet beluga whales.