Island Fox Recovery Program: Channel Islands National Park population trends (2006–2016) and recovery report

Dillon, Adam; Shaskey, Laura

doi:10.36967/nrr-2291279

In 2016, the National Park Service (NPS) continued to implement recovery actions for endangered island fox subspecies on San Miguel Island (Urocyon littoralis littoralis), Santa Rosa Island (U. l. santarosae), and (in partnership with The Nature Conservancy) Santa Cruz Island (U .l. santacruzae). The NPS is the lead for monitoring island foxes on San Miguel and Santa Rosa Islands, which are the focus of this report. Annual monitoring indicates that both populations have generally increased over time yet both populations decreased in 2016. The three subspecies were the object of intense recovery actions from 1999–2008; primary actions included relocation of predatory golden eagles (Aquila chrysaetos), captive breeding, and reintroduction of island foxes. From 1999–2006, 44 golden eagles were trapped on the islands and relocated to northern California, with a resultant increase in island fox survival. A 10-year program of captive breeding and release of all three island fox subspecies was completed in 2008. During that period, in which captive breeding was conducted separately on each island, 225 pups were born in captivity, and 254 foxes were released to the wild. Excellent reproduction in the wild, exceeding the per capita reproduction in captivity, was the primary reason to cease captive breeding. Also, predation by golden eagles had been almost completely mitigated during the 10-year time period, and wild island fox survival rose to 80–90% on all islands. The NPS island fox program is currently in an intensive monitoring phase designed to ensure that recovery continues, and to document attainment of criteria allowing delisting of the two subspecies from the U.S. Endangered Species Act. Island fox population status is monitored via estimation of island-wide fox abundance using capture-recapture statistics from island-wide grid trapping and estimation of fox mortality using radio-telemetry. Excellent survival and reproduction in the wild have previously permitted rapid growth of the small, recovering populations, however by the end of the 2016 “fox year” (5/1/2016–4/30/2017) the San Miguel island fox population declined to an estimated 267 adults and 24 pups, resulting in 291 total foxes (Figure 1). The San Miguel subspecies returned to pre-decline population levels in 2011 and may have reached or surpassed carrying capacity. In the 2016 “fox year” (5/1/2016–4/30/2017) the Santa Rosa population estimate was 1025 adults and 598 pups, resulting in 1623 total foxes. Although there was no formal population monitoring on Santa Rosa Island prior to bringing all island foxes into captivity in 2000, if pre-decline densities on Santa Rosa were similar to those on Santa Cruz then the current population on Santa Rosa has reached or possibly exceeded pre-decline numbers. When compared, the adult fox density on San Miguel (7.71 foxes/km2) is over 1.5 times that of Santa Rosa (4.76 foxes/km2).

Island Fox Recovery Program: Channel Islands National Park population trends (2006–2016) and recovery report

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