Technical Report

Development of methods for monitoring seabirds on Castle Rock National Wildlife Refuge

Castle Rock National Wildlife Refuge is a 6.4 ha island located 0.8 km off the coast of Crescent City, California. Castle Rock is the second largest seabird breeding-colony along the California coast, the largest Common Murre colony in the California Current, and hosts more than 100,000 seabirds. Eleven different species are known to have nested on this island including: Common Murre (Uria aalge), Brandt’s (Phalacrocorax penicillatus), Pelagic (Phalacrocorax pelagicus), and Double-crested Cormorants (Phalacrocorax auritus), Pigeon Guillemots (Cepphus columba), Cassin’s (Ptychoramphus aleuticus) and Rhinoceros Auklets (Cerorhinca monocerata), Tufted Puffins (Fratercula cirrhata), Western Gulls (Larus occidentalis), and Fork-tailed (Oceanodroma furcata) and Leach’s Storm Petrels (Oceanodroma leucorhoa) (Carter et al. 1992). The Aleutian Cackling Goose (Branta hutchinsii leucopareia) population has recovered from less than 800 in 1974 to more than 100,000 today (U.S. Fish and Wildlife Service 2001) and large numbers (~20,000) of geese have been roosting overnight on Castle Rock in the spring. These geese have the potential to alter habitats at the island. Brown Pelicans (Pelecanus occidentalis) have also roosted at Castle Rock in great numbers (1-2,000) during the fall. Castle Rock is situated midway along the U.S. Pacific Coast between two other major seabird colonies in the California Current: the Farallon Islands off the central California coast to the south and Three Arch Rocks off the Oregon coast to the north. Unlike the Farallon Islands, Castle Rock is much closer to shore and thus closer to many types of anthropogenic threats. However it is far enough off shore that observations or protections from the mainland have been very limited. Research and monitoring have been taking place at the Farallon Islands since 1971 (Ainley and Boekelheide 1990) and Three Arch Rocks has been a National Wildlife Refuge since 1907, while Castle Rock does not have a history of monitoring or conservation-based management and was privately owned until 1979 (it became a National Wildlife Refuge in 1980). The status of breeding seabird populations at Castle Rock are poorly known (except Common Murres) and cannot be assumed to mimic those of other seabird colonies such as the Farallon Islands or Three Arch Rocks (Parker 2005). Good estimates of the abundance of Common Murres and cormorants using the surface of Castle Rock can be obtained using aerial photos (Capitolo et al. 2006). Pigeon Guillemots and Tufted Puffins have been surveyed by boat because they are active diurnally at and around Castle Rock (Carter et al. 1992; Jaques and Strong 2001). However the burrow-nesting nocturnal species (Rhinoceros and Cassin’s Auklets, Fork-tailed and Leach’s Storm-petrels) are not easily seen because they are at sea feeding during the day and only come to the island between sunset and sunrise when they relieve their mate of incubation duties or feed their young. Further, human activity on the island has been very restricted with very few landings (Table 1). The burrow systems can be destroyed or badly damaged when stepped on by people which necessitates restriction of human visitation and also limits access for monitoring. Common murres are sensitive to the presence of people during the breeding season which prevents researchers from visiting the island from April to August (Thayer et al. 1999; Carter et al. 2001). Therefore nocturnal species have posed a very difficult challenge to study or monitor for trends. Based on past surveys conducted after the nesting period, Castle Rock probably still hosts substantial numbers of these burrow-nesting species; Castle Rock was the largest breeding colony of Rhinoceros auklets in California at the time of the last state-wide seabird survey (Carter et al. 1992). Despite the importance of this colony to these species in California, the current status of auklets and storm-petrels on the island is unknown.