Marine Matters
Conservation Biology of Rhinoceros Auklets, Cerorhinca monocerata, on Año Nuevo Island, California, 1993-1999

Julie A. Thayer, Michelle M. Hester, William J. Sydeman
Point Reyes Bird Observatory, 4990 Shoreline Hwy., Stinson Beach, CA 94970

Abstract
Rhinoceros Auklets (Cerorhinca monocerata) were once plentiful in California, but presently only three offshore islands, Castle Rock, the Farallon Islands, and Año Nuevo Island, provide nesting habitat for approximately 95 percent of the California breeding population of Rhinoceros Auklets, which totals no more than 2,000 birds. Since 1992, we have studied and managed the population of Rhinoceros Auklets on Año Nuevo Island (ANI), part of Año State Reserve (ANSR), and within boundaries of the Monterey Bay National Marine Sanctuary. Annually, our goals are to assess population status, reproductive performance, and food habits of this recovering species Management efforts have involved protection and enhancement of habitat through the construction of boardwalks, implementation of an Island Research Protocol, and installation of nest boxes. The breeding population of Rhinoceros Auklets on ANI increased from 1993-1997, but dropped substantially in 1998 in response to the El Niño warm-water anomaly. This fluctuation was due mainly to a reduction of birds breeding in natural burrows. The population increased again in 1999, but still remained lower than
pre-El Niño levels. The success of nest boxes as a seabird restoration tool has been indicated by the immediate and continuing occupation of nest boxes, the return of chicks reared in boxes to breed in boxes, and the increased productivity (percent successful) of pairs breeding in boxes, fast approaching that observed in natural burrows. Rhinoceros Auklets on ANI have predominantly relied on northern anchovies (Engraulis mordax) to rear their chicks; in years of high anchovy abundance we have observed both high chick growth rates, fledging weights, and overall productivity. In relation to our conservation efforts, the El Niño of 1998 was a set-back, but the breeding population seems to have
started recovery in the subsequent cold-water La Niña year. However, loss of nesting habitat on ANI continues to pose some problems. We plan on continued Rhinoceros Auklet population monitoring, development of a habitat restoration/stabilization program to minimize erosion of auklet nesting areas, and development of a population viability analysis to look at the long-term progress for the population.