STATUS AND TRENDS OF THE ASHY STORM-PETREL ON SOUTHEAST FARALLON ISLAND, CALIFORNIA, BASED UPON CAPTURE-RECAPTURE ANALYSES

We conducted a capture-recapture study on the population size and tren ds of the Ashy Storm-petrel (Oceanodroma homochroa) on Southeast Faral lon Island (SEFI), California, based upon data collected in 1971, 1972 , and 1992. From March through August, birds were lured to fixed-site sampling locations using taped vocalization playback. Using program JO LLY, we estimated population size and evaluated statistical models usi ng goodness-of-fit and Likelihood Ratio tests. On the southwestern slo pe of Lighthouse Hill, amidst prime breeding habitat, numbers of breed ing birds decreased from 1,271 +/- 140 ((x) over bar +/- SE) in 1972 t o 710 +/- 117 in 1992, a decline of 44% (approximate 95% CI = 22-66% d ecline; lambda = -2.8% per annum); for a variety of reasons, we consid er this to be the most reliable indicator of population change. In 197 1, on a portion of SEFI relatively disjunct from the sampling area in 1972, 2,131 +/- 322 breeding birds were estimated. To produce an overa ll early 1970s estimate with which to compare to 1992, we summed popul ation estimates from 1971 and 1972. An overall value of 6,461 birds, o f which 3,402 (53%) were breeders, was obtained for the early period. In 1992, the overall population in roughly the same area was estimated at 4,284 +/- 409 birds, of which 1,990 +/- 408 (46%) were presumed br eeders. These results, encompassing peripheral as well as more central ly located storm-petrel habitat, indicate an overall population declin e of 34% and a comparable decline in breeding birds of 42% over the pa st two decades. However, oceanographic conditions varied between 1971- 1972 and 1992, and reduced food availability in 1992 may have influenc ed colony attendance and breeding effort. Nonetheless, the apparent po pulation decline over the past 20 years suggests that the species warr ants management and/or additional protective status.