ESR SPECIAL PrePrint (2009) - Abstract

ABSTRACT: Successful conservation of threatened species is often hindered by a lack of long-term data required to identify the vital rates contributing to population decline and the extrinsic factors influencing those rates. Museum collections can provide a valuable resource for reconstructing the historic demography and diet of otherwise elusive species. Here, we used age ratios (the relative number of hatch-year to after-hatch-year individuals) to examine the hypothesis that population declines in a threatened seabird, the marbled murrelet Brachyramphus marmoratus, are due to declines in reproductive success over the past 150 yr. We also used stable-nitrogen isotopes to examine the hypothesis that variation in reproductive success over this period is related to the quality of food received by young in the nest. Age ratios from at-sea surveys conducted from 1994 to 2001 were significantly lower than museum collection age ratios for the period 1860 to 1950. Stable-nitrogen isotope values indicated that the trophic feeding level of marbled murrelet nestlings declined significantly (–2.6‰) from 1854 to 2008. Our results suggest that the reproductive success of marbeled murrelets breeding in the Salish Sea has declined over the past 150 yr and that declines in nestling diet quality may be partly responsible. Overall, our results support the idea that managers should consider the quality of both nesting and marine foraging habitat as they attempt to improve reproductive success and population growth rate in this threatened species.

KEY WORDS: Marbled murrelet · Stable isotopes · Age ratios · Diet reconstruction · Museumspecimens · Brachyramphus marmoratus · Salish Sea