Decline of the Marbled Murrelet in Clayoquot Sound, British Columbia: 1982-1993

Abstract

In 1992 and 1993, we repeated at-sea surveys of marbled murrelets (Brachyramphus marmoratus) in Clayoquot Sound, British Columbia, that were conducted originally in 1982. Murrelet distribution was determined by counting birds in 341 contiguous l-km2 quadrats in fiord, channel, and inshore marine habitats. Counts for this area totaled 4500 birds in 1982, 2704 birds in 1992, and 2622 birds in 1993 which constituted a 40% decline in the population size from 1982-1993. Birds were clumped in all surveys, with the highest densities in inshore (28, 14.3, 7.85 birds/km2 in 1982, 1992, and 1993, respectively) and channel (11.8, 9.1, 14.2 birds/ km2 in 1982, 1992, and 1993, respectively) marine habitats. The average change in the number of birds in each quadrat between the 1982 and 1992-93 surveys was significant for all quadrats combined, and individually for inshore quadrats. We consider the loss of nesting habitat in oldgrowth forest to be the probable cause of most of the observed population decline. By 1992 approximately 15-24.5% of the originally forested area in Clayoquot Sound had been logged. Oil spills, gill-net fishing, and oceanographic conditions (E1 Nino) were also considered as possible factors contributing to the population decline. This study is the first in North America to document a decline in a marbled murrelet population using a single, standardized at-sea survey technique. Marbled murrelets (Brachyramphus marmoratus) nest on tree branches in the canopies of oldgrowth and mature forests from southern Alaska to California (see recent su maries in Singer et al. 1991, Carter and Morrison 1992, Ewins et al. 1993, Hamer and Nelson 1995a). Standardized methods have been developed to monitor the activity of birds at inland nesting locations (e.g., Paton et al. 1990), but nests are difficult to locate and population size must be determined and monitored using surveys at ocean foraging areas (Sealy and Carter 1984). Previous to 1982, few at-sea surveys had been conducted for this species, and population size and distribution still are incompletely known for most areas on the west coast of North America (Carter and Morrison 1992). Thus, it has been difficult to document population trends for this species related to the loss of old-growth forest nesting habitat and mortality from oil spills and gill-net fishing. These threats are cause for concern and have led to the recent federal listing of the marbled murrelet as threatened in British Columbia, Washington, Oregon and California (Rodway 1990, U.S. Fish and Wildlife Service 1992) and state listing as endangered in California (California Fish and Game Commission 1992). In British Columbia, portions of the coast have been surveyed for marbled murrelet populations (Carter 1984, Sealy and Carter 1984, Kaiser et al. 1991, Lawrence and Backhouse 1991, Savard and Lemon 1992). Provincial estimates of 45,000-50,000 breeding birds (Rodway et al. 1992) are poorly substantiated for the most part because many areas have not been surveyed and survey data have been difficult to convert into reliable population estimates (Rodway et al. 1992, 1995). In addition, marbled murrelet populations have been considered to have declined in some areas in the province, based on anecdotal evidence (Brooks 1926, Pearse 1946) and loss of nesting habitat, yet quantitative data have not been available to investigate the degree of possible declines. Estimates of population size for large geographic areas have been published only for Clayoquot and Barkley sounds on the west