POPULATION SIZE AND REPRODUCTIVE SUCCESS OF CALIFORNIA GULLS AT MONO LAKE, CALIFORNIA, IN 1999, WITH EMPHASIS ON THE NEGIT ISLETS

In 1999, nest counts estimated about 40,530 adult California Gulls (Larus californicus) were nesting at Mono Lake in late May. About 91% of Mono Lake's breeding gulls were on the Negit Islets, 9% on the Paoha Islets complex, and <0.07% on Negit Island. Twain Islet remained the most populous nesting island, holding 53% of Mono Lake's breeding gulls, followed by Little Tahiti Islet with 25%. The few gulls that have recolonized Negit Island, which became further isolated from the mainland, may reflect the continued presence of coyotes there in 1999. The fledging rate on the Negit Islets of 0.26 chicks per nest was the lowest since our studies began in 1983. An estimated 4786 young fledged from all the lake's nesting islands in 1999. The fourth consecutive year of low gull productivity likely reflects reduced primary productivity during a corresponding period of meromixis at Mono Lake. Although the lake’s brine shrimp (Artemia monica) population matured slowly in spring 1999, it is unclear if this directly affected chick diet or chick production. During a prior six-year period of meromixis in the 1980s, gull nesting success was low the first two years but increased thereafter. Any reconsideration of management alternatives should take a holistic, ecosystem-wide approach, and balance any short-term impacts of meromixis against the long-term prospects for improved productivity and the positive benefits of current stream flows and lake level rise to stream restoration, wetland restoration, alkali fly (Ephydra hians) productivity, and curtailment of air pollution from dust storms. INTRODUCTION In 1999 Point Reyes Bird Observatory (PRBO) completed the seventeenth year of a study of the California Gull at Mono Lake, California. The objectives of our ongoing study are to measure year-to-year variation in population size and reproductive success and to determine their relationship to changing lake levels. This report focuses on the Negit Islets, which currently support most of the lake's nesting gulls, and on Negit Island, which supported the majority until the gulls abandoned it in 1979. Negit Island was recolonized in 1985 and was abandoned again in 1991. The effects of recent changes in the Mono Lake ecosystem are of special interest to biologists (Patten et al. 1987, Botkin et al. 1988) and to public agencies charged with protecting the lake's valuable natural and scenic resources (Jones and Stokes 1993). Because a recent decision that protects the Mono Lake ecosystem will allow the lake’s surface elevation to rise to 6392 feet (SCWRCB 1994), there is a continuing need to monitor the lake's resources, including nesting gulls, to document their responses to the changing conditions. STUDY AREA AND METHODS The study area at Mono Lake has previously been described in Shuford (1985) and Shuford et al. (1984, 1985), though conditions that potentially could affect nesting gulls have changed considerably over time. Since 1941, the lake has dropped almost 45 vertical feet and nearly doubled in salinity because of diversions of its inflowing streams. Wet winters in the early and mid-1980s caused a temporary reversal of the downward trend. The winters of 1986-87 through 1993-94 averaged very dry, and the lake level fell to a surface elevation of 6374.5 feet by May 1992. Very wet winters returned in 1994-95, 1996-97, and 1997-98 and lake level rose to 6384.4 feet in May 1999 and 6384.7 feet in July 1999 (P. Kavounas in litt.). Consequently, the channel that reformed between Negit Island and the landbridge in 1995 continued to widen in 1999. Over the years, small numbers of gulls have intermittently initiated nesting on a peninsula of Paoha Island (immediately adjacent to the Paoha Islets), which is either partially or completely (e.g.,