Ecosystem modeling of species interactions affecting sockeye salmon production in Iliamna Lake, Bristol Bay, Alaska

摘要

Sockeye salmon (Oncorhynchus nerka) returns to Iliamna Lake, Bristol Bay, are unique for their historically strong cyclic nature. Prior to 1950 there were often two strong runs followed by three weak returns; in the 1980s and early 1990s there was one strong return followed by four years of weak returns. Studies to determine the mechanism for the cycles have been conducted for many years. The aim of this paper is to look at an ecosystem approach to help explain cyclic behavior and productivity differences during the peak and low years. Ecosystem models, such as the ECOPATH model developed by Christensen & Pauly (1992), help determine trophic relationships between species or species groups in the ecosystem being modeled by using mass balancing under steady-state conditions. ECOTRACE has been used to trace marine-derived nutrients through an aquatic ecosystem (WAtkinson 1998). ECOSIM, the next stage in the ECOPATH modeling project (Walters et al. 1997) can be a valuable tool for designing adaptive management experiments or looking at changes in the system from various perturbations. In anadromous lake systems, trophic structure analysis takes advantage of marine-derived nutrients spawning salmon transport into the lake system that are then dispersed throughout the system by decomposing carcasses (Larkin & Slaney 1997, Bilby et al. 1998). This dispersal of nutrients enriches the system, allows for increased general productivity, but also provides us with a tool to estimate trophic relationships of species or species groups in an ecosystem. The ratio of isotope to normal marine nitrogen, ~(15)N: ~(14)N, is specific to trophic level and, using known transfer coefficients from one trophic level to another, allows the trophic level of a species to be estimated (Minagawa & Wada 1984).