Spawning Distribution of Sockeye Salmon in a Glacially Influenced Watershed: the Importance of Glacial Habitats

—The spawning distribution of sockeye salmon Oncorhynchus nerka was compared between clear and glacially turbid habitats in Lake Clark, Alaska, with the use of radiotelemetry. Tracking of 241 adult sockeye salmon to 27 spawning locations revealed both essential habitats and the relationship between spawn timing and seasonal turbidity cycles. Sixty-six percent of radio-tagged sockeye salmon spawned in turbid waters ( 5 nephelometric turbidity units) where visual observation was difficult. Spawning in turbid habitats coincided with seasonal temperature declines and associated declines in turbidity and suspended sediment concentration. Because spawn timing is heritable and influenced by temperature, the observed behavior suggests an adaptive response to glacier-fed habitats, as it would reduce embryonic exposure to the adverse effects of fine sediments. Pacific salmon Oncorhynchus spp. spawn in a variety of habitats throughout their natural range in the northern basin of the Pacific Ocean. Their spawning locations are typically associated with clear, cool, and well-oxygenated water and may include rivers, streams, springs, and lake shorelines (for reviews, see Groot and Margolis 1991 and Quinn 2005). Glacier-fed rivers and lakes, which are characterized by high summer turbidity, variable flows, and large concentrations of suspended fine sediments (Koenings et al. 1986, 1990; Lloyd et al. 1987; Brabets 2002), are generally considered poor spawning habitat for Pacific salmon (Groot and Margolis 1991) because fine sediments can smother developing embryos (Phillips et al. 1975; Hausle and Coble 1976; Everest et al. 1987). More specifically, fine sediments can inhibit oxygenation, metabolic waste transfer, and fry emergence (Everest et al. 1987), resulting in higher mortality levels. Studies indicate that survival to emergence of salmonids is inversely related to the percentage of fine sediments in a redd (Holtby and Healey 1986; Chapman 1988). Although the adverse effects of fine sediments on salmon egg and embryo survival are well documented (Holtby and Healey 1986; see review by Chapman 1988), many glacial rivers and lakes support productive salmon stocks. Traditionally, spawning in these systems was assumed to occur in associated clearwater tributaries and lakes (e.g., Burger et al. 1995). However, recent studies suggest that glacially turbid spawning habitats are more important for Pacific salmon production than previously thought (Burger et al. 1985, 1995; Eiler et al. 1992). Because even low turbidity levels (e.g., 4 nephelometric turbidity units [NTU]) inhibit visual observation of spawning salmon (Cousens et al. 1982; Lloyd et al. 1987), the actual distribution of salmon in glacier-fed watersheds has probably been underestimated. The objectives of this study were to (1) identify the spawning locations of sockeye salmon O. nerka in the Lake Clark watershed and assess their distribution relative to water turbidity and (2) examine the relationship between spawn time and suspended sediment concentration (SSC) within individual tributaries. Such information is essential to conservation of contemporary salmon populations because it has implications for the definition and conservation of essential spawning habitats, abundance estimates, and population management (Knudsen 2000).