Sockeye salmon production relative to changes in rearing capacity of Crescent Lake, upper Cook Inlet

Edmundson, J

Beginning in the mid 1980s and through much of the 1990s, production of the Crescent River sockeye (Oncorhynchus nerka) salmon declined as evidence by lower adult returns and fewer adult recruits produced per spawner (RPS). For brood years 1984-1988 and 1990, RPS values fell below replacement. Concern over the decline in productivity initiated limnological studies in 1996 to determine whether sockeye salmon production was related to variability in changing conditions of the major nursery lake (Crescent Lake) of this system. The escapment goal range was lowered in 1998 from 50,000-100,000 to the current 25,000-50,000 range in part based on the low density of cyclopoid (Cyclops scutifer) copepods, the primary food resource (zooplanktion) for rearing sockeye salmon juveniles, and an apparent increase in water column turbidity. Since then (1998) turbidity has lessened and the euphotic zone depth has increased, but copepod abundance has not. However, sockeye salmon escapements exceeded the escapement goal in both 1999 (69,000), 200 (57,000) and 2001 (78,000). Therefore, we are uncertain whether reduced Cyclops abundance is due to top-down or over-grazing effects by rearing sockeye juveniles or from bottom-up effects of reduced primary production associated with increasing turbidity or a combined effect. Nonetheless, both mechanisms can serve to reduce sockeye salmon productivity. Based on turbidity levels and amount of food resources for rearing sockeye salmon juveniles in Crescent Lake, we feel taht the curent escapement goal of 25,000-50,000 spawners should be maintained because it provides a good balance between fry abundance and the quantity of available forage.

Sockeye salmon production relative to changes in rearing capacity of Crescent Lake, upper Cook Inlet

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