Spatial–Temporal Dynamics of Early Feeding Demand and Food Supply for Sockeye Salmon Fry in Lake Washington

Abstract

We compared temporal consumption rates by sockeye salmon Oncorhynchus nerka fry with food supply to evaluate how different survival and dispersal rates or additional hatchery production affected the winter carrying capacity of Lake Washington, Washington. Peak immigration of sockeye salmon fry into southern Lake Washington precedes the spring zooplankton bloom by 2–3 months. Zooplankton density, fish diet, and growth were sampled during winter and spring 2001, when a record 52.4 million fry entered the lake. Supplementary information on the distribution and dispersal of fry was collected in 2002 and 2003. We used bioenergetics modeling to estimate the temporal–spatial consumption by fry during early lake rearing. Cyclops bicuspidatus were dominant in the diet and zooplankton assemblage but declined from more than 30/L in late February and early March to fewer than 5/L during mid-March and early April. Fry ate significantly larger Cyclops than the average size in the lake; approximately 83% of the food items in the stomachs were larger than 0.8 mm, which was the minimum food item size. Hydroacoustics and midwater trawling surveys during 2002 and 2003 suggested that fry dispersed quickly over the southern half of the lake and used depths of 0–30 m. Under the most realistic scenarios for fry dispersal, feeding, and survival, total consumption of all prey by sockeye salmon fry represented 5% of the average monthly biomass of Cyclops during March and early April, when their availability was lowest. Potential bottlenecks in the availability of Cyclops only developed in simulations when fry were restricted to feeding in 0–10-m depths in the southernmost region of the lake. Despite the seemingly adequate food supply for sockeye salmon fry, other planktivores also consume significant fractions of the exploitable prey biomass, and the interannual availability of exploitable zooplankton varies considerably during winter and early spring. Either of these factors could impinge on the localized food supply for sockeye salmon and other species during some years and should be considered in the adaptive management of any enhancement program.