Modeling Seasonal Trophic Interactions of Adfluvial Bull Trout in Lake Billy Chinook, Oregon

Abstract

We examined the trophic interactions of bull trout Salvelinus confluentus in Lake Billy Chinook, Oregon, using a bioenergetics model combined with data on annual growth, seasonal diet, distribution, and thermal experience to determine the seasonal and size-specific prey requirements of bull trout and the influence of bull trout predation on some of their major prey species in the reservoir. Per capita estimates of consumption were expanded to population-level estimates based on estimates of size structure and the abundance of the bull trout population. Bull trout became progressively more piscivorous with increasing size, and fish were the primary prey of predators of 450 mm fork length (FL) or more. Kokanee (lacustrine sockeye salmon Oncorhynchus nerka) and other salmonids (predominantly bull trout and rainbow trout O. mykiss) represented the largest fraction of fish prey in the diet, although cyprinids, cottids, and catostomids were also consumed. Predation on kokanee occurred primarily in autumn and secondarily in winter−spring. Predation by bull trout of less than 450 mm FL was greatest on smaller bull trout and rainbow trout in the reservoir during June−September, whereas predation by bull trout of 450 mm or more on salmonids other than kokanee was most prevalent during winter−spring. Bull trout of all sizes were capable of eating fusiform fish up to 50% of their own length. Given the size structure and estimated abundance of bull trout of 200 mm FL or more in Lake Billy Chinook during 1993 (3,600) and 1994 (8,400), model simulations indicated that annual predation removed large fractions of the bull trout (19–44% of age-0 fish and 7–16% of age-1 fish) and kokanee (5–11% of age-0 fish, 1–2% of age-1 fish, and 9–59% of fish of ages 2–3) populations. Thus, cannibalism and prey supply are potentially important factors limiting the abundance and production of bull trout in Lake Billy Chinook.