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.

Abstract.-We evaluated bioelectrical impedance analysis (BIA) as a nonlethal means of predicting energy density and percent lipids for three fish species: yellow perch Perca flavescens, walleye Sander vitreus, and lake whitefish Coregonus clupeaformis. Although models that combined BIA measures with fish wet mass provided strong predictions of total energy, total lipids, and total dry mass for whole fish, including BIA provided only slightly better predictions than using fish mass alone. Regression models that used BIA measures to directly predict the energy density or percent lipids of whole fish were generally better than those using body mass alone (based on Akaike's information criterion). However, the goodness of fit of models that used BIA measures varied widely across species and at best explained only slightly more than one-half the variation observed in fish energy density or percent lipids. Models that combined BIA measures with body mass for prediction had the strongest correlations between predicted and observed energy density or percent lipids for a validation group of fish, but there were significant biases in these predictions. For example, the models underestimated energy density and percent lipids for lipid-rich fish and overestimated energy density and percent lipids for lipid-poor fish. A comparison of observed versus predicted whole-fish energy densities and percent lipids demonstrated that models that incorporated BIA measures had lower maximum percent error than models without BIA measures in them, although the errors for the BIA models were still generally high (energy density: 15-18%; percent lipids: 82-89%). Considerable work is still required before BIA can provide reliable predictions of whole-fish energy density and percent lipids, including understanding how temperature, electrode placement, and the variation in lipid distribution within a fish affect BIA measures.

This study quantified temporal, spatial, and ontogenetic trophic interactions in Strawberry Reservoir to determine whether salmonid sport fish production was limited by food supply or predation. We combined field data on growth, diet composition, distribution, abundance, survival, and thermal experience with species-specific bioenergetics models for salmonids in Strawberry Reservoir to quantify monthly consumption of all prey fish, Daphnia, and macroinvertebrates. Fish prey represented a minor fraction of the annual diet of large cutthroat Oncorhynchus clarki and rainbow trout O. mykiss, but acute episodic piscivory associated with stocking of juvenile cutthroat trout in autumn could account for the low (1%) initial survival of these hatchery cohorts in recent years. Daphnia and other macroinvertebrates were important to all species and sizeclasses of salmonids. Model simulations suggested that salmonid consumption represented 5-30% of the Daphnia biomass during the June-October growing season in 1996-1997, but planktivorous demand exceeded Daphnia biomass in late winter. The bottleneck in prey supply during late winter and early spring suggests that Daphnia may not be able to support community consumption during this period if planktivore populations increase, either as a result of the expanding cyprinid populations or as a result of increased salmonid stocking or recruitment. Defining the role and impact of piscivorous and planktivorous trophic interactions provided information for management decisions regarding stocking strategies, recruitment, carrying capacity, and other food web processes.

Walleye Sander vitreum eggs could be stored at 7 C for up to 30 h with 60% fertilization rates. Fertilization rates at 1 and 12 C fell to 17 and 5%, respectively. At spermatozoa : egg ratios <25 000 : 1, survival to the eyed stage declined precipitously. Beyond a ratio of 25 000 : 1, survival to the eyed stage did not increase >c. 70%. Therefore, a standard spermatozoa : egg ratio of 25 000 : 1 was established and is recommended for studies concerning fertilizing success. # 2005 The Fisheries Society of the British Isles

Otolith microchemistry is a commonly used tool for stock discrimination in fisheries management. Two key questions remain with respect to its effectiveness in discriminating among river-spawning populations. First, do larvae remain in their natal river long enough for their otoliths to pick up that system’s characteristic chemical signature? Second, are larval otolith microchemical differences between natal rivers sufficiently large to overcome spatiotemporal variation in water chemistry? We quantified how larval age, the ratio of ambient strontium to calcium concentrations (Sr:Ca), and water temperature influence otolith Sr in both lab-reared and wild-collected Lake Erie walleye (Sander vitreus). Otolith microchemistry shows promise as a spawning stock discrimination tool, given that otolith Sr in larval walleye (i) is more strongly influenced by ambient Sr:Ca than by temperature; (ii) reflects Sr:Ca levels in the natal environment, even in larvae as young as 2 days old; and (iii) can effectively discriminate between larvae captured in two key Lake Erie spawning tributaries, even in the face of short larval river residence times and within-year and across-year variation in ambient Sr:Ca.