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Bioenergetics of the rainbow trout (Salmo gairdneri) and the kokanee (Oncorhynchus nerka) populations of Marion Lake, British Columbia Sandercock, Frederick Keith

Abstract

The annual energy budget was determined for the rainbow trout and kokanee populations of a small (13.3 ha) British Columbia lake. Population size of the rainbow trout aged 1⁺ and older was 4000 based on a 55 catch-day Schnabel estimate. Number of kokanee present in the lake (9000) was based on the ratio of occurrence of kokanee to rainbow trout (2.25:1) in catch. Monthly instantaneous growth and mortality rates were applied to each age group. Seasonal change in gonad weight as a percentage of body weight was similar in both species (ovaries 0.5%-13%, testes 0.5%-4%). Sex ratios were generally even in kokanee but there was a higher percentage of females (57.7%) in the rainbow trout. The relationship between egg number per female (F) and fork length (L) was calculated (rainbow trout, F = 18.7 L•⁹⁵⁹; kokanee, , F = 2.84 L¹•⁴¹⁴). The percent volume of each major food item was determined by stomach analysis. In only one month (May) was the food item of highest % volume the same in both species. Seasonal change in energy content of the food was estimated. Calculation of daily ration (as a percentage of body weight) was based on turnover rates (y) of food consumed at different temperatures (y = .545 e•º⁹¹x, where x is in °C). Feeding experiments using natural foods indicated assimilation efficiencies of 85.5%. Continuous (20 hr) measurement of routine metabolism of individual fish was made with a respirometer situated in Marion Lake, between, August 1967 and October 1968. Rate of oxygen uptake (Q) in ml/hr, at 15 °C was plotted against weight (W) in kg (Q = 62.6 W•⁷⁶). To determine the amount of energy trapped within the two populations, sample fish were dried (gonads separately) and burned in a calorimeter. The percent moisture of the fish was 74-78% during the year with marked increases at spawning time (rainbow 82%, kokanee 88%). The seasonal changes in energy content of immature fish were small (rainbow trout 6050-6200 cal/ash-free g; kokanee 6150-6300 cal/ash-free g). However mature fish underwent sharp reductions in energy content prior to spawning (rainbow trout 6100 to 5500 cal/ash-free g; kokanee 6100 to 5^00 cal/ash-free g). The pattern of seasonal change in energy content of the gonads was similar to that of the body. Average biomass present in the lake was 1.53 x 10⁴ Kcal/ha (12.3 kg/ha) for the rainbow trout, and 2.23 x 10⁴ Kcal/ha (16.6 kg/ha) for the kokanee. Annual production for the two populations was 2.81 x 10⁴ Kcal/ha. The ratio of production to biomass for rainbow trout was 0.80, and for kokanee was 0.71.

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