Growth, Size, Biomass and Production in Contrasting Populations of Trout Salmo trutta in Two Lake District Streams

Abstract

SUMMARY (1) Resident brown trout Salmo trutta L. in Wilfin Beck were smaller than migratory trout (progeny of 'sea trout') of similar age in Black Brows Beck. This investigation seeks an explanation for the size difference and examines its effect on biomass and production. (2) Mean sizes and growth rates for different life stages varied considerably between year-classes in both populations; they were not density-dependent. Water temperature and fry weight were the chief factors affecting growth in the first 3 years of the life cycle in Wilfin Beck and all the freshwater phase in Black Brows Beck. Food was not a limiting factor, except for the first autumn-spring period of the life cycle and for adults over 3 years old in Wilfin Beck. (3) Variation in individual size (measured by C.V.) was not stable, but was inversely density-dependent in Black Brows Beck and decreased with age in Wilfin Beck, the latter relationship suggesting a greater intensity of natural (stabilizing) selection in Wilfin Beck. (4) Fish biomass at the end of the summer in Wilfin Beck (mean for 1966-72, 79 g m-2) was c. 60% that in Black Brows Beck (mean for 1967-83, 13-0 g m-2), but annual production in Wilfin Beck (mean 6.1 g m-2) was only c. 25% that in Black Brows Beck (mean 24-4 g m-2). Migratory behaviour, a major genotypic difference between the populations, ensured that larger trout did not remain to use resources such as food and space in Black Brows Beck, whereas these older trout remained in Wilfin Beck, forming a major part of the biomass whilst contributing little to annual production. (5) Agreement with a growth model indicated no genotypic differences in growth rates between the two populations. Smaller fry size at the start of the growth period, not lack of food, was chiefly responsible for the smaller trout in Wilfin Beck. The larger fry size in Black Brows Beck was related directly to egg weight which, in turn, was related to the larger spawning females and to a higher proportion of the female's resources being invested in egg production (30-37% of female's total energy content in Black Brows Beck, 16-17% in Wilfin Beck). Some genotypic differences were therefore responsible for fry size and hence size differences between the two populations.