Population Regulation for Different Life-Stages of Migratory Trout Salmo trutta in a Lake District Stream, 1966-83

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Abstract
From 1966-1983 in Black Brows Beck (English Lake District), most surviving trout spent 2 yr in fresh water, then migrated to the sea/estuary, and finally returned to spawn in fresh water as either males in their 3rd yr or males and females in their 4th yr. The sex ratio was .apprx. 1 for fish of similar age. Survivor density at 5 stages in the life cycle was related to egg density at the start of each yr-class; the density-dependent relationship was best described by the dome-shaped, Ricker-curve (from 6 stock-recruitment models fitted to the data). Losses during successive life-stages were effectively zero for the egg-alevin stage (loss-rate k0), density dependent in the 1st spring (k1) and summer (k2) of the life cycle, and density independent within the 1st winter (k3), 2nd summer (k4) and 2nd winter plus post-migration period (k5). Density-dependent survival of the young trout (k1, k2) was chiefly responsible for the stock-recruitment relationships for different life-stages; proportionate survival occurred between life-stages of older trout (k3, k4, k5). The key factor for population regulation was the spring losses (k1) soon after the young trout started to feed. A predictive equation was developed for total loss-rate (K = k0 + k1 + k2 + k3 + k4 + k5 = 4.399 + 0.665k1). Neither water temperature nor rainfall significantly affected loss-rates except those for 1 + parr (k4) in summer droughts. There were no obvious density interactions between trout of different yr-classes or between trout and other fish species in the stream. The relationship between total egg production by the surviving progeny and egg density at the start of a yr-class was density dependent; the equilibrium point (S*) was 80 eggs m-2 and the slope at this point indicated that S* should be stable. The age structure of the trout population in 1933-1934 was similar to that in the present study. The problems of separating losses due to mortality from those due to migration are discussed but not completely resolved. The implications of density-dependent population regulation and equilibrium theory are discussed and it is concluded that the population rarely exists in dynamic equilibrium because of environmental perturbations such as droughts.