Numerical Changes and Population Regulation in Young Migratory Trout Salmo trutta in a Lake District Stream, 1966-83

(1) This 18-year study of numerical changes in a trout population is part of a general study on the population dynamics, growth and distribution of migratory trout, Salmo trutta L., in Black Brows Beck, a stony stream in the English Lake District. (2) Earliest and latest spawning dates were 3 November and 7 December; larger females spawned earlier than smaller females. Eggs per redd and mean egg weight increased with female size and decreased with time from the start of the spawning period. Migratory trout produced more and larger eggs per unit length of fish than did resident trout in a neighbouring stream. (3) In laboratory and stream experiments, 50% of eggs hatched and 50% of alevins emerged after 444? days and 852? days from fertilization, respectively. Egg weight positively affected alevin size at hatching and emergence from the gravel, but not development times. Survival times of newly-emerged fish kept in the laboratory without food were positively related to weight at emergence. (4) Population densities of eggs and alevins were very similar within but differed between year-classes (range: 12-133 eggs m-2). The dominant factor affecting population densities of young trout was the population density of eggs, and hence alevins, at the start of each year-class. Alevin emergence time, weight of eggs or alevins, water temperature, numbers of older trout and numbers of other fish species (chiefly Cottus gobio L.) had no obvious effect. The number of recruits to each year-class was related to the initial egg density by the Ricker, dome-shaped, stock-recruitment curve. Maximum recruitment was estimated as 7.2 fish m-2 (95% CL = 6.5-8.0) soon after the fry stage ended in May/early June and 2*0 (? 0-5) fish m-2 in late August/September. Optimum egg densities for these values were estimated as 40.0 m-2 (95% CL = 38.5-41-7) and 62.5 m-2 (55.6-71-4) respectively. (5) Mortality was very low for eggs and alevins but high for young trout trying to establish feeding territories. The instantaneous mortality rate was linearly related to egg density and hence was density-dependent. The mortality rate for June to August was also negatively related to, and appeared to compensate for, the high mortality rate in May/early June.