The pattern of natural mortality throughout the life cycle in contrasting populations of brown trout, Salmo trutta L.

Abstract

Mortality is compared in two neighbouring streams: Black Brows Beck that serves as a nursery for the progeny of estuarine and sea-trout; Wilfin Beck is populated only by resident, non-migratory trout. The general pattern of mortality was similar in the two streams even though population densities in Black Brows were about ten times those in Wilfin Beck for similar life stages. Instantaneous mortality rates were highest in spring in both streams. Although spring mortality rates were positively density-dependent in Black Brows but not in Wilfin Beck, mean values were not significantly different in the two streams. Mortality rates for the rest of the life cycle were lower, and fairly stable between year-classes from the same population, but were higher in Wilfin Beck for both the first and second summers of the life cycle and higher in Black Brows Beck for the first winter and the second winter plus the period before adults first spawn. Exceptions to this pattern were very high mortality rates in a few year-classes in Black Brows Beck. These high values were all associated with summer droughts that had the most marked effect on trout in the second year of the life cycle. Comparisons between surviving and moribund trout in Black Brows Beck showed that the moribund fish were underweight for their length, had a higher water content and lower fat and energy contets, all being typical symptoms of starvation. A preliminary investigation of mortality at sea showed that 37% of the variation between year-classes in the number of females returning to spawn in Black Brows Beck could be explained by variations in the number of smolts produced in each year-class. The positive relationship between female density and smolt density indicates a trend towards constant proportional survival at sea and the similarity of sea mortality rates in many year-classes supports this trend. Exceptionally high mortalities at sea in three year-classes were associated with low smolt weights, these being due to poor growth in year-classes affected by summer droughts.