Growth, Size, Biomass and Production of Young Migratory Trout Salmo trutta in a Lake District Stream, 1966-83

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Abstract
The objectives were to identify the chief factors affecting the growth of 1st yr trout in 17 year-classes and to develop a population dynamics model as part of a larger investigation on migratory trout, S. trutta L., in Black Brows Beck. Mean size and total biomass of alevins at the start of the growth period varied considerably between year-classes and were chiefly determined by egg weight and abundance. Mean size and growth rate of 1st yr trout varied considerably between year-classes, were not related to egg or alevin density and were thus not density-dependent. Water temperature and alevin weight were the chief factors affecting growth; food was not a limiting factor in most year-classes. Individual variation around mean size was inversely related to initial density of eggs or alevins and was thus density-dependent. It was low for alevins, increased markedly during the 1st mo. of growth and remained fairly constant within each year-class during spring and summer. Biomass and production were related to egg or alevin density by a dome-shaped curve but were also affected by variations in growth rate, especially towards the end of the summer. Their large variation between year-classes was determined by density-dependent mortality and to a lesser extent by density-independent growth. Egg or alevin density, temperature, time and alevin weight were thus the chief factors affecting the population dynamics of 1st yr trout and these were collated in a preliminary model that was a good fit to the data in all year classes except those for the dry summers of 1969, 1976 and 1983. The problem of predicting catastrophic droughts and their effects on the population dynamics of the trout cannot be resolved in an investigation over only 18 yr and illustrates the need for even longer analyses before populations can be modeled realistically.