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The natural regulation of numbers and growth in contrasting populations of brown trout, Salmo trutta , in two Lake District streams
Author(s) -
ELLIOIT J. M.
Publication year - 1989
Publication title -
freshwater biology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.297
H-Index - 156
eISSN - 1365-2427
pISSN - 0046-5070
DOI - 10.1111/j.1365-2427.1989.tb01344.x
Subject(s) - salmo , brown trout , trout , biology , ecology , population , population density , fishery , demography , fish <actinopterygii> , sociology
SUMMARY. 1. This short review summarizes a long‐term investigation of brown trout in two populations that probably represent opposite extremes of life histories in this polymorphic species; Bhick Brows Beck serves as a nursery for the progeny of migratory trout (mixture of sea and estuarine trout) and Wilfin Beck is populated by resident trout. 2. Population density in Black Brows Beck was always much higher than that in Wilfin Beck, and was regulated by density‐dependent survival in the early stages of the life cycle. There was no evidence for similar density‐dependent regulation in Wilfin Beck; simple proportionate survival occurred with fairly constant loss‐rates. Survival was reduced in both populations by summer droughts and also by spates in Wilfin Beck. 3. Black Brows trout were always larger than Wilfin Beck trout of similar age; fry size at the start of the growth period was chiefly responsible for these differences. Variations in water temperature were chiefly responsible for differences in growth rates between year‐classes within each population. Food intake was not a limiting factor, except in the first winter of the life cycle and for adults over 3 years old in Wilfin Beck. Variation in individual size was inversely density‐dependent in Black Brows Beck and decreased with age in Wilfin Beck, these changes being due to natural (stabilizing) selection. 4. There is strong evidence for genotypic differences between the populations. The implications of this are discussed, especially the need to conserve different populations that may contain unique genetic material, and the importance of restocking with fish reared from the indigenous population that should always contain the optimum genotypes for a particular habitat. Restocking with juveniles should be done with caution because it could lead to a decrease in both numbers and size variation when the population is regulated by density‐dependent mechanisms. 5. One major objective of future work should be the development and improvement of mathematical models that can be used to predict the optimum density for trout in different populations, the maximum attainable growth rate in different habitats, and the effects on trout populations of environmental changes due to natural causes (e.g. droughts and spates) or human activities.