Abstract
SUMMARY (1) A 22-year study (1967-88) of migratory trout, Salmo trutta L., in Black Brows Beck (English Lake District) showed that population density is regulated chiefly by densitydependent survival in the early life-stages. Sampling in 8 years was more frequent than in the long-term study and these samples provide the information used in the present investigation. (2) In the long-term study, survival appeared to be negatively density-dependent in the first spring and positively density-dependent in the first summer of the life cycle, but the more frequent sampling revealed that this puzzling reversal was simply the result of infrequent sampling. (3) The more frequent samples showed that there was a critical survival time (CT days
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