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First-year survival of brown trout in three Norwegian streams


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Monthly survival rates during the first year of life were estimated for the 1999 cohort of stream-resident brown trout Salmo trutta in three Norwegian streams, using capture-mark-recapture methods and the Cormack–Jolly–Seber model. It was hypothesized that reduced survival would occur during the winter. For one of the study populations, the data did support seasonal variation in survival, with monthly survival rates being lower during winter than during summer (0·65 v. 0·99). For the remaining two populations, there was no evidence for seasonal variations in monthly survival rates, but monthly survival rates were significantly different (0·87 v. 0·99). No evidence was found for size-dependent winter survival. Some marked individuals emigrated from the study sites, suggesting that survival rates were underestimated and that different survival rates among populations were partly due to different emigration rates. Net immigration of brown trout was evident at all three study sites.