Abstract— Age determinations of whitefish (Coregonus lavaretus (L.)) were compared in two different tests. In the first test, the readers determined the age of each individual from one calcified structure (scale, otolith, or opercular bone) at a time. The samples from three populations, 50 specimens in each, were mixed so that the readers did not know which population each calcified structure was from. A sample of known‐age whitefish was used in the second age determination test, where information such as time of catch, length, weight, and sex was available to the readers. In each of the 50 envelopes the reader got scale impressions and two otoliths, one of which had been burned and ground. In the first test, the precision of the readers was low both between readers and between different structures. In the samples of slow‐growing populations, the determinations made from the otoliths showed older ages than the determinations from the scales. In the second age determination test the results were better; 73‐90% (average 82%) of the determinations were correct. The use of two calcified structures and the knowledge of the material were considered to improve the accuracy. Age determination bias may occur that affects the age distribution: even though 80% of the fish were aged correctly, an exceptionally strong or weak year class could remain unidentified. The estimation of growth rate seemed less sensitive to incorrect age determination than age distribution.

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Wild and non‐native hatchery‐reared brown trout, Salmo trutta L., released when 2 summers old, were caught in the littoral habitat of Vinstervatna Reservoir, southern Norway. Hatchery‐reared brown trout grew more slowly and had a smaller asymptotic length (293 ± 71 mm CL) than native fish (391 ± 56 mm CL). Hatchery‐reared brown trout also exhibited significantly shorter life spans than native fish. This category consisted mainly of individuals aged 2+ and 3+ years, and only 1.5% of the specimens were aged ≥5 years. The ages of the native fish in the sample were between 2 and 8 years, and the most abundant age groups of trout were 4+ and 5+ years. It is suggested that the differences in life‐history characteristics are related to adaptations by the native trout to the local environmental conditions. In this reservoir, which has a limited food supply as a result of water level fluctuations and a high level of inter‐ and intraspecific competition, environmental effects might be significant.

No abstract