THE RELIABILITY OF WHITEFISH (COREGONUS-LAVARETUS (L)) AGE-DETERMINATION - DIFFERENCES BETWEEN METHODS AND BETWEEN READERS

Age determinations of whitefish (Coregonus lavaretus (L.)) were compar ed in two different tests. In the first test, the readers determined t he 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 wh itefish was used in the second age determination test, where informati on 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 fir st test, the precision of the readers was low both between readers and between different structures. In the samples of slow-growing populati ons, the determinations made from the otoliths showed older ages than the determinations from the scales. In the second age determination te st the results were better; 73-90% (average 82%) of the determinations were correct. The use of two calcified structures and the knowledge o f the material were considered to improve the accuracy. Age determinat ion 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.