Evaluating the effectiveness of stocking vendace (Coregonus albula (L.)) eleutheroembryos by alizarin marking of otoliths

Lake Wcrbcllin (804 ha. 18 m mean depth). a mesotrophic lake in north-eastem Germany yielded between <] and 14 tons (1.2 and 17.4 kg/ha) of vendace during the] 970's and 1980"'.;. For the last 20 years. the lake has been stocked annually with 0.5 3.3 million eleutheroembryos (620 4100 per ha) in early spring. but the population is presumably self-sustaining. The contribution of stocking to commercial yields has never been assessed. In spring \994. 1.5 million embryos were marked by immersion In Alizarin Red S ( ]50 mg/] for .' h) prior to stocking. Survival rare was better than 90 9; after 48 h (76 ()( in one tank where the concentration had been higher initially) and ranged from 74 to 83 o/e after 24 d (21 o/r in one tank). Mark quality was excellent. as was mark retention after almost two years. In autumn 1995. more than one thou­ sand specimens of the 1994 year-class were sampled from the lake. Lapilli and sagittae were removed. embedded in epoxy revrn. ground and polished. and examined under ultraviolet light. Only 1.3 % of fish (14 specimens) were marked. We conclude that natural reproduction in Lake Werbellin was successful in 1994. The cost/benefit ratio of stocking was above unity. Evaluation of stocking programs in which cleutheroembryo-, are added to self-sustaining coregonid populations is strongly recommended.