Proportion of river- and sea-spawning whitefish in catches at the Åland Islands (Baltic Sea), estimated from gill raker counts

The Åland Islands are feeding grounds for river- and sea-spawning whitefish (Coregonuslavaretus), including also hatchery-reared released whitefish. Management of fisheriesrequires knowledge of whitefish origin from their catches, but such information is deficientat the Åland Islands. The numbers of gill rakers had unique (discretized) Gaussianfrequency distributions in all three forms studied. This justified the usage of Gaussianfinite mixture models on gill raker data for estimating the proportions of the forms inmixed catches during the non-spawning season. From 14 catches at the Åland Islandsfrom 2012–2013, on average 54.8% were river-spawning whitefish, while the presenceof hatchery-reared, sea-spawning whitefish was low but uncertain. Using a hierarchicalvariant of the model, we showed that the proportion of river-spawning whitefish decreasedfrom 69.6% in June to 38.3% in August. Our study provides feasible methods for monitoringspatio-temporal variation in the proportions of river- and sea-spawning whitefish.