Gill raker counting for approximating the ratio of river- and sea-spawning whitefish, Coregonus lavaretus (Actinopterygii: Salmoniformes: Salmonidae) in the Gulf of Bothnia, Baltic Sea

Background. The ability to distinguish between stocks in mixed fi sheries is a prerequisite for a sustainable fi sheries management. In the Gulf of Bothnia the relative contribution of endangered river-spawning and sea-spawning whitefi sh, Coregonus lavaretus (Linnaeus, 1758), to fi sheries catches are currently not well known. This also applies to the southern Åland Islands, a major feeding ground for river-spawning whitefi sh from northern rivers. Riverand sea-spawning whitefi sh are mixed while away from the breeding grounds and off the spawning season, and cannot be distinguished based on external features. Materials and methods. Analysis on gill raker numbers of river-spawning (n = 480) and sea-spawning (n = 456) whitefi sh from twelve locations at the Finnish west coast and the Åland Islands was performed. In whitefi sh sampled from feeding grounds at the Åland Islands the strontium concentration was analysed in otoliths from fi sh (n = 20) with low (27) and high (30) number of gill rakers. Results. A marked difference in the mean gill raker number of the riverand sea-spawning whitefi sh stocks was observed. The weighted mean of gill rakers of whitefi sh caught at spawning locations showed that the number of gill rakers of fi sh from rivers and the sea were 29.9 ± 2.14 (n = 480) and 26.7 ± 2.21 (n = 456), respectively. The difference between the two groups was highly signifi cant (t = 22.50, df = 934, P < 0.0001). The means differed by 3.20 (2.92–3.48, 95% CL) indicating the groups are well separated. In whitefi sh sampled at feeding grounds at the Åland Islands, otolith strontium concentration was higher (t = 2.09, df = 18, P = 0.04) in fi sh having 27 gill rakers (3.86 ± 0.30 mg · g–1, n = 10), compared to those having 30 gill rakers (3.54 ± 0.35 mg · g–1, n = 10). Otolith strontium analysis thereby supported the utility of gill raker counting data for estimating the proportion of riverand sea-spawning whitefi sh in mixed populations. As expected, the gill raker counting method successfully indicated temporal alterations in the proportions of riverand sea-spawning whitefi sh on feeding grounds. Conclusion. Gill raker counting is an easy, fast, and inexpensive method that can be used to estimate the spatiotemporal occurrence and migratory patterns of riverand sea-spawning whitefi sh at the southern feeding grounds in the Gulf of Bothnia, and thereby aid in a sustainable management of whitefi sh stocks.