Challenges in mixed-stock fishery management: The case of the European whitefish in Kvarken, northern Baltic Sea

Fisheries management units—i.e. ’stocks’—frequently match poorly with biological variation. This mismatch poses a management challenge particularly when within-species units differ in their resilience to harvesting. It is therefore essential to both identify the distinct biological units and adjust fishing pressure accordingly. In the northern Baltic Sea, the European whitefish (Coregonus lavaretus) has two ecotypes, anadromous and sea-spawning, harvested primarily with gillnets. The former is endangered due to river connectivity issues and fishing pressure, while the latter is faring better. However, the key elements of effective management, accurate stock component identification and stock-component-specific fishing pressure adjustment, have not been assessed in this mixed-stock fishery. To address this knowledge gap, we compared ecotype identification methods and examined how fishing depth, season, and gillnet mesh size influence the whitefish catch composition. First, we assessed how well the ecotypes could be identified using phenotypic traits (gill rakers and growth) and genetic data (SNP genotyping). While both approaches were useful, many individuals exhibited intermediate characteristics and were not reliably identified, with phenotypic and genetic identification approaches agreeing only moderately. The prevalence of individuals with intermediate traits may reflect past stocking practices, habitat degradation, or inherently inconsistent homing behaviour. Nevertheless, we caught a higher share of sea-spawner-like individuals from deeper waters, earlier in the season, and with gillnets of smaller mesh size. The fact that it was possible to markedly affect the catch composition suggests that spatial and temporal fishing regulations have potential in the management of whitefish mixed-stock fisheries.