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The size distribution of fish populations is of key importance both for population viability and food web function. Yet few fully operational indicators of size structure have been developed. Further, developing thresholds for indicators of the size distribution of fish species has proven challenging, and this lack of quantitative reference points limits the usefulness of size-based indicators in management. Here, we focus on the length at the 90th percentile of the size distribution (L90), an indicator that responds to anthropogenic pressures, requires no prior life-history information, and can be estimated with high precision and accuracy even from relatively small sample sizes. Building on a previous method that developed an operational threshold for L90 in perch in the Baltic Sea, we apply this method to two additional nationally managed fish stocks along the East coast of Sweden and in Swedish lakes: pikeperch (from coastal areas and lakes) and whitefish (along the coast). We also extend the analyses of perch by incorporating monitoring data on perch size structure in Swedish lakes. Using standardized monitoring data from coastal and lake ecosystems, we estimated annual L90 values while accounting for methodological sources of variation related to sampling gear, season, and habitat (coast versus lake) through linear mixed-effects models. To place L90 in a biologically meaningful context, we combined size-structure analyses with estimates of female length at sexual maturity (Lmat), derived from logistic regression models of maturity status. This allowed us to evaluate which proportion of individuals have reached reproductive size at a proposed L90 threshold.

We propose species-specific and, where relevant, gear- and season-specific threshold values for L90 that are indicative of good environmental status. For perch, the proposed thresholds range from 23 to 25 cm depending on gear type and season, which closely match estimates of length at 90% maturity. For pikeperch, clear differences between coastal and lake populations, together with a declining temporal trend in coastal L90 and relatively high maturity lengths, support the use of a precautionary coastal threshold of 39 cm. For whitefish, a threshold of 37 cm was identified, exceeding the estimated length at 90% maturity and thereby allowing most females to reproduce before being subject to fishing mortality. Our results show that L90, when evaluated jointly with life-history information such as length at maturity, may provide a robust, ecologically relevant, and practical indicator of population status. The proposed framework allows for context-dependent threshold setting while emphasizing the importance of avoiding shifting baselines. As such, this approach offers a transparent and useful tool for integrating size-based indicators into national and regional monitoring and assessment frameworks for fish populations.

Nyckelord

indicator; coastal fish; status assessment

Publicerad i

Aqua notes
2026, nummer: 2026:11
Utgivare: Department of Aquatic Resources, Swedish University of Agricultural Sciences

SLU författare

Associerade SLU-program

Kust och hav

UKÄ forskningsämne

Ekologi
Zoologi

Publikationens identifierare

  • DOI: https://doi.org/10.54612/a.36efr8lvp4
  • eISBN: 978-91-8124-197-6

Permanent länk till denna sida (URI)

https://res.slu.se/id/publ/146578