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Abstract

The status of nationally managed fish stocks in Sweden is assessed following the temporal development of three indicators reflecting their abundance, size structure and mortality. The assessments are conducted by either comparing the indicator values in relation to reference points set by management, or if reference points do not exist, by investigating indicator values’ trends over time. In practice, time trends are the most used approach.For the European perch (Perca fluviatilis) populations along the Swedish coast of the Baltic Sea, assessments have mainly been based on the two indicators representing abundance and size structure. Although an indicator reflecting total mortality has also been included as part of the assessment in more recent years, the temporal variability of this indicator is currently not assessed. Moreover, despite that a rigorous length-stratified sampling program, which includes data collection of age, sex, and many other biological variables of perch individuals, is conducted along the Swedish coast of the Baltic Sea, this valuable data set is currently not used to a larger extent in the assessment of local perch populations.In this report we develop a population assessment model to be used for female part of perch populations along the Swedish Baltic Sea coast. The model is age-based and it can be used to track variability in mortality, recruitment and abundance of females in the populations. Moreover, temporal changes in indicators can be assessed per age-class, enabling a more detailed description of population status than done previously, where mortality was estimated at the whole population scale. Furthermore, uncertainty in population indicators naturally follow as an output from the modelImplementations of the model on multi-mesh gillnet data from eleven local perch populations along the Baltic Sea coast revealed some general patterns. For example, the model suggests that total mortality has increased over time in several local perch populations along the southern Swedish Baltic Sea coast and that age-class specific mortality in the populations increase with age. Four explanations are specifically laid out as potential reasons behind the observed pattern of increasing mortality with age for local perch populations. First, size selectivity in the multi-mesh gillnets may cause smaller and younger individuals to be under-represented as compared to larger and older individuals in gillnet catches, inferring that higher mortality of younger individuals and hence a less pronounced age-mortality pattern would be observed if size selectivity was accounted for compared to when it is not. Second, fishing mortality that mainly affects large individuals in the populations may increase the mortality of older individuals as compared to young individuals. Third, size selective predation mortality may cause a general age-specific pattern in total mortality. Fourth, changes in habitat utilisation of perch individuals may change during ontogenetic development, causing older individuals not to be caught representatively in the multi-mesh gillnets. The relative importance of the proposed explanations cannot at present be disentangled, and we hence conclude that the observed age-mortality relationship observed here likely results from multiple reasons, both described and to date unknown. More studies are therefore needed to address the relative importance of the reasons proposed here.Overall, the population dynamic model developed here is a tool that can be used to study variability in indicators, both at the population level and on a more detailed age-class specific level. Hence, it can be used to assess the status of local European perch populations, results of which may be used to pinpoint actions needed for strengthening and conserving local perch populations. Such results are of importance both for Fisheries and Marine Environmental Management.

Keywords

European perch; age-base model; population indicator

Published in

Aqua notes
2026, number: 2026:1
Publisher: Department of Aquatic Resources, Swedish University of Agricultural Sciences

SLU Authors

Associated SLU-program

Coastal and sea areas

UKÄ Subject classification

Agricultural Science
Fish and Wildlife Management

Publication identifier

  • DOI: https://doi.org/10.54612/a.7bs7klrgkg
  • eISBN: 978-91-8124-157-0

Permanent link to this page (URI)

https://res.slu.se/id/publ/145320