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Report - Peer-reviewed, 2023

Seasonal variation in thermal habitat volume for cold-water fish populations : implications for hydroacoustic survey design and stock assessment

Axenrot, Thomas; Degerman, Erik; Asp, Anders


For accurate stock assessment, survey design must consider fish behavior and ecology. Yearlings and older individuals of the commercially exploited cold-water species vendace (Coregonus albula) are found below the metalimnion through periods of thermal stratification. These stratification periods generally last for 3-4 months, from the middle of summer to early autumn. In lakes with heterogeneous distribution of depths, the habitat volume for vendace vary drastically within and across years, which affects the distribution and population densities. Variable thermal habitat volumes, with food and oxygen depletion in the hypolimnion through the period of stratification, may act as a population size-regulating factor.

Using hydroacoustics in combination with trawl data and temperature profiles, we examined the distribution of vendace through annual periods of thermal stratification. We found that yearling and older vendace these periods were confined to cold-water habitat volumes representing less than 10 % of the total water volume of Lake Mälaren, the third largest lake in Sweden. By introducing stratification to the design of hydroacoustic surveys supported by midwater trawling, seasonal aggregations of fish in temporally restricted thermal habitat volumes can be used to lower survey effort and improve the precision in estimates of population size. Temporally restricted habitat volumes may induce risks for the populations to over-fishing and sensitivity to environmental changes that potentially may call for directed management.


Habitat volume; thermal stratification; survey design; stock assessment; vendace; Coregonus albula

Published in

Aqua reports
2023, number: 2023:7
eISBN: 978-91-8046-847-3
Publisher: Department of Aquatic Resources, Swedish University of Agricultural Sciences