Epsilon Archive for Student Projects

Abstract

Due to its wide distribution and predatory role, Perca fluviatilis is a key component of the Swedish
fish fauna, contributing to the food web and ecosystem functionality. Nevertheless, little is known
about the factors that influence the size of large individuals within perch populations. Few studies
have examined and compared spatial and temporal patterns of the size of large perch between
different aquatic systems. The data used for this thesis are from SLU Aqua's NORS and KUL
databases, covering three ecosystem types: coastal areas, large lakes, and small lakes. The main
objective of the thesis was to determine the mean L90, a size indicator used for large perch, and to
identify possible size-based spatial gradients in the study systems. Data were selected from gillnet
surveys, with a minimum perch size selection of 15 cm. Results showed a slight increase in L90
over time and a difference across the systems. The 2016-2022 mean size of a large Swedish perch
was 27.3 cm (respectively, coast: 26.1 cm, large lakes: 27.2 cm, and small lakes: 28.7 cm). A
significant difference in the mean L90 between the small lake and coastal systems was detected
during these six years. Also, the analysis of L90 averages revealed no significant spatial gradient
within systems, although mean differences were observed in several geographically close
monitoring areas. These variations suggest the existence of local growth patterns, unique habitat
characteristics, environmental factors, differences in the level of human impact, and/or geographical
isolation. Using the L90 indicator in the three systems studied raised some points. Although suitable
for fish populations in coastal areas and applicable to large lakes, its practical use would require
appropriate adjustments to the system. Indeed, further studies would be needed to determine whether
the L90 threshold value in the other two systems should be identical to that in coastal areas. This
study is the first L90 analysis in this system for small lakes since large fish are not involved in
assessing the good status of this ecosystem. As the size and number of fish caught in many small
lakes were below the data inclusion threshold, only a small subset of all lakes caught was included
in the analysis. Of the 2,000 perch lakes sampled in the SLU database, only a maximum of 86 small
lakes were included, representing less than 5% of the total number of lakes. To conclude, by
conducting L90 comparisons between coastal, large, and small lake systems for the first time, the
analysis revealed several interesting patterns with new and significant results. Furthermore, this
thesis confirms the need for future research on large individuals to focus on specific sections that
contain local populations with distinct mean L90 despite close geographical proximity to understand
how environmental, genetic, and anthropogenic factors influence large perch size.