Epsilon Archive for Student Projects

Abstract

The sex ratio, or the female proportion in a population, is one of the most basic demographic prop-erties that directly influences reproductive output and population dynamics. Spatial and temporal variation in sex ratio has been largely overlooked in the case of the ecologically important species like Eurasian perch (Perca fluviatilis) in the coastal part of the Baltic Sea. In this thesis gillnet monitoring data from up to 14 coastal locations (2007–2024) were analyzed with generalized linear mixed-effects models and principal component analysis to examine the variation in sex ratio across time and space along the Swedish Baltic Sea coast. The main aim was to identify biological and environmental factors that contribute to any potential sex ratio variation.

I found clear size-dependent sexual dimorphism, with a greater proportion of females in the larger size classes and a greater proportion of males in the smaller size classes. Neither latitude nor time had strong predictive power for explaining sex ratio variation observed.

The lack of a latitudinal gradient, the results imply that patterns in sex ratios are more likely driven by local factors, such as population abundance, fishing pressure, habitat quality, and the thermal environment. Moreover, I also found indications of a flattening of the sex ratio curves with size in several locations during the recent sampling years, suggesting that, there is shift towards more balanced sex ratio in most locations along the Swedish Baltic Sea coast.

In all, the results of my thesis suggests that water temperature and perch population abundance might be important local factors affecting the sex ratio variation in perch. This is relevant for fisheries man-agement and monitoring of the ecosystem, since size selective fishing and climate change-induced shifts in growth conditions can indirectly affect the reproductive capacity through changes in the sex ratio.