Large changes in fish community structure are expected under changing climate. In the boreal region, the interplay between temperature and water color in aquatic systems is especially important because under warming conditions brownification (TOC) is expected to increase and experiments have shown conflicting results on the relative importance of temperature and TOC and the interaction between the two on fish communities. Fish using both littoral and pelagic habitats are of particular interest since both temperature and TOC is suggested to affect spatial habitat use of fish.
The objective of this study was to use catch data from a wide variety of Swedish lakes to examine large scale trends in perch catch in relation to temperature and TOC. We aimed to explore shifts in habitat use and determine the relative importance of both temperature and browning on each population.
Our study uses catch data from multi-mesh gillnet sampling of Swedish lakes, aggregated for the 2016 Sweden and Norway Intercalibration Report, specifically lakes sampled using both benthic and pelagic nets. We split each perch catch into sub-populations of pelagic and littoral perch for analysis, which allowed us to examine changes in catch and biomass in littoral and pelagic perch along a latitudinal gradient.
We focused our analysis on catch per unit effort (CPUE), biomass per unit effort (BPUE) and Average Weight, because these measures are able to give insight into changes in the community structure and account for both fish size and number. Our model shows that the biomass of pelagic perch populations is controlled by altitude, mean lake depth, and total phosphorous, all of which are acting on the number of individuals, not their average weight. Littoral populations on the other hand were impacted by latitude and water color, with latitude influencing average individual weight, and water color influencing number of individuals. Interestingly, average water temperature (0-6m), did not explain a significant amount of variation in catch or average weight, though perhaps this is because the temperature only reflects a specific moment in time and not water temperature over the course of the entire year. Generally, our findings show some support for past studies linking fish catch and body size to temperature (using latitude as a proxy for temperature), though we are also able to show that brownification changes the strength of this relationship and has differing impacts within different lake habitats.