PhD defence: Emil Kristensen

Freshwater fish in new lakes – Colonization, Species richness, Management, and Environmental factors

Supervisor: Kaj Sand-Jensen, Dept. of Biology, University of Copenhagen

Co-supervisor: Theis Kragh, University of Southern Denmark

Dean Jacobsen (Chair) Dept. of Biology University of Copenhagen
Anders Nilsson Aquatic ecology Lund University
Johanna Mattila Department of Aquatic Resources Swedish University of Agricultural Sciences

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Land-use changes have reduced the number of shallow lakes and wetlands throughout both North America and Europe between 1750 and 1980, where up to 80% have been drained and in many instances claimed for agricultural land. To counter this development, the establishment of new lakes has gradually gained momentum throughout Europe and North America during the last 40 years as a way to remove nutrients, manage extreme rain events, establish recreational areas, and increase biodiversity. Globally, more than 184,000 square kilometers of new permanent water bodies have appeared between 1984 and 2015, mostly due to the construction of reservoirs. In Denmark alone, more than 70 lakes over 10 ha have been established during the preceding 40 years with many more planned for establishment. An important species group in lakes are freshwater fish which to a large degree influence trophic interactions. In this thesis, I investigate how the colonization of fish takes place, the determinants for fish species richness, management possibilities, and potential pitfalls for the fish communities in newly created lakes. Concerning colonization of the new lakes, we found that fish migrated quickly into a newly established lake from nearby refugia with fast initial colonization followed by saturation of the species pool. The fish community composition showed increasing similarity over time compared to that in natural lakes in the region and even more to the two directly connected lakes regarding both species occurrence, catch per unit of effort (CPUE), and biomass per unit of effort (BPUE). On a broader scale, we found that species richness was not different in new lakes compared to natural lakes if they were connected to the stream network. However, the nonconnected new lakes had significantly fewer species than the natural lakes.

The species in 892 drainage basins, 179 natural lakes, and 52 new lakes, consisted most often of perch (Perca fluviatilis), roach (Rutilus rutilus), and pike (Esox lucius) and we generally found the same species in natural and new lakes. However, some species differed between the two lake types. Species such as ruffe (Gymnocephalus cernua), bream (Abramis brama), and white bream (Blicca bjoerkna), were in fewer of the new lakes compared to the natural lakes. However, species like tench (Tinca tinca), crusian carp (Carassius carassius), and common carp (Cyprinus carpio), were found more often in new lakes than in natural lakes. Following the immigration pattern in a single new lake, we found that the first colonizer was three-spined stickleback (Gasterosteus aculeatus) followed by roach and pike, which dominated the fish biomass in the following years. However, perch immigrated in low numbers and showed, together with pike, limited recruitment success. This could prove problematic since large predatory fish species are important for the trophic structure in lakes.