Since its first appearance in Lake Constance in 2016 the invasive quagga mussel Dreissena rostriformis bugensis has come to dominate the mussel community and now occurs in hyperabundant densities over the whole lake bottom. A lake‐wide field study was conducted between 2019 and 2020 to obtain a systematic insight into the importance of this novel source of potential prey for the native fish community. In total 664 fish of 20 different species were caught and their digestive tracts were analysed. Meanwhile, quagga mussels were sampled to assess their calorific energy value by size and season. Regressions of septum length on maximum valve length were used to evaluate size‐specific mussel consumption by the three dominant quagga‐eating fish species. The study shows that nearly all benthivorous fish are able to forage efficiently on quaggas. However, in the case of one keystone species with very high commercial interest, the benthic whitefish Coregonus spp., quagga consumption is more limited as only individuals larger than 35 cm consumed quaggas in relevant amounts. The energy content of quagga mussels is positively size dependent with seasonal effect and elevated values during summer for medium‐ and large‐sized mussels. Even at its peak, the calorific value of quaggas by weight is much lower than that of endemic aquatic invertebrates, while single mass is high. Future implications for fish stocks, food web integrity and possible fishery management options are discussed.
Scanning electron microscopy (SEM) has been widely used to describe interspecific differences in egg quality of teleost freshwater fish, but potential intraspecific differences are poorly studied. Eggs of many rheophilic cyprinids are covered with adhesive structures such as attaching villi facilitating egg attachment at substrates of spawning grounds with high currents. Recent findings indicate that the egg quality of the rheophilic cyprinid common nase (Chondrostoma nasus L.), a target species of conservation, differs in the adhesiveness between spawning populations, potentially explaining differences in recruitment success. In this study, a SEM image‐based standardized protocol was established to assess egg surface quality of Chondrostoma nasus eggs. Multivariate statistics detected significant differences of egg surface properties among individual females and among three different populations. These differences were mainly attributed to length variability and merging of adhesive villi as well as to coating and filament‐like connections of these structures. The findings of this study highlight the need for further investigations to better understand the relationship of egg surface properties, egg stickiness and hatching success to understand the recruitment ecology and performance of early life stages in freshwater fish.
The three-spined stickleback Gasterosteus aculeatus was introduced into Lake Constance in the 1940s and occupied a limited range until late 2012. Since then the species has expanded from a solely littoral habitat in Upper Lake Constance, but now makes seasonal migrations into the pelagic zone. This behavioral change has been accompanied by a drastic increase in stickleback abundance. In order to integrate information about feeding of sticklebacks in Upper Lake Constance over two consecutive years, stomach content analysis was combined with seasonal stable isotope analysis on two types of tissue (muscle and liver). Isotope values were also obtained for zooplankton, whitefish larvae and eggs. We calculated the contribution of potential food sources for sticklebacks’ diet using a Bayesian mixing model (SIMMR). Furthermore, we determined stickleback trophic position, and δ15N and δ13C values were compared with those of other fish species of Lake Constance. The results of the Bayesian model as well as the stomach content analysis showed clear evidence of stickleback predation on fish eggs and larvae. Stickleback δ15N values were elevated during winter and comparable to those of piscivorous pike, while δ15N values of zooplankton were reduced, and those of whitefish larvae were similar to those of sticklebacks after accounting trophic fractionation of N isotopes. Trophic position calculations further identified sticklebacks as piscivorous, while the δ13C values of the liver and stomach content analysis suggests that a benthic-pelagic species pair may exist in Lake Constance. These findings support the hypotheses that sticklebacks in Lake Constance can display piscivorous feeding behaviour on sympatric fish species, most likely whitefish larvae and eggs.
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