Fish communities in Italian sub-alpine lakes: Non-native species and anthropogenic pressures increase community dissimilarities.

In European lakes, anthropogenic pressures have increased significantly since the 1950s, facilitating colonisation by non-native species and increasing the potential for further invasions. Here, we determined the effects of anthropogenic pressures (i.e., habitat alterations and introduction of non-native species) on the fish communities of Italian sub-alpine lakes. We hypothesised that established non-native species would have more competitive traits against anthropogenic stressors, such as habitat alteration, than native species. Thus, we expected that non-native species would dominate lake communities and reduce native species occurrence and abundance depending on the degree of anthropogenic alterations. Overall, we predicted that the increase in anthropogenic pressures after the 1950s had led to homogenisation of the fish communities of the lakes in the region. We tested these hypotheses using data on 15 sub-alpine lakes, covering a broad geographical and morphological gradient, and compared the 2007-2014 fish community composition (sampled according to the CEN protocol plus point-abundance electrofishing) with variables of lake habitat and anthropogenic pressures (based on the Lake Habitat Survey, a method to evaluate the hydromorphological conditions of lakes according to the European Water Framework Directive) and fish communities before 1950, the latter based on bibliographic information. Following our hypothesis, non-native species showed higher prevalence of traits that increase their competitiveness against anthropogenic alterations (e.g., tolerance to pollution). In addition to lake morphology, the community composition of non-native fish determined as abundance (NPUE) and biomass (BPUE) was positively related to anthropogenic pressures. Since the 1950s, 19 non-native species have colonised the Italian sub-alpine lakes, and the occurrence of native species has decreased by ~27%. However, contrary to our expectation, these changes have increased the ß-diversity of the fish communities in the lakes.

What's the effectiveness of stocking actions in small creeks? The role of water discharge behind hatchery trout downstream movement.

Fish stocking to enhance freshwater fisheries or to improve the conservation status of endangered fish species is a common practice in many countries. Little is known, however, of the effectiveness of these practices in spite of the high efforts and investments required. The movement of subadult/adult hatchery-released brown trout Salmo trutta L. was studied by passive telemetry in a small tributary of Lake Lugano (i.e., Laveggio Creek, Canton Ticino, Switzerland). Hatchery fish, together with some resident wild individuals sampled during electrofishing surveys, were tagged with Passive Integrated Transponders (PIT) tags. Hatchery fish were released upstream and downstream a submersible monitoring antenna, which was anchored to the streambed in a pass-over orientation. The number of hatchery fish detected daily by the antenna (divided between fish released upstream and downstream the antenna) was analyzed in relation to the daily water discharge, to search for similar patterns in their fluctuation over time. Only the movement of fish released upstream the antenna displayed a significant relationship with water discharge, with the highest number of fish detected during periods of high-water flow, occurring after heavy rains. High-water discharge events had a significant role in hatchery trout downstream movement in our study site, likely acting as a driver for the downstream migration to Lake Lugano. Such events contributed to the poor effectiveness of stocking actions in this small tributary, providing further evidence against stocking strategies based on subadult/adult fish.