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Jůza T, Blabolil P, Čech M, Draštík V, Kubečka J, Muškaa M, Prchalováa M, Sajdlováa Z, Tušera M, Vašeka M, Vejříka L, Peterkaa J. 2024. Fish predation on newly stocked juvenile predatory fish in a biomanipulated reservoir. Lake Reserv Manag. 40:325–337. Juveniles of pikeperch, asp, and northern pike were stocked into the Římov Reservoir. Subsequently, potential fish predators were monitored by gillnets at the stocking sites and their diet was analyzed for presence of stocked fish. Asp and European perch were predominant predators, while pikeperch and northern pike were less abundant in gillnet catches. Digestive tract analysis revealed the highest percentage of ingested fish in northern pike, followed by pikeperch, asp, and European perch. The amount of stocked fish did not correlate with the number of predatory fish caught in gillnets. When asp was stocked, this species was strongly preferred by northern pike and partly by pikeperch and 0.3 individual of stocked asp was eaten per 1 kg of predatory fish captured by gillnets. During the northern pike stocking, this species was mainly preyed upon by asp and occasionally by northern pike, and 0.8 individual of stocked northern pike was eaten per 1 kg of predatory fish. When pikeperch was stocked, it was strongly preferred by pikeperch, northern pike, and European perch, it was almost absent in the diet of asp, and 1.1 individual of stocked pikeperch was eaten per 1 kg of predators. Our results indicate that pikeperch juveniles had the highest mortality rate due to predation after stocking, mainly due to sedentary behavior and the high proportion of cannibalism. In northern pike, cannibalism was found in only one case, and was never found in asp, which was under the lowest predation pressure. [ABSTRACT FROM AUTHOR]

The management of Invasive Alien Species (IAS) is often hindered by ecological, social and economic factors, resulting in inadequate biodiversity protection and inefficient use of public money. A clear example of such inefficient management in aquatic ecosystems is the European catfish Silurus glanis L. in southern Europe. Native to central Eurasia, S. glanis is an emblematic and controversial freshwater fish, being the subject of extensive and profitable trophy angling in central Europe and of commercial fishing in eastern Europe. Concurrently, in western and southern Europe where it was introduced in the XIX century, S. glanis is considered a problematic invader. The lack of comprehensive information on S. glanis invasive populations has limited effective management, which is critical to successfully control the spread and minimize negative impacts on native ecosystems and species. LIFE PREDATOR, started in September 2022 with a budget of € 2.85 million and a consortium of six partners from three countries, aims at developing a multidisciplinary and transnational approach to control established populations of S. glanis, and prevent further spreading and future introductions in southern European lakes and reservoirs. The project will develop and test an early warning system based on eDNA and citizen science and identify the most effective and selective capture techniques to reduce the abundance of catfish, particularly in Natura 2000 lakes, actively involving anglers and professional fishermen on this. Massive raising awareness campaigns will be conducted targeting anglers but also the general public, and protocols and best practices will be transferred to management authorities. For the long-term sustainability of the project, a South European Management Group will be created. Additionally, in northern Italy, where the catfish invasion is more advanced, a local circular economy will be implemented, involving the increase in fishing pressure by encouraging catfish consumption as food. [ABSTRACT FROM AUTHOR]

Diet plasticity is often studied in Eurasian perch (Perca fluviatilis), a species commonly described as having generalist populations composed of specialised individuals. Perch diet was examined using gut content analysis (GCA) and stable isotope analysis (SIA), and individual specialisation was calculated in two study lakes within 2 years. Mostly only one diet category was present in the perch stomach, with more variation in the diet in the Most lake compared to the Milada lake between 2013 and 2014. The calculated degree of individual specialisation indicated higher specialisation in the Most lake. Interestingly, despite the different or almost uniform diet composition between the years, the total niche width (based on SIA) of the population remained similar in both lakes. This suggests that the overall variation in the sources utilised by the entire population remained consistent between the years. GCA mostly indicated zooplankton as the prevailing food source, whereas SIA indicated significant utilisation of YOY fish earlier that year, an information that was completely missed by the GCA of fish caught in September. The differences between GCA and SIA results could be attributed to the different time intervals reflected by the methods, but possibly to the conversion of the diet into the body tissues that is reflected by SIA and may depend on the diet's nutritional values rather than the proportion of different prey consumed. [ABSTRACT FROM AUTHOR]

Year to year fluctuations in 0+ fish cohort strength are a common phenomenon. Many factors can affect cohort strength during the fish's early life period. In this study, development of a 0+ pikeperch Sander lucioperca cohort in the pelagic zone was studied by trawling for 50 days from first larvae hatching, in two consecutive years. In 2007, an abundant S. lucioperca cohort collapsed suddenly soon after hatching. After the incident, slow-growing S. lucioperca prevailed in the catch. In 2008, the catch gradually increased during the whole study period because of prolonged hatching. Environmental factors differed mainly in a slower temperature increase, higher water level and higher zooplankton abundance in 2008 compared to 2007. Our study revealed that a strong 0+ S. lucioperca cohort at the time of hatching might not result in a strong S. lucioperca cohort in general.