10.1007/s10750-012-1182-1Contact CEH NORA team at noraceh@ceh.ac.ukThe NERC and CEH trademarks and logos ('the Trademarks') are registered trademarks of NERC in the UK and other countries, and may not be used without the prior written consent of the Trademark owner. form, nor will it be during the first three months after its submission to Hydrobiologia."Corresponding author: Erik Jeppesen (ej@dmu.dk)We dedicate this paper to the late Prof. Jürgen Benndorf, a true pioneer and mentor in lake and reservoir management oriented research, who inspired a number of us to initiate longterm comprehensive experimental ecological studies on lakes and reservoirs. AbstractFish play a key role in the trophic dynamics of lakes. With climate warming, complex changes in fish assemblage structure may be expected owing to direct effects of temperature and indirect effects operating through eutrophication, water level changes, stratification and salinisation. We reviewed published and new long-term (10-100 years) fish data series from 24 European lakes (area: 0.04-5648 km 2 ; mean depth: 1-177m; a north-south gradient from 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 4 Sweden to Spain). Along with an annual temperature increase of about 0.15-0.3 °C per decade profound changes have occurred in either fish assemblage composition, body size and/or age structure during recent decades and a shift towards higher dominance of eurythermal species.These shifts have occurred despite a reduction in nutrient loading in many of the lakes that should have benefited the larger-sized individuals and the fish species typically inhabiting cold-water, low-nutrient lakes. The cold-stenothermic Arctic charr has been particularly affected and its abundance has decreased in the majority of the lakes where its presence was recorded. The harvest of cool-stenothermal trout has decreased substantially in two southern lakes. Vendace, whitefish and smelt show a different response depending on lake depth and latitude. Perch has apparently been stimulated in the north, with stronger year classes in warm years, but its abundance has declined in the southern Lake Maggiore, Italy. Where introduced, roach seems to take advantage of the higher temperature after years of low population densities. Eurythermal species such as common bream, pike-perch and/or shad are apparently on the increase in several of the lakes. The response of fish to the warming has been surprisingly strong and fast in recent decades, making them ideal sentinels for detecting and documenting climate-induced modifications of freshwater ecosystems.

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The fish kill in lake Peipsi (Estonia/Russia) during the extraordinarily hot summer of 2010 evoked an investigation into the effects of environmental extremes and long-term eutrophication on the fish community of the lake. Current data on lake Peipsi indicate that temperature extremes and synergistic interactions with eutrophication have led to a radical restructuring of the fish community. Commercial landings of lake smelt, Osmerus eperlanus eperlanus m. spirinchus (Pallas), the previous dominant species of the fish community, have decreased dramatically since the 1930s, these declines being coupled with summer heat waves coinciding with low water levels. Gradual decline in smelt stock and catches was significantly related to a decline of near-bottom oxygen conditions and to a decrease in water transparency. The first documented fish kill in 1959 occurred only in the southern, most shallow and eutrophic lake (lake Pihkva). Recently, summer fish kill have become more frequent, involving larger areas of the lake. In addition to the cold-water species, e.g. smelt and vendace Coregonus albula (L.), the abundance of bottom-dwelling fishes such as ruffe Gymnocephalus cernuus (L.) and juvenile fish have significantly decreased after the 2010 heat wave probably due to hypoxia and warm water temperatures. This study showed that fish community structure in large shallow lakes may be very vulnerable to water temperature increases, especially temperature extremes in combination with eutrophication

Pikeperch is a major predator in the pelagic zone of eutrophic waters, such as the large north-temperate lowland lakes Võrtsjärv (Estonia) and Peipsi (Estonia/Russia). The size and structure of the pikeperch population is strongly influenced by their success at the juvenile stage. Therefore, we investigated the diet and prey selection of pikeperch fry caught in the ice-free period in lakes Peipsi and Võrtsjärv in 2007 and 2008. We analysed the stomach contents of 635 pikeperch from Lake Peipsi and 202 pikeperch from Lake Võrtsjärv, and compared our findings with similar data from the 1950s (Erm, About Biological and Morphological Differences of Pikeperch. Hydrobiological Researches II (in Estonian), 1961). Analysing 4-20 cm long fry, we studied differences in prey size, seasonal diet patterns and the ontogenetic diet shift. In both lakes, 0? pikeperch feed mostly on large predatory zooplankters. However, in Lake Peipsi the stomach content weight and the average number of food items in stomach were higher, and the food spectrum was wider than in Lake Võrtsjärv. There was also a difference in the type of food that dominated fry's stomach content (calculated by weight) in the two lakes. In Lake Peipsi, chironomids larvae, as well as zooplankters Daphnia galeata and Bythotrephes longimanus dominated, while in Lake Võrtsjärv zooplankters Mesocyclops leuckarti and Leptodora kindti. Seasonal analysis showed that cladocerans dominated in pikeperch fry stomach content in summer and at the beginning of September, but copepods were dominant in autumn and spring. In contrast to the studies carried out from 1952 to 1958

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