Accelerated growth of freshwater fish during anthropogenic eutrophication has been attributed almost exclusively to the increased nutrient content, while density-dependent effects have been largely neglected. We evaluated the relative importance of these factors by studying the growth of 43 consecutive year classes of common whitefish (Coregonus lavaretus) from Upper Lake Constance. This prealpine lake underwent eutrophication from the 1950s to 1970s, followed by reoligotrophication. Because whitefish are harvested with gill nets in a strongly size-selective way, we used back-calculated lengths of average fast-growing fish to compare growth among cohorts. Standing stock biomass was estimated based upon virtual year-class strengths. Multiple linear regression analysis revealed that growth of whitefish during their second year was most strongly related to standing stock biomass followed by PO 4 -P content during spring turnover and by calendar year, which was incorporated as a third independent variable (adjusted R 2 = 0.84). The negative correlation between whitefish growth rate and calendar year is interpreted as evidence of an evolutionary response to the highly size-selective fishery during at least four decades. We conclude that density-dependent effects on whitefish growth are more important than had been realized previously and that the impact of eutrophication on growth of whitefish needs to be reconsidered.Résumé : La croissance accélérée des poissons d'eau douce dans les conditions d'eutrophisation anthropique a été attribuée presque exclusivement à l'augmentation du contenu en nutriments; en revanche, les effets reliés à la densité ont été en grande partie négligés. Nous évaluons l'importance relative de ces facteurs dans une étude de la croissance de 43 classes d'âge consécutives de corégones lavarets (Coregonus lavaretus) du lac supérieur de Constance. Ce lac préalpin a subi une eutrophisation des années 1950 aux années 1970, suivie d'une ré-oligotrophisation. Parce que les corégones sont récoltés dans des filets maillants qui font une forte sélection en fonction de la taille, nous avons utilisé les longueurs obtenues par rétro-calcul de poissons moyens à croissance rapide pour comparer la croissance entre les cohortes. Nous avons estimé la biomasse du stock d'après les forces des classes d'âge virtuelles. Une analyse de ré-gression multiple linéaire montre que la croissance des corégones durant leur seconde année est surtout reliée à la biomasse du stock, puis au contenu de P-PO 4 durant le brassage du printemps et à l'année du calendrier qui a été incorporée à l'analyse comme troisième variable indépendante (R 2 ajusté = 0,84). Nous interprétons la corrélation négative qui existe entre la croissance des corégones et l'année du calendrier comme une indication de la réponse évolutive à la pêche très sélective au cours d'au moins de quatre décennies. Nous concluons que les effets reliés à la densité sur la croissance des corégones sont plus importants qu'on ne le croyait antérieurement et qu'...

The year-class strength (YCS) of Blaufelchen (Coregonus lavaretus) in deep Upper Lake Constance was analysed for a 52-year period, from 1947 to 1998. Despite strong anthropogenic influences on the species' population dynamics due to cultural eutrophication and oligotrophication, intense fishing, and large-scale stocking, the influence of climate variability associated with the North Atlantic Oscillation (NAO) is apparent in the data set. This influence is significant although large-scale stocking of cold-bred larvae was performed to avoid a mismatch of larvae with their food. The importance of stocking on YCS, however, is unclear and was only detectable when analysing a subset of the data. In addition to climate variability a yet unidentified factor related to zooplankton suitability as food for fish larvae, and density-dependent mortality probably related to cannibalism do significantly influence YCS. The NAO seemed to influence YCS twofold, through temperature effects on egg development time and on larval growth rate. The first of these two mechanisms is related to the NAO via a time lag of 1 year due to the specific mixing dynamics of warm monomictic Lake Constance. Hence, a warm winter in the year before spawning results in earlier hatching of larvae, that is, hatching is decoupled from the actual meteorological conditions. This should make the larvae very prone to mismatch the dynamics of their food. However, we found no evidence for such a mismatch in this 52-year data set.

Considerable uncertainty exists regarding the ability of reach-scale habitat restorations to promote ecological integrity and affect community composition in degraded streams and rivers and the time scales at which these effects take place. Restoration of habitats on the reach scale (hundreds of meters to a few kilometers) is expected to support threatened species because many of them are habitat specialists. In contrast, generalist species are predicted to be replaced in restored reaches. We used a large data set for 62 reach-scale restoration projects in 51 stream systems in Germany, Switzerland, and Liechtenstein and analyzed the changes in fish community composition induced by the restorations in terms of species richness, species turnover, Brillouin diversity index, total fish abundance, and proportion of alien and endangered species. We further analyzed the temporal dynamics of the fish community recovery over a period of 19 y postrestoration. Species richness and Brillouin diversity index increased in most of the restoration projects (66 and 57%, respectively), but recovery to historical reference conditions was not achieved. Total abundance was enhanced by most of the projects. Species composition in restored reaches underwent directed shifts for at least 10 y, with high and variable species turnover in the first years that decreased over time. The effects of restoration on Brillouin diversity index were highly variable in the first few years after restoration, but tended to increase over time. These dynamics must be considered more carefully in future protocols for evaluating restoration results, and final evaluation of restoration outcomes on fish communities should not be made too early. Our results indicate that reach-scale habitat restorations may be a suitable tool for increasing local fish abundance and slightly enhancing species diversity. However, more targeted approaches are needed to support threatened species and repress alien species.

Size‐selective fishery harvest leads to phenotypic changes in fish reproductive traits. When these changes represent an evolutionary response of a stock, they may have severe consequences for future stock dynamics and yields. In freshwater ecosystems, reproductive traits may also be affected by other human impacts such as changes in system productivity. The present study uses regression analysis to evaluate the impacts of changes in lake trophy and of an intensive size‐selective harvest over several decades on the reproductive traits of common whitefish in Lake Constance between 1963 and 1999. Fecundity was strongly linked to lake trophy but was also related to the calendar year, suggesting an evolutionary response to size‐selective harvest and to massive stocking of the lake with hatchery‐reared larvae. The present study is an example of how fish reproductive traits are influenced by the combined action of various human impacts: changes in system productivity, size‐selective harvest and massive stocking.

The ecosystem of Lake Constance in central Europe has undergone profound modifications over the last six decades. Seasonal and inter-annual changes in the vertical distribution patterns of whitefish were examined and related to changes in biotic and abiotic gradients. Between 1958 and 2007, the average fishing depth in late summer and autumn was related to two factors influencing food supply of whitefish -lake productivity and standing stock biomass. In years with low food supply, whitefish were harvested from greater depths, where temperatures were up to 4°C lower. The whitefishÕs distribution towards colder water might be a bioenergetic optimisation behaviour whereby fish reduce metabolic losses at lower temperatures, or it may result from a reassessment of habitat preference under conditions of limited food supply, according to the ideal free distribution theory.