Fisheries-induced evolution (FIE) has been reported for several intensively exploited fish stocks worldwide. Most studies focused on marine populations of high commercial interest, but FIE may also occur in intensively harvested freshwater fish stocks. An example is the whitefish (Coregonus lavaretus) stock of Lake Constance. Previous evidence for FIE in this stock, through the analysis of long-term data sets, showed a long-term effect on growth and reproductive traits in addition to the effects of environmental factors. In this study, we tested whether growth rates of Lake Constance whitefish were partially hereditary, and whether the results of this experiment support the fisheries-induced evolution scenario for Lake Constance whitefish. In a laboratory experiment using whitefish larvae, we hypothesized that slow-growing females would produce slow-growing offspring. Our experimental setup allowed rearing of 24 different batches of larvae over 40 days. Growth of larvae was inversely correlated to their weight-at-hatch, and larvae with larger yolk sac volume grew slower. We attribute this result to the intensive size-selective harvest that has occurred in Lake Constance during the past several decades because fish that allocate more energy to reproduction at the expense of growth have an advantage under the present harvest regime over fish that follow the opposite allocation strategy. Our results are in line with FIE theory and suggest a genetic component to life-history traits of Lake Constance whitefish, whereby slow-growing females tend to produce larvae of a lower growth performance.

P>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 degrees C lower. The whitefish's 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.

The near-surface distribution of young-of-the-year perch was observed with a towed upward beaming echosounder system (SIMRAD EK60 with a circular 7 transducer). Perch aggregated densely in the epilimnion during daytime and dispersed evenly below the surface at night. Shoaling commenced in late June when perch metamorphosed from the larval to juvenile stage. Average shoal width was 6.6 m and average shoal height was 2.35 m in July, when perch were observed for the last time in the pelagic zone of Lake Constance.Upward echosounding revealed the presence and near-surface distribution of pelagic juvenile perch and therefore this method can be used as a complementary survey tool to get more precise information about the distribution, behaviour and abundance of near-surface fish. (C) 2008 Elsevier B.V. All rights reserved.

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.

Accelerated growth of freshwater fish during anthropogenic eutrophication has been attributed almost exclu- sively 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 bio- mass 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 PO4-P content dur- ing 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.

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.