Eckmann, Reiner

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Reiner
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A review of the population dynamics of coregonids in European alpine lakes

2013, Eckmann, Reiner

The population dynamics of coregonids in European alpine lakes have been studied intensively during recent decades. Many of these studies have focussed on the impacts of anthropogenic eutrophication and subsequent re-oligotrophication. These changes in productivity represent the single most important perturbation of coregonid habitats since the mid-twentieth century. While the problem of eutrophication has been successfully addressed in many European alpine lakes, new forms of anthropogenic challenge such as climate change and the colonization of lakes by non-native aquatic species continue to impact on coregonid populations and demand research. The picture that emerges from a compilation of primary research papers suggests that abiotic and biotic factors are most infl uential on year-class strength during the larval phase, exerting more moderate effects during the embryonic and reproductive phases, and only a weak infl uence during the growth phase. In contrast, individual biomass is mainly determined during the growth phase, but moderate effects on growth may also occur during the larval and reproductive phases. Changes in lake productivity infl uence the various life-history phases of coregonids in both positive and negative ways, for example by improving feeding conditions and via deterioration of spawning habitat quality. Climate change, on the other hand, seems to influence mainly the embryonic and larval phases, with the available studies suggesting that higher temperatures and earlier onset of stratifi cation in spring tend to boost year-class strength. These effects, however, appear insuffi cient to balance the marked effects of re-oligotrophication, which has caused and continues to cause a marked depression of growth in many European alpine lakes. The bumper fish harvests obtained during the peak of anthropogenic eutrophication are thus a thing of the past.

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Yields of European perch from Upper Lake Constance from 1910 to present

2006, Eckmann, Reiner, Gerster, Stefan, Kraemer, A.

Commercial yields of perch, Perca fluviatilis (L.), from Upper Lake Constance changed markedly during the 20th century. The perch boom in the 1950s and 1960s is attributed to higher fishing intensity and lake eutrophication. Decreasing yields in recent decades are mainly because of slower growth of perch, while a gradual decrease in year-class strength, and an overall reduction of fishing intensity, might also have contributed to yield reduction. Slower growth is mainly attributed to lake re-oligotrophication and infestation of perch with the pike tapeworm Triaenophorus nodulosus (Pallas). Competition for food with non-native ruffe, Gymnocephalus cernua(L.) is likely of minor importance, while the invasion of the lake by alien macrozoobenthos species has the potential to reduce the food supply for benthivorous perch. Perch yields are assumed to remain at the low level attained at the beginning of the 21st century; however, under changing climate conditions, fluctuations around the long-term mean will probably be wider than in the past.

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Seasonal and long-term changes in fishing depth of Lake Constance whitefish

2010, Thomas, Gregor, Rösch, Roland, Eckmann, Reiner

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 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.

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The influence of eutrophication and population biomass on common whitefish (Coregonus lavaretus) growth : the Lake Constance example revisited

2007, Thomas, Gregor, Eckmann, Reiner

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 PO4-P content during spring turnover and by calendar year, which was incorporated as a third independent variable (adjusted R2 = 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 eutro- phication on growthof whitefish needs to be reconsidered.