Person: Eckmann, Reiner
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Long-term changes in littoral fish community structure and resilience of total catch to re-oligotrophication in a large, peri-alpine European lake
1. The littoral zone of lakes is used as spawning, shelter, or feeding habitat for many fish species and hence is of key importance for overall lake functioning. Despite this, hardly any studies exist examining the long‐term dynamics and response of the littoral fish community, composed mostly of juvenile fish, to environmental change. Here, we study the response of total catch per unit effort (CPUE) and individual species CPUE of such a community to 17 years of oligotrophication and examine whether the species responses can be characterised as synchronous or asynchronous.
2. We analyse a data set of beach seine catches carried out during morning and twilight, late spring and late summer at three sites in large and deep Lake Constance from 1997 to 2014. Generalised additive mixed models were used to explore changes in CPUE of the overall community and of the most frequently occurring species, and Kendall's W test was applied to examine whether the dynamics of fish species were synchronous or asynchronous.
3. Species‐specific and total CPUE strongly differed between morning and twilight and between spring and summer indicating an important role of behavioural and life cycle adaptations of species for CPUE. In addition, also the CPUE of some species seeking shelter behind larger stones was lower at sites without these.
4. Total CPUE did not decline suggesting the overall abundance of littoral fish was resilient to declining nutrients. In contrast, fish community composition changed strongly during the study period due to increases in some species (dace, loach, perch) and decreases in others (bream, burbot, chub, ruffe), indicating response diversity of fish to oligotrophication. The type of community dynamics was scale‐dependent, whereby significantly synchronous dynamics according to Kendall's W were observed when taking seasonal variability into account. In contrast, significantly asynchronous species dynamics were observed when only the low‐frequency variability of species dynamics was considered separately for spring and summer time series.
5. Resilience of littoral fish total CPUE to oligotrophication might have important consequences for ecosystem dynamics and ecosystem services beyond the littoral zone. As small fish often impose strong predation pressure on zooplankton, their resilience might sustain a high top‐down control on zooplankton resulting in a further reduction of zooplankton biomass. This could contribute to delayed food web responses and reduced growth of fish with oligotrophication.
The impact of density-dependant growth on whitefish production in re-oligotrophic lakes : a bioenergetics simulation study
Following a period of eutrophication in the 1960s and 1970s, the productivity of many European lakes was restored to near-pristine levels at the end of the 20th century, a process termed re-oligotrophication. Lower phosphorus concentrations, however, lead to slower growth of fish due to a reduced food base, and eventually to smaller harvests. This problem is particularly serious in perialpine lakes whose fish communities are dominated by whitefish. In these lakes fishermen benefitted from enhanced lake productivity under eutrophic conditions but are now confronted with smaller whitefish yields under oligotrophic conditions. As growth of lake whitefish is densitydependent, manipulation of standing stock biomass has the potential of increasing growth rate and eventually harvests. This possibility is evaluated through bioenergetics modelling, simulating growth and yield of whitefish under various population densities and zooplankton food concentrations. These simulations clearly indicate that not only individual growth but total biomass production increase with decreasing population size, and the increase in total biomass production is even more pronounced at low ration levels. Decreasing whitefish population density therefore has the potential of increasing fishery yields. The unresolved problem however persists, how whitefish population density can be manipulated in the desired way.
The Effect of Water Hardness on Mortality of Zebrafish (Danio rerio) During Exposure to Oxytetracycline
Marking of fish otoliths with oxytetracycline and tetracycline is a widely used method to evaluate the effectiveness of stocking operations. Available protocols for the labeling of fish specify a number of factors influencing mark quality and potential risk for fish during marking. This study investigates the influence of water hardness on mortality of freshwater fish during marking with OTC. In order to pursue this question complexation of OTC with Mg2+ and Ca2+ cations was measured spectrophotometrically. Furthermore, zebrafish (Danio rerio) were immersed in OTC solutions (1200 mg/L; 48 h immersion) combined with varying levels of water hardness (5.5, 15.5, 25.5, 32.5°dH). The amount of OTC-Mg-Ca-complexes was positively correlated to water hardness. Moreover, it could be demonstrated that mortality of zebrafish during marking varied as a factor of water hardness. Highest mortalities occurred at the lowest (5.5°dH) and the highest (32.5°dH) tested levels during marking with OTC.
Impact of Fishing and Stocking Practices on Coregonid Diversity
Fish species diversity can be lost through interacting stressors including habitat loss, stocking and overfishing. Although a multitude of stressors have played a role in the global decline of coregonid (Coregonus spp.) diversity, a number of contemporary studies have identified habitat loss stemming from eutrophication as the primary cause. Unfortunately, reconstructing the role of fishing and stocking practices can be difficult, because these records are incomplete or appear only in hard-to-access historic grey literature. Based on an illustrative set of historic and contemporary studies, we describe how fisheries management practices may have contributed to coregonid diversity loss in European and North American lakes. We provide case studies examining how fishing and stocking may reduce coregonid diversity through demographic decline and introgressive hybridization. In some lakes, fisheries management practices may have led to a loss of coregonid diversity well before issues with habitat degradation manifested. Our review suggests that fish conservation policies could beneficially consider the relative importance of all stressors, including management practices, as potential drivers of diversity loss.
Reconstructing the build-up of a pelagic stickleback (Gasterosteus aculeatus) population using hydroacoustics
A lake wide fishing survey of Upper Lake Constance revealed that in September 2014 three-spined sticklebacks (Gasterosteus aculeatus) made up 96% of the pelagic fish community in terms of number. To narrow down the point in time when sticklebacks started to colonize the pelagic zone and to reconstruct the build-up of this unprecedented population size, we analysed 18 hydroacoustic surveys from March 2009 to March 2018. As current analysis techniques do not allow discriminating echoes from different fish species of similar size, we considered as sticklebacks all fish smaller than a certain single-target threshold minus the average abundance of small fish well before the sticklebacks invaded the pelagic zone. Our data suggest that sticklebacks colonized the pelagic zone in 2012, one year before they appeared as by-catch in test fisheries. Population density fluctuated between 1280 and 7990 individuals/ha from November 2012 to March 2018 but never fell below 2900 individuals/ha in recent years. These estimates do not include shallow waters where densities might be even higher because the nearshore zone was excluded from our surveys.
Incorporation of dietary carotenoids into the fins of yellow- and red-finned Eurasian perch Perca fluviatilis
The colour of the ventral, anal and caudal fins of Eurasian perch can range from pale yellow to intensered. Within spatially separated populations, however, individuals are usually very uniform in colour.We fed astaxanthin- and canthaxanthin-enriched dry feed to juvenile perch from a yellow-finned and ared-finned population to compare the influence of dietary carotenoids on fin colour between these twopopulations. In this way we wanted to test whether fin colour in perch is predominantly a phenotypicallyplastic trait or whether differently coloured individuals represent colour morphs. The ventral fins of perchfrom the red-finned population always exhibited significantly more intense redness when their feed wassupplemented with either one or both carotenoid additives. Perch from both populations deposited morecanthaxanthin in their ventral fins than astaxanthin, red-finned perch in particular. Yellow-finned perchprobably converted canthaxanthin into -carotene, which was the dominant carotenoid in their ventralfins, whereas the fins of red-finned perch contained only trace amounts of -carotene. We conclude thatthese two populations represent colour morphs that differ fundamentally in their ability to metaboliseand deposit dietary carotenoids into their ventral fins. Considering the multiple physiological functionsof carotenoids, these fundamental differences in carotenoid metabolism between perch colour morphsmay have far-reaching consequences for the performance of different populations, for example in theirresponse to parasite infections.
Absence of intrinsic post-zygotic incompatibilities in artificial crosses between sympatric coregonid species from upper Lake Constance
A full factorial crossing experiment with five females and five males of each of two coregonid species from upper Lake Constance was used to test for intrinsic post-zygotic incompatibilities during early ontogeny. Up until shortly before hatching, there was no difference in embryo mortality between homo and heterologous crosses. Amaternal effect on mortality was found in both species, but paternal effects and female–male interactions were absent. Thus, genetic incompatibility during early ontogeny does not appear to prevent introgressive hybridization, suggesting that genetic divergence between these species is maintained primarily by pre-zygotic barriers. The recent genetic homogenizations of coregonid species flocks in European alpine lakes may have been caused by a flattening of adaptive landscapes through eutrophication, but intensive stocking with larvae obtained in hatcheries from artificially fertilized eggs is also likely to be a contributing factor. To safeguard diversity among sympatric coregonids, it is important to re-establish ecological conditions conducive to species divergence and to revise traditional management strategies.
Rapid niche expansion by selection on functional genomic variation after ecosystem recovery
It is well recognized that environmental degradation caused by human activities can result in dramatic losses of species and diversity. However, comparatively little is known about the ability of biodiversity to re-emerge following ecosystem recovery. Here, we show that a European whitefish subspecies, the gangfisch Coregonus lavaretus macrophthalmus, rapidly increased its ecologically functional diversity following the restoration of Lake Constance after anthropogenic eutrophication. In fewer than ten generations, gangfisch evolved a greater range of gill raker numbers (GRNs) to utilize a broader ecological niche. A sparse genetic architecture underlies this variation in GRN. Several co-expressed gene modules and genes showing signals of positive selection were associated with GRN and body shape. These were enriched for biological pathways related to trophic niche expansion in fishes. Our findings demonstrate the potential of functional diversity to expand following habitat restoration, given a fortuitous combination of genetic architecture, genetic diversity and selection.
Cross-realm assessment of climate change impacts on species' abundance trends
Climate change, land-use change, pollution and exploitation are among the main drivers of species' population trends; however, their relative importance is much debated. We used a unique collection of over 1,000 local population time series in 22 communities across terrestrial, freshwater and marine realms within central Europe to compare the impacts of long-term temperature change and other environmental drivers from 1980 onwards. To disentangle different drivers, we related species' population trends to species- and driver-specific attributes, such as temperature and habitat preference or pollution tolerance. We found a consistent impact of temperature change on the local abundances of terrestrial species. Populations of warm-dwelling species increased more than those of cold-dwelling species. In contrast, impacts of temperature change on aquatic species' abundances were variable. Effects of temperature preference were more consistent in terrestrial communities than effects of habitat preference, suggesting that the impacts of temperature change have become widespread for recent changes in abundance within many terrestrial communities of central Europe.
Individual identification of Eurasian perch Perca fluviatilis by means of their stripe patterns
In this study we show that the number, position and shape of stripes were sufficiently unique to demonstrate that juvenile Eurasian perch Perca fluviatilis can be individually identified based on their stripe patterns. The stripe patterns in perch and also other species may thus be used in experiments as an alternative to conventional marking techniques that frequently cause stress to the fish.