Climate change effects on hatching success and embryonic development of fish: Assessing multiple stressor responses in a large-scale mesocosm study.

Climate change threatens freshwater fish species due to predicted changes in thermal, sedimentary and hydrological properties of stream ecosystems. Gravel-spawning fish are particularly sensitive to such alterations as warming, higher inputs of fine sediment and low-flow all have potentially negative effects on the functionality of their reproductive habitat, the hyporheic zone. Multiple stressors can interact in synergistic and antagonistic manners, causing surprise-effects that cannot be predicted from the additive consideration of individual stressors. For obtaining reliable, yet realistic data on the climate change stressor effects warming (+3-4 °C), fine sediment (increase in <0.85 mm by 22 %) and low-flow (eightfold discharge-reduction), we constructed a unique large-scale outdoor-mesocosm facility consisting of 24 flumes to study individual and combined stressor responses in a fully-crossed, 3-way-replicated design. To acquire representative results reflecting individual susceptibilities of gravel-spawning fish species due to taxonomic affiliation or spawning seasonality, we studied hatching success and embryonic development in the three fish species brown trout (Salmo trutta L.), common nase (Chondrostoma nasus L.) and Danube salmon (Hucho hucho L.). Fine sediment had the most significant single negative effect on both hatching rates and embryonic development (-80 % in brown trout, -50 % in nase, -60 % in Danube salmon). When fine sediment was combined with one or both of the other stressors, we observed strongly synergistic stressor responses, being distinctly stronger in the two salmonid species than in the cyprinid nase. Danube salmon was most susceptible to synergistic effects due to warmer spring water temperatures exacerbating the fine sediment-induced hypoxia, hence leading to complete mortality of fish eggs. This study highlights that individual and multiple-stressor effects depend strongly on life-history traits of respective species and that climate change stressors have to be assessed in combination to obtain representative results due to the high level of synergisms and antagonisms detected in this study.

Can top-down effects of cypriniform fish be used to mitigate eutrophication effects in medium-sized European rivers?

Eutrophication seriously threatens the ecological quality and biodiversity of running waters. In nutrient-enriched streams and shallow rivers, eutrophication leads to excessive periphyton growth and, in turn, biological clogging, oxygen depletion in the hyporheic zone and finally a reduction in the hyporheic habitat quality. Top-down control of the food-web by manipulating fish stocks, similar to the biomanipulation successfully applied in lakes, offers a promising approach to mitigating the effects of eutrophication in shallow rivers, especially those in which major reductions in nutrient input are not feasible. We conducted a reach-scale experiment over 4 years in a medium-sized eutrophic river to assess whether the top-down effects of two important large European cypriniform fish species, herbivorous common nase (Chondrostoma nasus) and omnivorous European chub (Squalius cephalus), would mitigate the effects of eutrophication. The enhancement of fish stocks was expected to reduce biological clogging, via the top-down control of periphyton by benthic grazing and enhanced bioturbation, thus increasing oxygen availability in the hyporheic zone as well as water exchange between the surface water and the hyporheic zone. As expected, enhancing the stocks of nase and chub increased both oxygen availability and vertical exchange flux of water in the upper layer of the hyporheic zone. However, periphyton biomass (chlorophyll a) was significantly reduced only in deeper pool habitat. Thus, while experimental biomanipulation in a shallow river significantly mitigated the effects of eutrophication in the hyporheic zone, top-down effects on periphyton biomass were rather small. Overall, to our knowledge, our results provide first evidence that the biomanipulation achieved by enhancing herbivorous and omnivorous fish stocks has the potential to mitigate the effects of eutrophication in medium-sized European rivers.

Effects of large-river restoration measures on ecological fish guilds and focal species of conservation in a large European river (Danube, Austria).

River engineering measures of the last two centuries have severely impacted the fish coenoses of all larger rivers in Europe by creating river barriers, replacing natural shores with riprap-reinforced banks, and installing artificial structures that changed the hydrology of inshore habitats. As a result, fluvial specialists have declined and were replaced by generalists. Consequently, there is an urgent need to develop functioning means and measures to restore stream habitats and re-establish near-natural conditions. In this study, we analyze the effectiveness of four different restoration measures to increase the abundance and ratio of the rheophilic fish guild in general and of two fluvial focal species of conservation, the barbel and nase, in particular. The sampling took place in a free-flowing part of the Austrian Danube, during the monitoring program of a large hydro-engineering project with both economic and ecological targets. We applied a BACI design and used data from three extensive sampling years. The ecologically orientated measures included groynefield adaptations, a bank re-naturalization, and a side arm re-connection. All measures increased the abundance of nase and barbel and also provided suitable conditions for young-of-the-year fishes. The bank re-naturalization, significantly increased both the abundance and the ratio of rheophilic fishes. The side arm re-connection significantly improved the ratio of rheophilic fishes. Both groyne adaptation measures showed positive, but non-significant effects. Consequently, replacing riprap sections with near-natural gravel bars and re-connecting former side arms can be recommended as appropriate measures to support the rheophilic fish guild, whereas groyne adaptations appear to be of limited value. The evaluated restoration measures significantly improved conditions for local populations of target species and coenoses and can be used as building blocks in upscaled, river-wide restoration efforts. A sustainable restoration of the whole river ecosystem, however, requires a holistic approach that also addresses floodplains and catchment-wide issues.