Effect of Artificial Regime Shifts and Biotic Factors on the Intensity of Foraging of Planktivorous Fish

It is still to be confirmed whether global warming with its predicted elevated water temperature will cause an increase in predation and alter phenological and physiological processes leading to changes in the size of aquatic organisms. In an experimental system of water column stratification simulating a natural combination of field conditions, we created artificial abiotic factors that mimicked the natural environment, i.e., light intensity, oxygen conditions, and thermal stratification. Subsequently, we added biotic factors such as algae, Daphnia, and planktivorous fish. We studied the intensity of foraging of planktivorous fish on individuals of Daphnia per min in different conditions of biotic and abiotic gradients. We demonstrated a possible scenario involving the risk of elimination of large prey within macrocladocera communities by predatory pressure as a result of climate change. A higher intensity of foraging of planktivorous fish caused or increased the occurrence of larger groups of planktonic animals with a smaller body size. The mechanisms of a future scenario were discovered at a higher trophic level in the aquatic environment.

Ecosystem approach to harvesting in the Arctic: Walking the tightrope between exploitation and conservation in the Barents Sea

Projecting the consequences of warming and sea-ice loss for Arctic marine food web and fisheries is challenging due to the intricate relationships between biology and ice. We used StrathE2EPolar, an end-to-end (microbes-to-megafauna) food web model incorporating ice-dependencies to simulate climate-fisheries interactions in the Barents Sea. The model was driven by output from the NEMO-MEDUSA earth system model, assuming RCP 8.5 atmospheric forcing. The Barents Sea was projected to be > 95% ice-free all year-round by the 2040s compared to > 50% in the 2010s, and approximately 2 °C warmer. Fisheries management reference points (FMSY and BMSY) for demersal fish (cod, haddock) were projected to increase by around 6%, indicating higher productivity. However, planktivorous fish (capelin, herring) reference points were projected to decrease by 15%, and upper trophic levels (birds, mammals) were strongly sensitive to planktivorous fish harvesting. The results indicate difficult trade-offs ahead, between harvesting and conservation of ecosystem structure and function.

Interactions between a planktivorous fish and planktonic microcrustaceans mediated by the biomass of aquatic macrophytes

Habitat complexity is recognized to mediate predator–prey relationships by offering refuge or not. We investigated the availability of planktonic microcrustaceans and the diet of a planktivorous fish (Hyphessobrycon eques) at different levels (low, intermediate and high) of aquatic macrophyte biomass. Sampling was carried out in a river with low flow speed, located in a Neotropical floodplain. We collected fish and microcrustaceans in macrophyte stands with variations in biomass. There were no differences in microcrustacean density in the water among the levels of macrophyte biomass, but microcrustacean richness and diet composition of H. eques differed. Microcrustacean richness and trophic niche breadth of the planktivorous fish were higher in high biomass stands. There was high consumption of a small cladoceran species in low macrophyte biomass, which was replaced by larger species, such as copepods, in intermediate and high biomass. Thus, the selection of some species was different among the biomass levels. These results suggest that plant biomass plays an important role in the interaction between fish and microcrustaceans, and prey characteristics such as size, escape ability and energy value make them more or less subject to predation by fish according to habitat structuring.

ISHRANA GRABLJIVIH VRSTA RIBA I NJIHOV UTICAJ NA ODRŽANJE STABILNOSTI AKUMULACIJE BOVAN

The aim of this study was to investigate the diet of adult predatory fish (pikeperch, perch, pike, and catfish) that inhabit the Bovan reservoir and determine their impact on the ecological stability of the reservoir. The results showed that predators most often used planktivorous fish species in their diet, including bleak, roach, and perch. These species are dominant prey to predators, and at the same time important links in food chains. By feeding on planktivorous fish, predatory fish can contribute to the reduction of the eutrophication of the reservoir and keep this ecosystem stable.

Dispersal

Dispersal of benthic crustaceans primarily occurs by larvae, which can be transported far from parents. However, larval dispersal is reduced by depth regulation in a sheared water column, where surface and bottom currents flow at different rates or directions, and navigation by postlarvae recruiting to adult habitats. Larvae undertake migrations between adult and larval habitats that range from retention near adult habitats to cross-shelf migrations. The extent of these migrations is regulated by depth preferences and vertical migrations that are timed exogenously or endogenously by diel and tidal cycles over planktonic development. Depth regulation is cued primarily by gravity, hydrostatic pressure, and light, and secondarily by temperature, salinity, and turbulence. Settlement stages navigate to suitable settlement sites using hierarchies of acoustic, chemical, visual, and celestial cues that are effective at different distances. The extent of larval migrations between adult and larval habitats as well as diel vertical migrations may be set by the vulnerability of larvae to abundant planktivorous fish in estuaries and nearshore waters. The timing of larval release and vertical swimming by larvae changes across tidal regimes to conserve migrations between adult and larval habitats across species ranges while minimizing predation.

Foraging success in planktivorous fish increases with topographic blockage of prey distributions

Banks and shelves are productive zones of the ocean, and often home to large fish stocks. Can shallower bottom topographies improve foraging opportunities for pelagic fish by blocking zooplankton from hiding in deep, darker water? We use mechanistic principles of visual prey search and an extensive dataset of zooplankton depth distributions to model foraging success in planktivorous fish across a large marine ecosystem. Our results show that zooplankton distribute deeper with deeper bathymetry, and that fish find exponentially less food with increasing bottom depths. Over shallow banks, zooplankton are forced into higher light exposure, providing higher prey encounter rates for fish despite lower abundance of prey. Stomach data analyses from a key planktivore support these predictions and suggest that fish foraging on copepods are more successful over shallower grounds. Our study demonstrates that prey availability for planktivorous fish is not proportional to zooplankton abundance, while the bottom depth is an important factor in fish foraging success and zooplankton mortality rates.

Body size adaptions under climate change: zooplankton community more important than temperature or food abundance in model of a zooplanktivorous fish

One of the most well-studied biogeographic patterns is increasing body size with latitude, and recent body size declines in marine and terrestrial organisms have received growing attention. Spatial and temporal variation in temperature is the generally invoked driver but food abundance and quality are also emphasized. However, the underlying mechanisms are not clear and the actual cause is likely to differ both within and among species. Here, we focused our attention on drivers of body size in planktivorous fish that forage through vision. This group of organisms plays a central role in marine ecosystems by linking the energy flow from lower to higher trophic levels. Using a model that incorporates explicit mechanisms for vision-based feeding and physiology, we investigated the influence on optimal body size of several biotic (prey size, prey energy content, and prey biomass concentration) and abiotic (temperature, latitude, and water clarity) factors known to affect foraging rates and bioenergetics. We found prey accessibility to be the most influential factor for body size, determined primarily by prey size but also by water clarity, imposing visual constraints on prey encounters and thereby limiting feeding rates. Hence, for planktivores that forage through vision, an altered composition of the prey field could have important implications for body size and for the energy available for reproduction and other fitness-related tasks. Understanding the complicated effects of climate change on zooplankton communities is thus crucial for predicting impacts on planktivorous fish, as well as consequences for energy flows and body sizes in marine systems.