Diets and Trophic Structure of Fish Assemblages in a Large and Unexplored Subtropical River: The Uruguay River

Water-level fluctuations lead to changes in the diet of an omnivorous fish in a floodplain

Abstract The hydrological regime leads to fluctuations in the availability of food resources for fish, which may reflect in their diet variation. Our main goal was to analyze the relationship between water-level fluctuations and the diet and the body condition of Trachelyopterus galeatus in the Upper Paraná River floodplain. The fish used in the study were sampled quarterly along nine years, at nine sampling stations. Our results showed that diet of T. galeatus varied in response to oscillations in the hydrometric level, with the consumption of terrestrial invertebrates being positively related to hydrometric level, while the consumption of aquatic invertebrates was negatively related. The trophic niche breadth and feeding activity were not affected by hydrometric level, but the fish body condition was positively affected. The high food plasticity allows T. galeatus to consume the most available resources (e.g., terrestrial invertebrates in high water levels), which probably contributed to the increase in its body condition in higher hydrometric levels. In summary, the hydrological dynamic is fundamental for the diet variation of omnivorous fish, which can take advantage of the resource availability according to the hydrometric level, increasing its body condition when allochthonous resources as terrestrial invertebrates are supposed to be more available.

Ecological and Phenotypic Diversification after A Continental Invasion in Neotropical Freshwater Stingrays

Habitat transitions are key potential explanations for why some lineages have diversified and others have not - from Anolis lizards to Darwin's finches. The ecological ramifications of marine-to-freshwater transitions for fishes suggest evolutionary contingency: some lineages maintain their ancestral niches in novel habitats (niche conservatism), whereas others alter their ecological role. However, few studies have considered phenotypic, ecological, and lineage diversification concurrently to explore this issue. Here, we investigated the macroevolutionary history of the taxonomically and ecologically diverse Neotropical freshwater river rays (subfamily Potamotrygoninae), which invaded and diversified in the Amazon and other South American rivers during the late Oligocene to early Miocene. We generated a time-calibrated, multi-gene phylogeny for Potamotrygoninae and reconstructed evolutionary patterns of diet specialization. We measured functional morphological traits relevant for feeding and used comparative phylogenetic methods to examine how feeding morphology diversified over time. Potamotrygonine trophic and phenotypic diversity are evenly partitioned (non-overlapping) among internal clades for most of their history, until 20-16 mya, when more recent diversification suggests increasing overlap among phenotypes. Specialized piscivores (Heliotrygon and Paratrygon) evolved early in the history of freshwater stingrays, while later trophic specialization (molluscivory, insectivory, and crustacivory) evolved in the genus Potamotrygon. Potamotrygonins demonstrate ecological niche lability in diets and feeding apparatus; however, diversification has mostly been a gradual process through time. We suggest that competition is unlikely to have limited the potamotrygonine invasion and diversification in South America.

Cascading impacts of urbanization on multitrophic richness and biomass stock in neotropical streams

The conversion of natural streams to urbanized systems with the intention of supplying the cities' water demand causes species loss across many trophic groups, with negative consequences for ecosystem functioning. High levels of watershed urbanization cause environmental changes through water quality deterioration and loss of habitat heterogeneity. However, it remains unclear how environmental changes resulting from urbanization affect the diversity of multiple trophic groups and ecosystem functions, such as biomass stock in streams. Here, using a dataset from Neotropical streams, we investigate the cascading effects of urbanization (via impoverishment of water quality and habitat heterogeneity) on richness of multiple trophic groups of fish, and their consequences to biomass stock of streams. The increase in urbanization decreased the richness and standing biomass of carnivores, omnivores, and detritivores across streams. Urbanization also decreased habitat heterogeneity and water quality, which driver a huge cascading decrease in the richness of carnivores, omnivores, and detritivores, and ultimately reduced the whole-community standing biomass. Our analysis revealed that urbanization expansion induces a cascading reduction of multitrophic diversity and standing biomass in Neotropical streams. Therefore, the predicted increase in urbanization in the coming decades should impacts the richness of multiple trophic levels, with potential negative consequences to ecosystem functioning of streams.

Small-sized omnivorous fish induce stronger effects on food webs than warming and eutrophication in experimental shallow lakes

Warming, eutrophication, and increased omnivory by small-sized fish are global change processes that induce major effects on the food web structure and primary producers of shallow lakes. Despite the key relevance of phytoplankton and periphyton in freshwaters, the combined and potential synergistic effects of fish omnivory, warming and eutrophication, especially on periphyton, remains little addressed, particularly for subtropical shallow lakes. We experimentally tested the food web effects on phytoplankton and periphyton induced by small visually feeding omnivorous fish (Rhodeus ocellatus), high nutrient enrichment and warming (+4.5 °C) in thirty-two 1000 L-mesocosms simulating littoral conditions of subtropical shallow lakes. We aimed at analysing the mechanisms and responses of periphyton and phytoplankton to these experimental factors. All mesocosms included the submerged macrophytes Vallisneria denseserrulata and Potamogeton lucens and artificial plants at 50% plant volume inhabited, plankton and macroinvertebrates. Small-sized visually feeding omnivorous fish enhanced phytoplankton dominance and periphyton loss. These changes coincided with a decrease in zooplankton biomass and a diversity loss of both zooplankton and macroinvertebrates as well as an increase in snail abundance. Fish presence led to a collapse of cladocerans, thereby releasing the grazing pressure on phytoplankton, and predator and collector macroinvertebrates were replaced with small snails (Radix peregra < 0.5 cm) resulting in enhanced grazing on periphyton. Eutrophication reinforced the fish effects, while warming had weak or no effects. Our results indicate that omnivory by small-sized visually feeding fish may induce stronger effects on the food webs of shallow lakes, towards phytoplankton-dominated states, than the combined effect of nutrient enrichment and warming under the present experimental conditions.

Long-term trends of fishery landings and target fish populations in the lower La Plata basin

Abstract The La Plata basin is the second largest basin of South America and has supported important river fisheries for more than a century. In this paper, we evaluate for the first time the historical trends of landings of 21 fish taxa and the recent population trends of 27 species of commercial fishes in the lower La Plata basin (Argentina). We compiled three kinds of data sets: Total fishery landings (between 1934 and 1986) and exports (1994‒2019), fisheries monitoring programs of Chaco and Santa Fe provinces in the Paraná River (2009‒2019), and surveys of fish populations in the Upper (Corrientes, 1993‒2020) and Middle (EBIPES, 2005‒2020) Paraná River. The analysis of the historical landings showed more species declining in the lower portion of the basin than in the upper basin. Regarding recent population trends, Pimelodus spp., Hoplias spp., Salminus brasiliensis, Luciopimelodus pati, and Ageneiosus spp. declined in more than one region, while Megaleporinus spp., Pterodoras granulosus, and Oxydoras kneri showed stable to positive trends, with the other species varying in their trends between regions. These tendencies could be associated to a combination of factors such as overfishing and environmental changes that would require an ecosystem approach for their adequate management.

Short-Term Interactive Effects of Experimental Heat Waves and Turbidity Pulses on the Foraging Success of a Subtropical Invertivorous Fish

Sudden increases in temperature and turbidity in aquatic ecosystems are expected for different regions in the future, as a result of the more frequent extreme climatic events that are predicted. The consequences of these abrupt changes in the outcomes of predator–prey interactions are unknown. Here, we tested the effects of a heat wave and a turbidity pulse on the foraging success of a subtropical cichlid fish (Gymnogeophagus terrapurpura) on amphipods (Hyalella curvispina). We carried out a short-term experiment combining treatments of turbidity (3 and 100 nephelometric turbidity units [NTU]) and water temperature (19.2, 22.2, 25.2 and 27.0 °C), considering potential differences given by fish length. Changes in water temperature did not promote significant changes in prey consumption. Higher turbidity, in contrast, decreased prey consumption. Also, we found that fish with different body lengths consumed a similar amount of prey under clear waters, but, in turbid waters, bigger individuals were more efficient than the smaller individuals. This finding is an empirical demonstration that the effect of increased turbidity on predation rate depends upon predator body size, and it suggests that bigger body sizes may help overcome turbidity-associated limitations in finding and capturing prey. Our short-term results suggest that, if turbidity pulses and heat waves become more frequent in the future, the outcome of fish–invertebrate interaction can be affected by local characteristics such as fish population size distribution.