Abstract
Due to the heterogeneous distribution of resources along large rivers, understanding prey utilization by basin‐scale fish assemblages remains a challenge, and thus, recognizing regional fish trophic guilds and indicator species is important. We analyzed the stomach contents of 96 fish species along the subtropical East River in China and identified 8 prey items (29 subcategories). Site‐specific differences in fish diet composition (DC) revealed longitudinal shifts in utilized prey taxa, from upstream lotic to downstream semi‐lentic items, and these were characterized by a decrease in the proportions of epilithic diatoms and aquatic insect larvae (Ephemeroptera and Chironomidae) accompanied by an increase in bivalves (Corbicula and Limnoperna), shrimps and fishes, and organic sediments. The relative prey consumption weighted by fish abundance and biomass indicated that decreasing insect consumption and increasing detritus consumption were two fundamental vectors governing fish‐centered feeding pathways. Seventeen prey‐oriented fish guilds that were clustered based on DC matrix determined the spatial variation in the fish trophic structure. The cumulative presence of (a) upstream guilds reliant on insects and epiphytes, (b) midstream guilds reliant on hydrophytes, molluscs, and nekton, and (c) downstream guilds reliant on detritus, annelids, and plankton resulted in a longitudinal increase in guild richness, but this continuity was interrupted near the industrialized estuary. The most abundant 28 fish species across the guilds were selected as trophic indicator species; their spatial distribution significantly (p < 0.05) explained >80% of the environmental and prey variables identified. These species signified the availability of predator–prey links in distinct habitats and the key environmental factors supporting these links. With a high contribution (>51%) of exotic species, an increase in detritivores downstream distinguishes the subtropical East River from temperate rivers. Particularly, in the disturbed lower reaches, the dominance of detritivores prevailed over the predicted increase in other feeding groups (e.g., omnivores and carnivores).
Highlights
Conceptualized as a series of physical gradients, river systems pro‐ vide selective pressures that strongly influence aquatic communi‐ ties (Horwitz, 1978; Peres‐Neto, Bizerril, & Iglesias, 1995; Seegert, Vondruska, Perry, & Dixon, 2013; Sheldon, 1968)
We aimed to demonstrate that (a) there are longitudinal shifts in prey utilization by fish species and the feeding pathways of fish assemblages, (b) guild classification based on utilized prey taxa by fishes could re‐ veal crucial information that traditional feeding classifications have failed to bring to light, and (c) fish species that represent key trophic links could be used to indicate changes in both environmental fac‐ tors and prey distribution
Experiments were designed to address the following four objectives: (a) to reveal the downstream shifts in prey utilization through basin‐scale fish stom‐ ach content analysis, (b) to cluster prey‐oriented fish guilds based on the diet com‐ position (DC) of individual species, (c) to analyze the longitudinal vari‐ ation in the composition and structure of fish trophic guilds, (d) to quantify the consumption of prey by site‐specific fish assemblages, and (e) to explore the use of the dominant fish species in each guild as indicator species of predator–prey links and environmental fac‐ tors
Summary
Conceptualized as a series of physical gradients, river systems pro‐ vide selective pressures that strongly influence aquatic communi‐ ties (Horwitz, 1978; Peres‐Neto, Bizerril, & Iglesias, 1995; Seegert, Vondruska, Perry, & Dixon, 2013; Sheldon, 1968). The river continuum concept (Vannote et al, 1980), dynamic landscape model (Schlosser, 1991), and riverine productivity model (Thorp & Delong, 1994) have been used to predict the downstream geographical distribution of invertebrates, fish, and algae to max‐ imize resource use As these hypotheses emphasize independent biotic assemblages and their responses to the envi‐ ronment, the evidence for longitudinal changes in fish‐centered feeding pathways and the trophic connections within food webs remains limited. Experiments were designed to address the following four objectives: (a) to reveal the downstream shifts in prey utilization through basin‐scale fish stom‐ ach content analysis, (b) to cluster prey‐oriented fish guilds based on the DC of individual species, (c) to analyze the longitudinal vari‐ ation in the composition and structure of fish trophic guilds, (d) to quantify the consumption of prey by site‐specific fish assemblages, and (e) to explore the use of the dominant fish species in each guild as indicator species of predator–prey links and environmental fac‐ tors. Compared with concepts and models based on temperate and tropical rivers, our results provide novel insights into the trophic dynamics of aquatic food webs and ecological assessments of large river systems in the subtropics
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