Zoobenthos are minor dietary components of small omnivorous fishes in a shallow eutrophic lake

Abstract

We examined whether small omnivorous fishes (smaller than ,100 mm long) integrate littoral, pelagic and benthic pathways in a shallow, eutrophic lake (Lake Izunuma, Japan). The surface of the lake was covered by a dense vegetation of floating-leaved macrophytes, and small species dominated the icthyofauna. We determined the d 13 C and d 15 N ratios of five dominant species of small omnivorous fishes. Using a stable isotope analysis in the R mixing model, we determined the possible contribution of three potential food sources (epiphytic algae, zooplankton and zoobenthos (larval chironomids)) to omnivorous fish tissue compositions. Four omnivorous fishes (Gnathopogon elongatus elongatus, Pseudorasbora parva, Biwia zezera and Tridentiger obscurus) subsisted largely on epiphytic algae and zooplankton, whereas zoobenthos contributed little to their diets. Acheilognathus rhombeus subsisted mostly on epiphytic algae. Thus, in this shallow, eutrophic lake, omnivorous fishes incorporated both littoral and pelagic production into the food web, but rarely benthic production. The dominant benthic chironomid larvae often burrow several centimetres into the sediment, and the low dietary contribution of zoobenthos to small fishes may be due to inefficiency at foraging on buried benthos associated with fish body size.Additional keywords: chironomid, cyprinid, d 13 C and d 15 N, isotope mixing model, methane-oxidising bacteria.

Ontogenetic dietary shifts of largemouth bass do not increase trophic position in a shallow eutrophic lake in Japan

“…2(a)). In Lake Izunuma, pelagic producers (mainly phytoplankton) had lower d 13 C values than did littoral producers (epiphytes and aquatic plants) (Yasuno et al, 2014;Yasuno et al, 2016). This suggests that the contributions of the pelagic food chain to the diet of largemouth bass decreased, while those of the littoral food chain increased as the bass became larger.…”

“…We collected three species of small omnivorous fishes, red swamp crayfish and the two species of dragonfly used in this study. We used reference isotopic data for the other five species of small omnivorous fishes and zooplankton from Yasuno et al (2016) and for larval chironomids from Yasuno et al (2013). Largemouth bass generally forage on zooplankton during their early lifehistory (<ca.…”

“…Our results showed that the trophic positions of small omnivorous fishes were as low as, or lower than, those of copepod zooplankton, although the mean trophic levels of omnivorous fishes and zooplankton were similar (Table 4). In Lake Izunuma, small omnivorous fishes forage on epiphytes (producer; lower trophic level) as well as zooplankton (Yasuno et al, 2016). Foraging on epiphytes may explain the lower trophic positions of small omnivorous fishes.…”