Body size–trophic position relationships among fishes of the lower Mekong basin

Abstract

Body size is frequently claimed to be a major determinant of animal trophic interactions, yet few studies have explored relationships between body size and trophic interactions in rivers, especially within the tropics. We examined relationships between body size and trophic position (TP) within fish assemblages in four lowland rivers of the Lower Mekong Basin in Cambodia. Stable isotope analysis (based on δ15N) was used to estimate TP of common fish species in each river, and species were classified according to occupation of benthic versus pelagic habitats and major feeding guilds. Regression analysis yielded strong correlations between body size and TP among fishes from the Sesan and Sreprok rivers, but not those from the Mekong and Sekong rivers. The Mekong fish assemblage had higher average TP compared with those of other rivers. The relationship between body size and TP was positive and significantly correlated for piscivores and omnivores, but not for detritivores and insectivores. The body size–TP relationship did not differ between pelagic and benthic fishes. Body size significantly predicted TP within the orders Siluriformes and Perciformes, but not for Cypriniformes, the most species-rich and ecologically diverse order in the Lower Mekong River. We conclude that for species-rich, tropical fish assemblages with many detritivores and invertivores, body size would not be an appropriate surrogate for TP in food web models and other ecological applications.

Climate effects on fish body size–trophic position relationship depend on ecosystem type

“…Jardine (2016) proposed that increasing body size would lead to a greater consumer biomass reliance on allochthonous material in freshwaters. This mechanism is supported by the strong land-water connectivity generally observed in inland aquatic ecosystems (Jardine 2016), which could decrease the trophic position of larger fish in freshwaters (Ou et al 2017). Allochthonous material in freshwater systems is mostly derived from terrestrial plant detritus rich in carbon compared to nutrients (Hecky et al 1993), especially at lower latitudes (Boyero et al 2017).…”

“…Allochthonous material in freshwater systems is mostly derived from terrestrial plant detritus rich in carbon compared to nutrients (Hecky et al 1993), especially at lower latitudes (Boyero et al 2017). Therefore, the association among the higher relative species richness of omnivorous fish in tropical waters (González-Bergonzoni et al 2012), the higher reliance of large aquatic consumers on terrestrial carbon and the high availability of carbon-rich plant-derived allochthonous material in tropical freshwaters may help elucidate the lower trophic position of larger tropical freshwater fish species (Ou et al 2017), compared to their temperate counterparts.…”

Environmental niche patterns of native and non‐native fishes within an invaded African river system

“…For the native fishes that co‐occurred in the downstream section of the study systems, it is likely that their coexistence within similar habitat conditions may be mediated by trophic niche differentiation mechanisms. Studies elsewhere have shown that lower reaches of rivers are characterised by high richness that is mediated by trophic diversification that allows different fishes to occupy different trophic positions (Guo et al, ; Ou et al, ). Secondly, the general absence in the Great Fish River's upper mainstem section of the E. anoplus , which was historically widespread (Laurenson & Hocutt, ), suggest the likely impact of the IBWT in influencing both habitat availability for this species, together with the proliferation of multiple non‐native fishes, which include generalist predators such as C. gariepinus (Kadye & Booth, , ).…”

Consumer trophic positions respond variably to seasonally fluctuating environments

“…Additional data for foraging behavior, habitat, mobility and physiology of Tonle Sap species would help address these ideas, but body size (Ou et al. ) and seasonal feeding strategies are known to vary widely within fish functional groups from other tropical floodplains (Novakowski et al. ).…”

“…The capacity to forage across spatially complex habitats has also been linked to cognitive capacity (brain size) both within and among freshwater fish species (Edmunds et al 2016), suggesting there could be physiological limits on the types of habitats and resources used by fish. Additional data for foraging behavior, habitat, mobility and physiology of Tonle Sap species would help address these ideas, but body size (Ou et al 2017) and seasonal feeding strategies are known to vary widely within fish functional groups from other tropical floodplains (Novakowski et al 2008). While difficult to predict, the variable nature of seasonal foraging strategies therefore appear characteristic of fishes inhabiting these systems.…”