Niche expansion by indigenous fish species following the introduction of rainbow trout (Oncorhynchus mykiss) in a subtropical river system, upper Blyde River, South Africa

Abstract

The introduction of alien predatory fish such as rainbow trout (Oncorhynchus mykiss) can have a significant effect on indigenous fish communities such as altering the structure and dynamics of food webs. Quantifying the trophic niche utilised by the alien fish species is therefore important to aid our understanding of how their feeding strategies might aid establishment, define their functional role and inform on potential impacts. This study assessed food web interactions between fish communities in river reaches that are invaded by O. mykiss and sections that are free of invasions in the upper Blyde River catchment, South Africa. It specifically evaluated the hypothesis that O. mykiss invasion is likely to lead to a decrease in the trophic functional diversity through predation and that indigenous fish will shift their trophic niche to either minimise competition with, or avoid predation, by O. mykiss. Gut content and stable isotope analyses were used to determine trophic interactions. Fish communities in uninvaded areas utilised fewer and similar food sources and occupied lower trophic levels, than fish communities in invaded areas. Oncorhynchus mykiss fed mainly on invertebrates and at trophic levels similar to and or lower than indigenous fishes. This suggests that their current impact on indigenous fish communities is mainly through competition for resources. We posit that O. mykiss invasions reduced the abundance of indigenous fishes and thereby reduced predation pressure on aquatic invertebrates, with a knock-on effect on the trophic interrelationships among fish assemblages. Our findings are consistent with other studies in South Africa that have shown that the impacts of O. mykiss invasion can lead to a decline and fragmentation of indigenous species populations and niche shifts.

Living on the edge: Reservoirs facilitate enhanced interactions among generalist and rheophilic fish species in tributaries

“…Another example is the construction of the Three Gorges Reservoir, China, which led to a reduction in rheophilic fishes in the tributaries and increased competition with 18 non-native generalists that proliferated in the modified reservoir environment (Liao et al, 2018;Lin et al, 2018;Liu et al, 2019). Maimela et al (2022) revealed that adverse impacts on community structure and function were observed due to competition for food and space between O. mykiss and indigenous species in the upper Blyde River Catchment, South Africa. Psedudobarbus verloenii, a tropical rheophilic species, is reported to decline in South Africa due to reservoir construction.…”