Trophic ecology of co‐occurring fishes in the Sundays River Valley irrigation ponds, assessed using stable isotope and gut content analyses

Abstract

The analysis of food web structures has increased the understanding of the dynamics of organisms belonging to different trophic levels. In this study, the diet of two native species, Glossogobius callidus and Gilchristella aestuaria, was assessed in the presence of two non-native species, Oreochromis mossambicus and Gambusia affinis, in irrigation ponds, Eastern Cape Province, South Africa. The proportion of dietary items consumed and assimilated by the four fish species were inferred from gut contents and carbon and nitrogen stable isotope analysis. Stable isotope analysis revealed that both G. affinis and O. mossambicus had a larger isotopic niche size than G. callidus and G. aestuaria. Although G. callidus fed on benthic resources and G. aestuaria fed on phytoplankton, gut content analysis showed that G. callidus, O. mossambicus and G. affinis fed predominantly on benthic resources, whereas G. aestuaria fed mainly on plankton resources. Considerable niche overlap corroborates the view that resource competition is a major factor shaping the composition of the four fish species. This study highlighted the low diversity of the food web within the Sundays River Valley irrigation ponds, where food items are shared by all the small-bodied fishes.

A systematic review of poecilid fish invasions in Africa

“…Stable isotope analysis in Sundays River Valley irrigation ponds nds G. a nis to exhibit a broad dietary niche, overlapping considerably with the native Mozambique tilapia (Oreochromis mossambicus) (Mofu et al, 2023). Negative spatial interactions incur from resource competition between G. a nis and G. callidus (J.…”

“…Negative spatial interactions incur from resource competition between G. a nis and G. callidus (J. South -unpublished data; Mofu et al, 2023). However, the varying diets of G. a nis size classes indicate greater predator e ciency and prey-switching throughout the water column, potentially in attempt to avoid competition with River Goby (Mofu et al, 2023).…”

“…Therefore, biotic homogenisation is a likely outcome of poecilid invasions, which reduces both taxonomic and functional biodiversity, and will be exacerbated by multiple stressors of climate change and degradation (Dallas and Rivers-Moore, 2014). 2016) and the ecological niches of multiple species(Mofu et al, 2023) will provide the data necessary to inform policy decisions.Climate change and future perspectivesClimate change is in uencing the likelihood of invasion and establishment of non-native freshwater species; a trend set to continue in the future(Seebens et al, 2020; van Wilgen et al, 2023). Given that invasive poeciliids inhabit many sub-optimal habitats in Africa(Lawal et al, 2012;Viskich et al, 2016; Gomes-Silva et al, 2020a), dominate hypoxic and brackish ecosystems over a wide distribution, and adapt to higher temperatures(Mofu et al, 2019a;Jourdan et al, 2021); deteriorating environmental conditions will continue to favour their establishment.…”

Low redundancy among caddisflies sharing a mid‐mountain stream

“…Carbon (δ 13 C) and nitrogen (δ 15 N) stable isotope analysis enables the characterisation of consumers' resource use and trophic niche structure respectively (Matich et al., 2017), and complements trophic studies assessed using gut content (Mofu et al., 2023). Through a progressive increase along trophic chains in 15 N/ 14 N (difference between a consumer and its diet is typically 3–4‰; DeNiro & Epstein, 1981; Peterson & Fry, 1987), it is possible to describe a continuous measure of the trophic position of consumers within community food webs.…”