Predation by introduced largemouth bass Micropterus salmoides on indigenous marine fish in the lower Kowie River, South Africa

The predatory impact of introduced largemouth bass Micropterus salmoides on the juveniles of estuary-associated indigenous marine fish species in the lower Kowie River and estuary headwaters was investigated in 2012–2013. Stomach contents and stable isotope analyses were employed to assess the dietary composition of small, medium and large bass during 2012 and 2013. Small bass were more dependent on estuary-associated fish species as prey than were larger bass, with the latter relying more on freshwater crabs and aquatic insects as primary food sources. This was reflected in the stomach contents analyses of the three different size classes of bass, as well as in the δ¹³C and δ¹⁵N isotope results. The study highlights the need for careful consideration prior to the introduction of predatory alien fish into South African waterbodies in future. In particular, future introductions should take into account their possible impacts on indigenous marine fish, especially those species that migrate between estuarine and freshwater ecosystems as juveniles.