The lesser of two evils: the role of an instream barrier to control the distribution and invasive potential of non-native fish species in the Mooi River, South Africa

Abstract

Context Recreational angling has a history of species introductions causing significant loss of native biodiversity. Despite this, it represents a large economy supporting many livelihoods. The Mooi River, South Africa, supports trout fisheries in its upper reach. A weir was constructed to mitigate the loss of a natural barrier, now inundated by a large impoundment. Aims We evaluated the Inchbrakie Weir’s efficacy in restricting movement of non-native fish. Methods We evaluated the weir as a fish barrier by using fish surveys. We used species distribution modelling to assess the potential for bass to invade the upper reaches should they make their way past the barrier. Key results Our surveys detected three bass species in the study area, including Micropterus salmoides upstream of the fish barrier. Micropterus dolomieu was the main concern with the highest invasion potential, but was not found upstream of the fish barrier. Distribution modelling showed that suitable habitat was available for all three detected bass species, with M. salmoides showing the greatest potential in the upper Mooi River. Implications Invasion by Micropterus spp. concerns the local trout fishery and native fauna persistence. We suggest a long-term continuous monitoring plan to detect the early invasion of M. dolomieu and that a pre-emptive management plan is in place.