Changes in the fish community of the St Lucia estuarine system (South Africa) following Cyclone Gamede, an episodic cyclonic event

Abstract

Lake St Lucia, the largest estuarine lake system in Africa, has been closed since 2002 except for a five month period in 2007 when the mouth was breached by an episodic event, Cyclone Gamede. Mouth opening in 2007 was preceded by five years of low lake levels and hypersaline conditions, which changed the fish species composition and greatly reduced marine fish species diversity. Mouth opening in March 2007 caused the system to partially fill and marine salinities were restored, allowing recruitment of marine species. This study assessed changes in the fish community during the period 2008–2011 following mouth closure after the Cyclone Gamede breaching event. A total of 19,726 fish representing 61 species were collected using seine netting at six sites across the system during biannual sampling between 2008 and 2011. After mouth closure in August 2007, salinities increased and North Lake became hypersaline during 2009–2010. Marine species initially dominated in terms of species number, but freshwater and estuarine species became progressively more abundant, notably Oreochromis mossambicus and Ambassis ambassis. The number of species decreased from 51 in 2008 to 37 in 2011, with 22 marine and two estuarine species being lost from the system. The abundance of marine species decreased by >75% as the system became progressively more saline and the closed mouth limited recruitment to infrequent overtopping events. Changes in the fish community were largely driven by salinity changes across the three main regions of the system. Although management efforts to increase freshwater inflow from the Mfolozi estuary was successful, rapid siltation of the mouth area is a concern.