Fish breeding in, and juvenile recruitment to, the St Lucia Estuarine System under conditions of extended mouth closure and low lake levels

Abstract

Ongoing drought conditions along the northern coast of KwaZulu-Natal resulted in the St Lucia Estuary mouth closing, and to evaporation leading to extremely low lake levels and the development of hypersaline conditions in most parts of the system. The mouth closed in June 2002 and has remained closed for more than three years. The St Lucia Estuarine System is an important nursery ground for juveniles of many marine fish species. Ezemvelo KZN Wildlife initiated a once-off survey to investigate the effects of the drought on the fish fauna and to establish if any fish were breeding in the system under the existing drought conditions. Fish were sampled at six sites throughout the system, using seine and gill-nets. In December 2004, the system had become partitioned into four isolated compartments, and at this time a total of 4 475 individuals comprising 30 species were recorded. An analysis of size class data indicated the presence of juveniles of at least 12 species, including nine estuarineassociated marine species, which had spawned after mouth closure. Although some estuarine species had spawned within the system, most appeared to have recruited into the Narrows and lake sections of the St Lucia system during January 2004 when the adjacent Mfolozi River, which is linked to the open Mfolozi Estuary, flooded and overtopped into St Lucia Estuary. Should the St Lucia mouth remain closed for four or more years, it can be expected that the lack of recruitment of first-time spawners from the St Lucia nursery grounds into the off-shore marine breeding stocks will have major impacts on these estuaryassociated marine species.

Keywords: drought, environmental impacts, environmental monitoring, estuary, nursery ground

African Journal of Aquatic Science 2006, 31(1): 83–87