The comparative ecology of Natal's smaller estuaries.

Abstract

Fifty one of Natal's 73 estuaries have been almost completely overlooked in terms of any scientific study, despite which a marine nursery function has commonly been attributed to each of them. In the knowledge that many of these systems were normally closed and others were in a seriously degraded state, this study was undertaken with the aims of examining their present day community structure in order to provide a basis upon which their future condition can be monitored and to provide a classification of these coastal resources. The study area incorporated 62 different systems extending over 240 km of the Natal coastline south of the Tugela River. During the three year study period (Sep 1979 - Nov 1982) 82 515 specimens, comprising an assemblage of 125 different species, were caught by means of a small beam trawl. These comprised 86 species of fish, 21 species of crabs and 18 species of prawns. The data obtained were correlated to abiotic variables such as mouth condition, salinity, temperature, dissolved oxygen, water transparency, depth, nature of the substratum and peripheral vegetation. Based on the ability of biota to synthesize environmental variables into one common response, multi-variate analysis is used to demonstrate the similarity in community structure between open and closed systems (for example) or between fresh and saline systems, and thereby resolve an age-old argument about estuary classification. The data also suggest that in their present day condition only six of the systems studied make a significant contribution to the recruitment of estuarine-dependent marine stocks (sensu stricto) principally because of their open mouth condition. Closed systems, deliberately classified as lagoons, have a different resource value, being utilized primarily by resident species that can complete their life cycle within the system. An appreciation of this salient difference helps to reinforce the critical need for an effective management strategy to be implemented to prevent Natal's dwindling estuarine resources from deteriorating any further. The term 'estuarine-dependence' is critically examined in this context to show that a species more dependent on estuaries than any other, is man. It is argued that man's continued abuse of these resources is shortsighted, and that the most serious threat of all is sedimentation, accelerated in this instance beyond the geological norm by catchment mismanagement. The practical application and value of classification to planning and management is demonstrated and a methodology proposed, based on community responses, for the monitoring of the future environmental condition of each estuary and lagoon in Natal.