A comparison of the ichthyofaunas in two permanently open eastern Cape estuarie

<p><strong>ENGLISH ABSTRACT: </strong>The Kowie and Great Fish estuaries are situated less than 30 km apart, yet they differ considerably in terms of riverine inflow, turbidity, food resources and habitat availability. The ichthyofauna of the two estuaries were sampled using plankton, seine and gill nets. A greater ichthyofauna richness (R) was recorded in the Kowie estuary and this is attributed to the wider range of habitats and greater degree of marine influence in this system. In contrast, all three sampling gears revealed an approximate 3:1 ratio between fish abundance in the Great Fish and Kowie estuaries. The higher abundance of fishes in the Great Fish estuary is partly attributed to the large organic and nutrient inputs into this system when compared with the Kowie system, and the influence of these inputs on estuarine primary and secondary production. In dividual fish species are affected differently by turbid water conditions. In dications from this study were that piscivorous fishes (e.g. Lichiaa mia) which rely mainly on visual foraging methods were adversely affected by the high turbidity conditions within the Great Fish estuary, whereas piscivores (e.g. Argyrosomus hololepidotus) which rely mainly onnon-visual methods were unaffected. Macrobenthic predators (e.g. Pomadasys commersonnii) and detritivorous fish species (e.g. Mugil cephalus) also appear to be unaffected by high suspensoid levels and were usually more abundant in the Great Fish than in the Kowie estuary. The length-frequency distributions of some of the dominant fish species occurring in both estuaries rea presented.</p><p>********</p><strong>AFRIKAANSE OPSOMMING:</strong> Die Kowie en Groot Vis riviermondings is minder as 30 km van mekaar af geleë, maar verskil aansienlik in terme van varswaterinvloei, watertroebelheid, voedselbronne en beskikbare habitat. Die visfauna van die twee riviermondings was met plankton, trek en kiefnette gemonster. ‘n Hoër visspesiesrykheig (R) is in die Kowie riviermond vasgestel en dit word toegeskryf aan die groter habitat verskeidenheid en groter see invloed op die sisleem. In teenstelling het al drie moniteringsmetodes ongeveer 3:1 verhouding tussen visgetalle in die Groot Vis en Kowie riviermondings aangetoon. Die groter aantal visse in die Groot Vis riviermonding word deelteliks toegeskryf aan die groot hoeveelheid organiese en voedingstoevoegings tot hierdie sisteem in vergelyking met die Kowie sisteem, en die invloed van hierdie toevoegings tot primêre en sekondêre produktiwiteit. Sommige visspesies word deur watertroebelheid aangetas. Afleidings van hierdie studie dui daarop dat visvretende visse (b.v. Lichia amia), wat hoofsaaklik op visuele metodes aangewys is, meer negatief deur troebelheid in die Groot Vis riviermonding beïnvloed word, terwyl visvreters (b.v. Argyrosomus hololepidotus), wat hoofsaaklik nie-visuele metodes gebruik, nie aangetas is nie. Bodemroofvisse (b .v. Pomadasys commersonnii) en detritus-vretende visspesies (b.v. Mugil cephalus) word klaarblyklik nie deur hoë troebelheid aangetas nie, en was gewoonlik meer talryk in die Groot Vis riviermonding. Die lengte-frekwensie verspreiding van die belangrikste vi spesies wat in beide riviermondings voorkom, word ook aangetoon.