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African Journal of Aquatic Science 2007, 32(2): 113–123 Printed in South Africa — All rights reserved
AFRICAN JOURNAL OF AQUATIC SCIENCE ISSN 1608–5914 doi:10.2989/AJAS.2007.32.2.2.199
Composition, abundance, distribution and seasonality of larval fishes in the Mngazi Estuary, South Africa P Pattrick1, NA Strydom2* and TH Wooldridge3 1
Department of Entomology and Zoology, Rhodes University, Grahamstown 6140, South Africa South African Institute for Aquatic Biodiversity, Private Bag 1015, Grahamstown 6140, South Africa 3 Department of Zoology, Nelson Mandela Metropolitan University, PO Box 1600, Port Elizabeth 6000, South Africa * Corresponding author, e-mail:
[email protected] 2
Received 31 March 2006, accepted 23 March 2007 We investigated the larval fish assemblage in the intermittently-open Mngazi Estuary, in the subtropical/warm-temperate biogeographic boundary region of South Africa. Larvae were collected by means of boat-based plankton tows in summer and winter for a period of three years between 2003 and 2005. Within the Mngazi Estuary, salinity ranged from 10.1–37.4, temperature ranged from 15.5°C–27.3°C, pH ranged from 6.8–8.3ppt and oxygen ranged from 2.6mg l–1–8.8mg l–1. In total, 8 343 larval fishes were collected, representing 18 families and 31 species. Clupeidae and Gobiidae were the dominant fish families whilst the dominant species was the estuary-resident Gilchristella aestuaria (Clupeidae). Estuary-resident larvae were the dominant group in the system. Larval fish density ranged from 0–38 600 individuals per 100m3 in summer and from 0 to 638 individuals per 100m3 in winter. Species diversity also varied seasonally. Salinity zones played a significant role in terms of species distribution. The euhaline zone supported the highest species diversity while the mesohaline zone supported the highest densities of larval fishes. Salinity, temperature and turbidity were identified as important variables governing larval fish dynamics in the system. Keywords: biogeographic boundary, estuary-dependent taxa, impoundments, physico-chemical variables, salinity zones
Introduction Estuaries are important as nurseries for the larvae and juveniles of many species of estuarine and marine fishes (Whitfield 1998, Strydom et al. 2003). Most estuary-dependent marine fishes enter these systems during the postflexion stage of their larval phase (Miskiewicz 1986, Strydom and Whitfield 2000). The abundance of food, shelter from planktonic marine piscivores and favourable temperature regimes found in estuaries allow larval fishes to complete their transition into juveniles with greater success. Some species remain in these nursery estuaries until they are sexually mature (Whitfield 1998). Research on larval fishes in South Africa has been conducted mainly in the warm-temperate region and, to a much lesser extent, in the subtropical KwaZulu-Natal region. These studies have focused on the structure and composition of larval fish assemblages in both single (Melville-Smith and Baird 1980, Melville-Smith 1981, Whitfield 1989a) and multi-estuary studies (Harris and Cyrus 2000, Strydom et al. 2003); selective tidal stream transport (Melville-Smith et al. 1981); habitat utilisation (Strydom 2003a), and tidal exchange (Beckley 1985, Whitfield 1989b, Strydom and Wooldridge 2005). To date, no larval fish studies have been conducted in the subtropical/warm-temperate boundary region of South Africa (Maree et al. 2000). Obtaining information about species composition, distribution and seasonal variations in previously unstudied areas is important in
understanding larval fish community dynamics along the South African coast. This study focused on a representative estuary within this boundary region. The Mngazi Estuary is situated in the former Transkei. There is very little published information available on this estuary, although temporal changes in the species composition, distribution and habitat preferences of adult fishes within it have been investigated (Mbande et al. 2002). The present study represents the first estuarine larval fish study in the South African subtropical/warm-temperate biogeographic boundary region, and aims to provide information on the composition, abundance, distribution and seasonality of larval fishes in this estuary. Materials and methods Study area Larval fishes were collected from the intermittently–open (Wooldridge pers. obs.) Mngazi Estuary (31°41NS, 29°27NE) situated south-east of Port St Johns, in the former Transkei region of South Africa (Figure 1). The climate here is classified as subtropical (Whitfield 2000), with rainfall occurring predominantly in summer and reaching a maximum in March (Day 1981). The Mngazi Estuary is approximately 6.5km in length, ranging between 50m and 170m in width and is generally shallow, with depths
