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10 Lockyear.Qxp African Journal of Aquatic Science 2006, 31(2): 275–283 Copyright © NISC Pty Ltd Printed in South Africa — All rights reserved AFRICAN JOURNAL OF AQUATIC SCIENCE EISSN 1727–9364 The distribution and abundance of the endangered Knysna seahorse Hippocampus capensis (Pisces: Syngnathidae) in South African estuaries Jacqueline F Lockyear1, Thomas Hecht1, Horst Kaiser1* and Peter R Teske2 1 Department of Ichthyology and Fisheries Science, Rhodes University, PO Box 94, Grahamstown 6140, South Africa 2 Molecular Ecology and Systematics Group, Botany Department, Rhodes University, PO Box 94, Grahamstown 6140, South Africa * Corresponding author, e-mail: [email protected] Received 7 July 2005, accepted 23 March 2006 The occurrence, distribution and abundance of the endangered Knysna seahorse Hippocampus capensis in 10 estuaries on South Africa’s warm temperate south coast, were investigated. Seahorses were found only in the Knysna, Swartvlei and Keurbooms estuaries. Sex ratios were even and, in most cases, more adults were found than juveniles. During the first year of study, seahorse densities were higher in the Swartvlei and Keurbooms estuaries than in the comparatively larger Knysna Estuary but, during the second year, seahorses were absent from the Keurbooms estuary, and the population size in the Swartvlei Estuary had decreased by more than 80%. These results suggest that, although the two smaller estuaries are able to support comparatively high densities of seahorses, population sizes may fluctuate considerably. Population size estimates for the Knysna Estuary were similar to those obtained in a previous study, suggesting that this estuary may represent a more stable environment and may thus be particularly important for the survival and conservation of this species. Keywords: freshwater floods, Keurbooms Estuary, Knysna Estuary, Swartvlei Estuary, patchy distribution, population size fluctuation, submerged vegetation Introduction The Knysna seahorse Hippocampus capensis Boulenger, be unsuitable for H. capensis because of the absence of 1900 was the first seahorse species listed as endangered submerged vegetation (Toeffie 2000). on the IUCN Red List (Hilton-Taylor 2000) because of its Construction developments are increasing along the limited distribution range, small population size and vulner- species’ available habitats (Skelton 1987), a recent exam- ability to natural and anthropogenic disturbances. Despite ple being the construction of a marina on Thesen’s Island in its conservation status, little information is available on its the Knysna Estuary (Figure 2a). The associated domestic, population sizes, population structure and habitat prefer- industrial and recreational activities, as well as pollution ences (Skelton 1987). Most studies on this species have events and other disturbances, may impact negatively on been conducted under captive conditions (Fourie 1997, the seahorse populations (Skelton 1987). Apart from human Lockyear et al. 1997, Tops 1999, Le Cheminant 2000), or activities, the seahorses are also threatened by freshwater have employed genetic methods (Toeffie 2000, Teske et al. floods. In the Swartvlei Estuary, for example, the breaching 2003, 2005), except for an in situ ecological study of its of the sand bar, which periodically separates this estuary abundance and distribution by Bell et al. (2003). from the sea resulted in a mass mortality that killed at least The Knysna seahorse is endemic to South Africa and has 3 000 seahorses. Seahorses inhabiting the shallow vege- been recorded in the Knysna, Swartvlei and Keurbooms tated areas of the estuary were affected by the sudden drop estuaries (Kok 1981, Whitfield 1989, Russell 1994, Figure in water level and were believed to have died as a result of 1). Anecdotal reports suggest that the species may also high temperature (Russell 1994). occur in the Klein Brak, Breede, Duiwenhoks and Goukou In the light of these potentially adverse impacts on the estuaries (Grange pers. comm.). It has never been reported seahorse populations, research on H. capensis was in samples collected outside estuaries. Knysna seahorses required to suggest a management plan for the conserva- are usually found in shallow water in association with tion of this endangered species. Here we present the aquatic vegetation (Bell et al. 2003) where its prehensile tail results of a survey that continued some of the work is used to grasp plants and other objects (Whitfield 1995). reported by Bell et al. (2003). The main aims were to deter- Previous studies indicated that the distribution of seahorses mine the estuaries in which Knysna seahorses occur and to within the Knysna Estuary was patchy (Bell et al. 2003), describe their distribution and abundance within these and that large areas of habitat in this estuarine system may systems. This information may be useful for identifying 276 Lockyear, Hecht, Kaiser and Teske 33°30 SOUTH AFRICA Goukamma Gamtoos AFRICA Groot Keurbooms Klein Brak Swartvlei Groot Kabeljous Knysna 34° Brak Krom Jeffreys Bay Sedgefield Knysna INDIAN OCEAN SOUTH 0 30 60 90km AFRICA Study 34°30 area 22° 22°30 23° 23°30 24° 24°30 25° Figure 1: Map of the southern Cape coast showing estuaries sampled for seahorses areas of high conservation value, where human activities sampled twice (Table 1b). Spot surveys were performed on should be managed accordingly. We also quantified a number of other estuaries in the region (Klein Brak, Groot seahorse population structures in the Knysna, Swartvlei and Brak, Goukamma, Groot, Kromme, Kabeljous and Gamtoos: Keurbooms estuaries and estimated population sizes to Figure 1) to determine the extent of the distribution of H. provide a baseline for monitoring future changes in popula- capensis. All of these estuaries are located in the warm tion size and composition. temperate coastal province (Day 1981) and, with the excep- tion of the Kromme and the Gamtoos estuaries, they may Materials and methods close temporarily during times of low freshwater inflow. Estuaries studied Population size estimations The Knysna, Swartvlei and Keurbooms estuaries (Figure 1) The surface area of each of the six sampling areas within were sampled during 2001–2003 (Tables 1a and b). In addi- the Knysna Estuary was estimated using an orthophoto map tion, to determine the extent of the distribution of divided into 1 sec2 geographical grid squares (approximately Hippocampus capensis, seven additional estuaries on South 8.3m2). Aerial photographs were used to identify areas not Africa’s south coast (Figure 1) were surveyed between inundated at spring low tide, which were excluded from March and May 2002 (Table 1). surface area estimations because they are unlikely to The Knysna Estuary, which was sampled over a period of provide habitat for seahorses. Surface areas of the Swartvlei 10 months, is the largest of the three systems in which H. (closed phase) and Keurbooms estuaries were determined capensis had previously been recorded. It can be divided using digital topographical data provided by the Chief into three major regimes: the mouth area represents the Director: Surveys and Mapping, South Africa. In the case of marine-dominated ‘bay regime’, the middle section repre- the tidal Keurbooms Estuary, the areas located above the sents the ‘lagoonal regime’ and the upper estuary repre- spring low tide mark (e.g. sandbanks, mudflats and islands) sents the ‘estuarine regime’ (Largier et al. 2000). To deter- were subtracted from the total surface area. Seahorse abun- mine the suitability of the different portions of the Knysna dance in each of the six sampling areas within the Knysna Estuary to provide habitat for H. capensis, the estuary was Estuary, and in the Swartlvlei and Keurbooms estuaries, divided into six sampling areas (A–F, Figure 2a), each of was estimated by determining seahorse densities during which could be assigned to one of the three estuarine surveys (see below). Seahorse density estimates were regimes: Area A represents the marine-dominated ‘bay converted to individuals m–2 and then multiplied by the corre- regime’, areas B–E correspond to the ‘lagoon regime’, and sponding surface area estimates. area F represents the ‘estuarine regime’. Environmental characteristics, including long-term salinity regimes, temper- Sampling procedure atures and approximate distributions of dominant organisms Visual transect surveys (used to divide the lagoon regime into four biologically In the Knysna Estuary, distribution and abundance of meaningful units) within each of the six sampling areas, are Hippocampus capensis were mostly determined by visual listed in Table 2. transect line counts using SCUBA, after Curtis et al. (2004). In comparison to the Knysna Estuary, the two smaller Grid squares were randomly selected and at least one site estuarine systems in which H. capensis has been recorded, was sampled in each. The starting position and compass the Swartvlei and Keurbooms estuaries, are characterised direction of each transect within a grid square were by more homogeneous environmental conditions: both are randomly selected and entered into a Garmin Etrex hand- characterised by shallow water and have seagrass beds held GPS. On reaching a selected GPS position by boat, the along their entire lengths (Whitfield et al. 1983, Duvenage anchor was released, and a diving reel was attached to it. and Morant 1984). Because of this and their comparatively Two divers were deployed and each searched for seahorses smaller size, the Swartvlei and Keurbooms estuaries were
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