Browsing by Author "Cowley, P D"

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    A multidisciplinary study of a small, temporarily open/closed South African estuary, with particular emphasis on the influence of mouth state on the ecology of the system
    (2008) Whitfield, A K; Adams, J B; Bate, G C; Bezuidenhout, K; Bornman, T G; Cowley, P D; Froneman, P W; Gama, P T; James, N C; Mackenzie, B; Riddin, T; Snow, G C; Strydom, N A; Taljaard, S; Terörde, A I; Theron, A K; Turpi, J K; van Niekerk, L; Vorwerk, P D; Wooldridg, T H
    In 2005/2006 a multidisciplinary research programme that included studies on the hydrodynamics, sediment dynamics, macronutrients, microalgae, macrophytes, zoobenthos, hyperbenthos, zooplankton, ichthyoplankton, fish and birds of the temporarily open/closed East Kleinemonde Estuary was conducted. Particular attention was given to the responses of the different ecosystem components to the opening and closing of the estuary mouth and how this is driven by both riverine and marine events. Using a complementary dataset of daily estuary mouth conditions spanning a 14-year period, five distinct phases of the estuary were identified, including closed (average = 90% of the days), outflow (<1%), tidal (9%) and semi-closed (<1%). The open-mouth phase is critical for the movements of a number of estuary-associated fish (e.g. Rhabdosargus holubi) and invertebrates (e.g. Scylla serrata) between the estuary and sea. The timing of this open phase has a direct influence on the ability of certain estuaryassociated fish (e.g. Lithognathus lithognathus) and invertebrates (e.g. Palaemon peringueyi) to successfully recruit into the system, with a spring opening (October/November) being regarded as optimal for most species. The type of mouth-breaching event and outflow phase is also important in terms of the subsequent salinity regime once the berm barrier forms. A deep mouth breaching following a large river flood tends to result in major tidal inputs of marine water prior to mouth closure and therefore higher salinities (15–25). Conversely, a shallow mouth breaching with reduced tidal exchange during the open phase often leads to a much lower salinity regime at the time of mouth closure (5–15). The biota, especially the submerged macrophytes, respond very differently to the above two scenarios, with Ruppia cirrhosa benefiting from the former and Potamogeton pectinatus from the latter. River flooding and the associated outflow of large volumes of water through the estuary can result in major declines in zooplankton, zoobenthos, hyperbenthos and fish populations during this phase. However, this resetting of the estuary is necessary because certain marine invertebrate and fish species are dependent on the opening of the estuary mouth in order to facilitate recruitment of larvae and post-larvae into the system from the sea. Slight increases in the numbers of certain piscivorous and resident wading bird species were recorded when the estuary mouth opened, possibly linked to increased feeding opportunities during that phase.
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    Open Access
    Alternate explanations of the dispersal pattern of galjoen Dichistius capensis
    (2005) Attwood, C G; Cowley, P D
    The movement behaviour of galjoen Dichistius capensis (>250mm total length) was studied by using a tag and recapture technique. A total of 25 191 galjoen was tagged at four sites in South Africa. Three of the sites were in fully protected reserves (two in the De Hoop Marine Protected Area, one in the Tsitsikamma National Park), whereas the fourth site was at the Cape Peninsula where fishing is permitted. In all, 2 174 fish were recaptured, some of them several times. The vast majority of fish were caught at the site of release. After correcting for differences in recovery effort between the release sites and other areas, it was estimated that 95% of tagged fish remained at the release site. The remaining 5% moved throughout the South African range of galjoen, which was separated from the Namibian range. There was no evidence to suggest that movement is linked to season, age or sex. Although not territorial, galjoen do hold home ranges, which were estimated to be no larger than 1.38km in extent, but probably much smaller. Two likely models, the polymorphic and the tourist, are advanced to explain the movement behaviour. The polymorphic model is a combination of two movement patterns, one resident and another nomadic. The balance between the two may represent a mixed evolutionary stable strategy. The tourist model does not differentiate between fish. Each fish spends its time at a small number of widely separated sites, moving between them as conditions dictate. The tag and recovery data do not favour one model above the other unequivocally.