Fish communities in temporarily open/closed estuaries from the warm- and cool-temperate regions of South Africa: A review |
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Authors: | Nicola C James Paul D Cowley Alan K Whitfield Steve J Lamberth |
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Institution: | (1) Department of Ichthyology and Fisheries Science, Rhodes University, Grahamstown, 6140, South Africa;(2) Present address: South African Institute for Aquatic Biodiversity, Private Bag 1015, Grahamstown, 6140, South Africa;(3) Marine and Coastal Management, Private Bag X2, Rogge Bay 8012, Cape Town, South Africa |
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Abstract: | The majority of estuaries along the coastline of southern Africa are termed temporarily open/closed estuaries (TOCEs) and
are closed off from the sea for varying periods by a sandbar which forms at the mouth. It is therefore important to understand
the processes occurring within TOCEs and their importance to fishes in order to make sound management recommendations. Estuaries
along the coast of South Africa and their associated fish assemblages are biogeographically distinct and occur in either a
subtropical, warm-temperate or cool-temperate zone. There are 125 TOCEs found within the cool-temperate and warm-temperate
zones. Most fish species found in TOCEs are the juveniles of marine taxa that breed at sea. Permanently open estuaries generally
have a higher diversity of species than TOCEs, but TOCEs still provide important nursery areas for many marine species and
numerically often have a higher proportion of estuarine resident species. Important taxa in terms of abundance and biomass
in warm-temperate TOCEs include the sparids Rhabdosargus holubi and Lithognathus lithognathus, several mugilid species, estuarine residents (particularly Gilchristella aestuaria and Atherina breviceps) and the freshwater cichlid Oreochromis mossambicus. The diversity of fishes in cool-temperate TOCEs is low when compared with warm-temperate systems and Liza richardsonii tends to dominate catches by number and mass in most systems. Several species recorded in TOCEs show clear longitudinal distribution
trends. For example Atherina breviceps is generally more abundant in the lower reaches of estuaries. Mouth state, particularly the frequency, timing and duration
of mouth opening plays a key role in determining species richness, composition, diversity and abundance in TOCEs. Mouth state
is directly linked to freshwater input. Reduced river inflow leads to prolonged mouth closure and shorter open phases, which
inhibits immigration and emigration of marine fish species between estuaries and the sea. Understanding of the effects of
various processes occurring within these systems, particularly variation in freshwater input, on the biota of these important
systems facilitates the development of informed management recommendations. |
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Keywords: | Temporarily open/closed estuaries Fish communities Mouth opening Flow variation South Africa |
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