Fascinating fish of the Klein River Estuary

By Sue Matthews, Overstrand Estuary Management Coordinator

A few months ago I joined a research team from the Department of Agriculture, Forestry and Fisheries to spend a day sampling the Klein River estuary’s fish. Apart from the ubiquitous mullet, our trek-net hauls yielded some fascinating little fish that serve as ‘fodder’ for the estuary’s piscivorous birds and predatory fish.

For instance, the longsnout pipefish Syngnathus temminckii belongs to the same family as the Knysna seahorse, and has the same tube-shaped ‘snout’ - as its name implies - but lacks the seahorse’s bulging belly and arched back. Instead it has a skinny eel-like body that tapers down to a small tail fin. Like the seahorse, though, it is the male that goes through ‘pregnancy’. The female deposits her eggs into the male’s brood pouch, where fertilisation takes place and the embryos develop. 

Longsnout pipefish inhabit eelgrass beds, and their shape and coloration make them so well camouflaged that they can be mistaken for a blade of grass. They feed on free-swimming zooplankton, as well as amphipods clinging to the surface of plants, hoovering them up through the tube-shaped mouth. Both seahorses and pipefish are protected species under the Marine Living Resources Act, which prohibits their “fishing, collecting or disturbing”. 

The Cape halfbeak Hyporhamphus capensis [hypo = under, rhampus = beak or bill] has an even stranger mouth, its lower jaw being extended well beyond the upper to form a long ‘needle’. The function of this apparatus is controversial – some claim it allows the fish to manoeuvre filamentous plant matter along the length of the lower jaw, from where it can be drawn into the mouth, while others think it enables small prey items at the surface to be gathered up. The fish is indeed an omnivore, feeding on submerged aquatic vegetation and small crustaceans, bivalves and insects in the plant canopy. It is closely related to the marine garfish, and distantly related to needlefish and flying fish. Like the latter, it is known to leap along the water surface to evade predatory fish. 

The Bot River klipfish Clinus spatulatus was first recorded in the Bot River estuary in 1983, but is actually more abundant in the Klein River estuary. The most common fish in our nets were the Cape silverside Atherina breviceps and the estuarine roundherring Gilchristella aestuaria, which can only be told apart by the number of dorsal fins and the length of the stomach. Both have a bright silver stripe down the side of the body, hence the common name ‘silverside’ for Atherina (old Greek for smelt) breviceps (Latin for short head), while the roundherring is named after the Cape Colony’s first government-appointed marine biologist, John D. Gilchrist, who described the species in 1914. These shoaling fish are such an important food source for birds and larger fish that less than 1% of their populations survive beyond two years of age.  

The research team, headed by Dr Steve Lamberth, samples the estuary’s fish at least twice per year – in summer and winter – to monitor long-term changes in fisheries recruitment, as well as the response to reduced freshwater flow, which helps inform the mouth management policy for the estuary. The duration and frequency of open-mouth states is less than it was historically because runoff into the Klein River has been reduced by about 25% due to alien infestation and intensive agriculture in the catchment. This means that there is insufficient freshwater flow to prevent the mouth from being closed by marine sediments. In order to restore the estuary’s connection with the sea and maintain natural variability and diversity in its ecosystem, the mouth may need to be artificially breached periodically, but only at water levels high enough to ensure good scouring of accumulated sediments.