Zandvlei Trust

Keeping tabs on Fish.

(Extract article from the volume 2008/3 Newsletter)

Photographs by Charlie Ward and Cassandra Sheasby.

The winter fish survey in Zandvlei done during blustery wet weather in July, is part of an ongoing programme that is run in partnership between Marine and Coastal Management (MCM) and Biodiversity Management (Nature Conservation). The City of Cape Town's environmental resource management department conducts these fish treks each quarter to get a representative idea of the fish using the estuary each season.

Rowing one of the nets out across the vlei           The hard work starts, pulling the nets in.

MCM and Zandvlei Nature Reserve staff pulling in the net.

What have we got?                                                  Just some of the sample catch, which is checked,
                                                                              for species, sorted, sized, catagorised, weighed
                                                                              checked for condition and much more......

This data collection has been on going for a number of years and Steve Lambeth of MCM is busy analysing the data and writing a scientific paper on the trends.
MCM have a permit to use trek nets and gill nets in the estuary for scientific purposes. During the survey the fish that are identified, measured and indigenous fish such as Leervis are put back into the water as fast as possible. The fish that are removed make up a representive sample of the estuary and are a very small number in relation to the size of the population. The number of fish is then mulitplied out by using a formula. Its not an exact science, but is used to give an approximation and to work out the trends through the year and over a longer period of time.

What have we got?                                                                      Checking the gills.   

Sebastian with one of the carp.                                         The fish are sorted into sizes for research
                                                                                      information.

Sebastian checking the small fish.                   Fish being measured.

When Zandvlei was managed by Clifford Dorse in 1998, 29 fish species were found to be present in the estuary. These included some very important fish such as White Steenbras (endangered) Leervis or Garrick, White Stumpnose, Cape Stumpnose and indicator species such as Longsnout pipefish which help to indicate the health of the water body.

Cape Stumpnose                                                            A large Leervis or Garrick, which was released.

During the past 3 fish surveys, 14 species of the 29 have been recorded. The other 15 we still have to look for which will be part of our Fisheries Resource Management student Sebatian Osborne's project for this year. During the previous survey many adult mullet (Liza richardsonii)  were recorded. However during this survey (winter) the mullet caught were only juveniles and no adults were found. This indicates that the adults are migrating out to sea seasonally to spawn.

Pipe fish in a container                                  Close up of a pipe fish head.

Indicator species such as the Longsnout pipefish and Knysna Goby were also reorded indicating that the water quality is relatively good as these animals live close to the bottom sediments which trap the majority of heavy metals and pollution.

Knysna Goby

Black hand Sole                                               Cape Sole

Should any members of Zandvlei Trust wish to see what we do during the quarterly fish treks, they are most welcome to contact me. The fish treks are weather dependant but we will let any interested parties know the dates if they wish to help or spectate.

Cassandra Sheasby
Manager;  Greater Zandvlei Estuary Nature Reserve.

                                                                                                  

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