Category Archive Estuary

A survey in the Thukela estuary

The University of KwaZulu Natal (UKZN), Unit: Aquatic Ecosystem Research (AER) conducted a  survey to Thukela Estuary on the 19th-26th March 2017. The participants for this survey were Mphatheni Mthembu, Lungelo Madiya, Nombuso Gongo and Jennifer Cele. The Thukela estuary is situated approximately 100 km north of the city of Durban in KwaZulu-Natal. This estuary is shallow with relatively small surface area and has a large catchment area. It is one of an open river mouth estuarine system in South Africa thus making this entity more abundance and diverse in terms of species than temporal open/closed or closed estuaries. This entity is an importance estuary to humans and marine species supplying goods and services that can range from nursery grounds, fisheries, to recreational amenities. The purpose of the survey was to track and monitor changes in the Thukela estuary and ultimately determine the causes and effect for the identified changes.

OUTCOMES

The AER estuary team noted with concern the dynamism of this estuary in that the sediment deposited was slightly higher than previous surveys thus causing high sand exposure during low tide. The team suspected this years (2017) early flood for this increased sediment deposition in the estuary.

The Thukela estuary during low tide

The AER Team also noted that the Thukela estuary had a low salinity which indicated that this entity is dominated by the Thukela river input. This could be the cause for low marine fish species number and abundance found in the middle and upper reaches of the estuary.

The YSI water meter showing water quality reading of Thukela estuary

The AER estuary team used cast nets, fyke nets, gill nets and seine nets for sampling. It was noted that the seine net sampling efficiency was higher during cold days. The seine net also caught significant higher fish species than other nets combined. The fish caught were collected, measured, identified and recorded. The team found more fish species in the lower reaches of the estuary and the fish caught were dominated by juvenile marine taxa in those lower reaches of the estuary. These juvenile marine taxa were not high in species number as expected but were abundance. The most dominating species caught in this estuary were Mugilidae (Mullet), Caranx neberi (blacktip kingfish), Leiognathus equula (Slimy) Ambassis natalensis (Slender glassy) and Ambassis gymnocephalus (Bald glassy) although there are some species that are still to be identified which were not caught in large numbers.

The estuary team also collected benthos samples using the grab for laboratory analyzes. The water quality samples were also collected. The team noted with concern a variety of fishing activities taking place in the Thukela estuary, including illegal, largely subsistence gillnetting and they catch big fishes which were ready to spawn. These gillnetting are combined such that there could cover 2/3 or whole estuary across. These gillnetting efforts have been commercially driven by selling fishes to markets.

CONCLUSION

In conclusion, The AER group noted that the estuary was highly dynamic with high sediment exposure during low tide compared to previous survey. The group was also concerned with the extreme long gillnets used by locals for fishing. The group also noted high abundance of fish species as one move closer to the lower reaches of the estuary. This site survey proved to be useful for updating information in the Aquatic Ecosystem Research in University of KwaZulu Natal.