A Post-Graduate Research Project on fish diversity in the Knysna Estuary

Andrew Meiklejohn – Masters Candidate

Rhodes University, Department of Icthyhology and Fisheries Science

The final two samples for my master’s were made up of the second seine net sample set and second beam trawl sample set. Seine netting is a method of sampling used to target a wide variety of fish in shallow water habitats of estuarine systems. The use of a beam trawl is a new method of sampling in the Knysna estuary, it has however been used in other South African estuaries. Beam trawl sampling allows better targeting of early juvenile fish species. It was hypothesised that a large number of fish species would be present in this summer sample set as well as a large number of early juvenile species due to the sampling having taken place at the end of the spawning season.

The seine net sample recorded 36 different species including 1 species which had previously never been recorded in the Knysna Estuary, the Indian threadfin (Alectis indicus).

Fig 1: Indian threadfin (Alectis indicus) recorded for the first time in the Knysna Estuary during seine net sampling

Along with this unique species a large number of important fisheries species were present in the seine net, including a number of juvenile Shad/elf (Pomatomus saltatrix), White steenbraas (Lithognathus lithognathus), Blacktail (Diplodus sargus), Cape stumpnose (Rhabdosargus holubi) and a small number of Leervis (Lichia amia). These juvenile fish species are all known to be associated with estuarine habitats. Their presence in this system highlights the importance of conserving this estuary for the wider fish communities in the surrounding marine habitats.

The second beam trawl sample showed higher and more diverse catches than the first beam trawl sample, undertaken in mid-November, which is considered the start of the spawning season for the majority of estuarine associated fish species. This sample set produced 23 different species with a relatively high proportion of these being marine associated fish species as compared with other South African estuarine systems. One of the best examples was a Moontail bullseye (Priacanthus hamrur) which is a marine, reef-associated species which is very seldom associated with estuarine environments.

Fig 2: Marine, reef species, Moontail bullseye (Priacanthus hamrur) caught in a beam trawl.

The high biodiversity of marine-associated fish species makes the Knysna Estuary one of the most interesting and unique estuarine systems in South Africa. The permanently open Knysna Heads mouth system allows the easy movement of fish into the system to spawn and further acts as a nursery for these juvenile fish species.

With the seine net and beam trawl sample sets done, it ends the field work for my master’s project. With four very successful sampling trips and good catches, it should yield some important data on the fish communities within the Knysna Estuary. These results can be contributed to the relevant conservation plans on how best to protect the valuable species this estuary plays host to.