Page 1 of 3
2015 Update - Students from the Department of Zoology & Entomology of Rhodes University under the supervision of Professor Alan Hodgson have participated in a number of ongoing and new research projects during the year.
Melissa Pollard (2nd year M.Sc.) continued her investigations on fish utilization of the sea grass meadows in the vicinity of Thesen Islands. Using four GoPro cameras and non-destructive seine netting, Melissa has been able to document species abundance, and gained some insights into fish behaviour in this important and threatened habitat.
The fragile nature of sea grass was highlighted this year by a noticeable decline in the eelgrass at her research sites, a result of invasion by sea lettuce.
The study found that fish abundance is associated with water temperature in the lower reaches of the Knysna estuary where fish numbers were higher in spring and summer months and lower in winter and autumn months. The Sparidae (e.g. Black Tail) family was seen to utilise eelgrass meadows for feeding purposes and were found in this habitat more often than in bare unvegetated areas. The Mugilidae family (e.g. Mullets) in contrast was found to feed in the bare unvegetated areas but was also seen utilising the eelgrass meadows and often seen resting in this habitat. The comparison between seine net sampling and underwater video monitoring showed a similar pattern in fish species observed which shows that the non-destructive video method has great potential as an alternative to other more destructive sampling methods.
David Purchase (B.Sc. Honours student) has been using GoPro cameras along with fyke nets to begin to document what fish species are utilizing the canals of Thesen Islands Marina. The preliminary results of this non-destructive technique are most promising and the work will be continued by other students next year.
One of the most abundant invasive alien species in the Knysna embayment is the Mediterranean mussel, Mytilus galloprovincialis. We have estimated that this mussel arrived in Knysna in about 2002, and it is now found on all man-made hard structures. It is especially abundant from The Heads to the Railway Bridge. One reason for its success could be its high reproductive ability and Isabel Micklem (B.Sc. Honours) has been determining whether young mussels (called spat) are recruiting to populations throughout the year. To do this she has been putting ‘spat collectors’ in the embayment and counting how many young mussels are settling at different times of the year. This project is yielding interesting data and the work will continue as a longer term study to look for seasonal patterns of mussel recruitment.
The alien mussels are fierce competitors for space and will displace native species. However, mussel beds do provide a habitat for other organisms. Two third year students (Samantha Mannheim and Delsy Sifundza) continued a project looking at what invertebrate species are found in association with mussel beds throughout the embayment. We now have a considerable amount of data that needs to be analysed fully and prepared for publication.
In 2014 a study was begun to determine what species had colonized the gabions that form the vertical walls of the canals of Thesen Islands Marina. James Radloff and Stuart Stopford (B.Sc. third year) continued this work in 2015 and increased the number of sites sampled. They have helped us to build up a picture of invertebrate species richness and diversity as well as distribution in the different regions of the marina. Further sampling will continue in 2016 before publication of the results.
This work has been undertaken alongside a study by Murray Roodt (1st year M.Sc.) who is looking at what organisms have colonised the floating jetties in Thesen Islands Marina. Until recently these structures were home to an abundance of species. However, the recent September floods have caused significant mortality of many invertebrates owing to the decrease in salinity. This has provided an opportunity to study what has died, what has survived and how rapidly recolonization of the jetties occurs after a flood event. This will be the focus of Murray’s work for the next year.