Use of Gabion Baskets / Reno Mattresses as a Conservation Tool in Estuaries


The Knysna estuary on South Africa's south coast (34.06°S,23.05°E) is considered to be the most important estuary in that country in terms of conservation importance, roughly 42% of South Africa’s estuarine biodiversity being found within that one system.  It contains the largest Zostera beds in South Africa and is home to both the endangered Knysna seahorse (Hippocampus capensis) and the critically endangered limpet Siphonaria compressa, each dependent on seagrass habitat. These are just some of the features that highlight the ecological importance of this estuary, and the crucial need to ensure its long-term conservation.

The Knysna estuary is not only ecologically important, but also functions as an important tourist attraction, providing much needed economical support for the town and surrounding communities. Many local communities depend on it for subsistence in the way of fishing and/or bait collection. 

Increased development in the surrounding catchments and within the estuary itself have resulted, as elsewhere, in a marked increase in the occurrence of artificial hard structures. In a recent study, Reno mattresses (wire cages filled with rocks) used in the construction of Thesen Islands Marina, were shown to provide suitable habitat for the Knysna seahorse (Claassens, 2016, Estuar Coast Shelf Sci 180: 1-10). This specific construction material has the potential to provide much needed complex habitat (particularly as an alternative to conventional construction materials lacking complexity). Specifically, the Reno mattress structures within the marina have the potential to be used as a conservation tool for H. capensis (see discussion of Artificial Marine Micro-Reserves by Garcia-Gomez et al., 2011, Mar Ecol 32: 6-14). These structures are not only used by seahorses, but also by a whole myriad of other animals – an aspect which warrants further research to establish the overall ecological role such structures play in the Knysna estuary, as well as to establish their efficacy as an ecologically-friendly construction material in coastal and estuarine development generally.



The Knysna Basin Project, a joint initiative of staff from Rhodes and Cambridge Universities and an organisational member of the CCF, is seeking to undertake such a study and would welcome collaborative input from CCF and UCCRI members with an interest in this general field, especially from those with expertise in ecological engineering and experimental animal behaviour.  Expressions of potential interest could be directed to the KBP's Director Louw Claassens.

Other projects being planned include one on the effects of local algal blooms on seagrass ecology which may also be of interest (and in respect of which input from those with expertise in sediment geochemistry and modelling would be especially welcome).  The Cambridge representative on the Scientific Steering Committee of the KBPRichard Barnes, would be happy to provide any further information that CCF/UCCRI members might wish.