Estate Living Digital Publication Issue 11 November 2015 | Page 26

The Knysna seahorse (Hippocampus capensis) is an
iconic species found exclusively in only three estuaries
on the southern coast of South Africa (Bell et al. 2003;
Lockyear et al. 2006) but it is the Knysna Estuary
that is considered to be integral to the survival of H.
capensis (Teske et al. 2003). It is this limited range,
small population size and habitat vulnerability that led
to its Endangered classification on the IUCN Red Data
List (Hilton-Taylor 2000) – the first and only seahorse
to be classified as such.

INTEREST PIECE

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This beautiful fish, with an average size of 7 cm, can
be found holding on to submerged vegetation within
its estuarine home where it continuously feeds
on tiny crustaceans. The holdfast is of particular
importance as the seahorse is no Le Clos and
subsequently needs a stable structure to hold on to
in strong currents. During the breeding season (late
spring to early autumn) seahorse couples take part
in a lengthy courting dance by clasping their tails and
moving vertically through the water column (Grange
& Cretchley 1995). The female then transfers mature
eggs into the male’s broodpouch where fertilization
takes place. After a two to four week ‘pregnancy’
the male gives birth to a number of small juvenile
seahorses.
Active research on the Knysna seahorse has been
dormant for the last fourteen years (the Lockyear
et al. survey within the Knysna Estuary took place in
2001). During this dormant stage many changes have
taken place within the Knysna Estuary – the most wellknown the development of Thesen Islands Marina.
Two important aspects about Thesen Islands Marina
should be mentioned here: (I) the development can
be classified as an Artificial Water Body (European
Commission 2003) as aquatic habitat was added to
the estuary. This is as opposed to already existing
estuarine habitat transformed into an artificial system.
Twenty-five hectares of tidal habitat was added to the
Knysna Estuary in the form of marina canals. (II) The
canals of Thesen Islands Marina were constructed
out of gabion walls and lined with reno mattress
(wire cages filled with rocks) – which stands in stark
contrast with conventional marina and harbour wall
designs of solid concrete panels. These particular

features presented us with the perfect opportunity
to investigate the response of the Knysna seahorse to
anthropogenic changes to its environment (in this case
the development of a residential marina estate) and to
investigate the ecological suitability of gabions in such
developments. To answer these questions the Knysna
Seahorse Status (KySS) project was launched by the
Knysna Basin Project in February 2014 through the
generous financial support of Thesen Islands Home
Owners Association and residents. Thesen Islands
Home Owners Association also provided a small
laboratory on the estate from where the research is
taking place.
Since the start of the project three surveys have been
completed and a mean density range of 0.12 to 0.18
Individuals/m2 were found within the reno mattress
habitat. Seahorses were found throughout the marina
system – but the highest densities were restricted
to the western section. Together with the seahorses
various other animals can be observed within the
artificial mattress habitat ranging from urchins to
gobies. The utilization of the reno mattress habitat
by the Knysna seahorse is of particular interest and
we hope to pin-point the reason behind this use in
the next year. Focus will also be placed on population
trends across time so that a conclusive assessment
of the stability of Thesen Islands Marina’s seahorse
population can be made. The Knysna Basin Project is
an NGO and we are always in need of contributions to
keep our heads above (and below) water.
CONTACT
Louw Classens
www.knysnabasinproject.co.za