The Knysna seahorse, Hippocampus capensis, is the only one of nearly 40 species worldwide that are found in river estuaries and not in the ocean. Endemic to the Garden Route area, it occurs in only three estuarine systems: the Swartvlei (at Sedgefield), Knysna and Keurbooms (at Plettenberg Bay).
Their habitat is restricted almost exclusively to Cape eelgrass, Zostera capensis, which they cling to in relatively shallow water, at depths between 20-50 cm, sucking minute crustaceans off submerged plants or those floating by in moving water.
They reach maturity at one year and a pair mate for life (8-10 years). An elaborate courtship involves tail grasping while facing each other until the female deposits her eggs in the male’s inflated pouch. The eggs are safely sealed in the lining of the pouch while the male incubates them for roughly four weeks. As they hatch, he forcibly expels them from his pouch – up to 200 at a time – and they are left to fend for themselves. Within hours of the tiny Knysna Seahorse replicas exiting their father’s pouch, the female will place another batch of eggs in the male’s pouch. For the rest of his life he is a dedicated incubator.
Fortunately, the Knysna seahorses are able to camouflage themselves by changing colours. Some will be preyed upon by crabs, birds and fish, and even other seahorses, but many will survive. They cannot, however, protect themselves from environmental degradation like estuary pollution or the loss of their habitat.
In the Sedgefield estuary they are vulnerable in places to disturbance and trampling as fishermen push through the grasses to sandbanks they fish from and when people launch their canoes, kayaks etc., unaware of their presence.
Unlike Knysna and Keurbooms, the Swartvlei Estuary is an open/closed system and in the last few years the estuary has been more closed than open. This means that seahorses that have had time to establish themselves in an eelgrass area are vulnerable to being stranded when the closed estuary is opened to the sea. This happens when high rainfall inland swells the tributaries flowing into the Swartvlei and forces the river mouth to open. The water rushes out over three to five days, resulting in the water levels reducing substantially, exposing the eelgrass and flattening it as the water’s edge recedes.
Artificial opening of the river mouth also occurs at times when water in the estuary, closed to the sea, rises high enough to threaten low-lying residential areas and substantial rain is forecast which could threaten such suburbs. SANParks then has to make a judgement call, to allow the Municipality to breach the opening “artificially”.
Sedgefield/SANParks Knysna Seahorse Rescue
In November 2017 and 2018, the SANParks Citizen Science Program led by Clement Arendse engaged local volunteers to assist them to monitor these events. Some 4-5 days after the river mouth was breached, teams were created to check out the various eelgrass sites along the Swartvlei Estuary riverbands and attempt to rescue those seahorses stranded in the grasses. The aim was to return them to deeper submerged parts of their habitat but not too exposed so that they fell prey to hungry predators.
As a result, in 2017 SANParks staff with volunteers collected 900 seahorses, of which 450 were found alive and returned to their habitat. In 2018, they found only 11 and only one survived, which is indicative of certain trends, according to Arendse. In 2019, 1 300 seahorses were found of which 650 were alive.
Arendse explained that when the water level drops the seahorses “suddenly get exposed” and instinctively hang onto the eelgrass that flattens on the water’s edge. They then get buried in it and are unable to escape, resulting in their death.
Swartvlei estuary was breached again on 25 May 2021 and the Citizen Science programme activated thereafter. Volunteers searched for seahorses along assigned sections of the Swartvlei Banks. They first had to sign permits to do this work because it is illegal to interfere or remove seahorses from the estuary and you are not allowed to keep one even if it’s dead.
Information cards had to state how many live seahorses had been found and returned to the estuary and the dead ones (that didn’t revive after placing them in a small bucket of river water for 30 minutes) had to be kept in a bucket and handed in with the score card.
Sedgefield Knysna Seahorse Conservation Strategy
Arendse noted that the Swartvlei Estuary seems to experience population fluctuations and periodically, mass deaths.
I reckon this could be due to oil spills as the volume of speed boats (and skiing) increase in holiday periods, especially in areas close to eelgrass on the Island. Thoughtless residents have been known to spray insecticide on reeds in or near the water which could seriously affect seahorse populations.
My feeling is that people ought to be notified by estate agents that they are moving into a conservancy area, if they wish to buy property on the Island, and there are conservancy regulations to be considered.
This happens to be the only seahorse on the IUCN (International Union for the Conservation of Nature) Red Data List of Vulnerable Species. Locals need to be made aware of how endangered this seahorse is and be conservation-minded with the activities they pursue in the vicinity of its habitat. It would help too if SANParks erected a Knysna Seahorse Information Board on the Island or had prohibited areas demarcated in the eelgrass habitat.
SANParks Information Board Thesen Island; 2017 & 2018 Citizen Science Volunteers with the SANParks Knysna Seahorse Rescue Project
Read more at https://www.discover-sedgefield-south-africa.com/knysna-seahorse.html