The Knysna Seahorse (Hippocampus capensis)) is an endemic species to the south coast of South Africa and is found in only three estuaries, namely the Keurbooms, Knysna and Swartvlei estuaries. As adults they reach a length of 12cm and exhibit colour variation ranging from an olive brown to a mottled dark brown/black with observations of captive specimens demonstrating slight colour change according to background tones and hues.
A remarkable reproductive feature of seahorses is that the male has a brood pouch in which he receives the fertilized eggs from the female. After the eggs hatch, the babies develop in the pouch until they are large enough to fend for themselves, then the male releases them into the water. He will mate with a female within hours after giving birth to a brood of seahorses.
The Swartvlei estuary population has a few unique factors. Firstly, while the same species is found in the Knysna and Keurbooms estuaries, they have low genetic diversity compared to the other two populations which suggests early genetic isolation from the other two populations when sea levels began rising and separated the rivers which were connected on the now submerged Agulhas Bank.
Secondly, they are the only population that lives in a blind estuary (an estuary that closes when a sand bar forms across the estuary mouth). When closed, the water level rises and overflows from the channel over the banks. Depending on the duration that the mouth is closed and the amount of rainfall during that period, aquatic vegetation starts growing in the newly inundated area. When this happens, aquatic fauna disperse into these shallow waters. Then when there is heavy rain and the mouth opens, everything, including Knysna Seahorses and pipe fish living in the shallow areas can be stranded as a result of the sudden drop in water level.
Responding to these breaching events, SANParks has developed an efficient citizen science program to collect both seahorses and pipefish. Volunteers register with SANparks and are added to a database.
Once the mouth has been breached, SANParks then monitor the level of water, and when it has dropped approximately 1.6m, notifies the volunteers with dates and times for collection. Volunteers meet at a designated site in Sedgefield and are briefed by rescue operations manger, Clement Arendse, a SANParks marine scientist. Following the briefing, volunteers are split into groups headed by a ranger and sent to respective transects on both sides of the estuary.
The focus is to collect all seahorses and pipefish, both dead and alive specimens, but any unusual species are also collected. All specimens are placed into buckets with water. Essentially the water is for the live seahorses and pipefish, but from from previous rescues, it has been shown that some dried out dead looking seahorses can rehydrate and recover.
The search is focuses on the exposed estuary banks which are covered with dried and drying algal mats. Unlike most fish in the estuary which, when the water level begins to drop, swim to deeper water, seahorses and pipefish, being cryptic species, instead remain in the algal cover, which as the water subsides, bears down on them and traps them.
Sizing Up the Collection
The most recent breaching of the mouth was on the 22nd November 2021 and volunteers were notified that the rescue would take place over three day from the 25th to the 27th November. Our group of 6 was assigned the transect from the Fish Eagle Green slipway to the Blue Gums at Kob Hollow, approximately 800m.
On our transect, we collected 145 pipe fish (141 dead and 4 alive) and 63 seahorses (45 alive and 18 dead). Most noticeable on this rescue were the sizes of both the seahorses and pipefish, with the average size being smaller than previous occasions.
It wasn’t only seahorses and pipefish that we saw marooned on the bank. Fingerlings, swimming crabs, mussels and isopods has also become victims of the sudden drop in water levels.
Gift of Life
One of the seahorses that we recovered on the 26th was a dead male with a bulging pouch. It was placed in the bucket and the intention was to see if it would be possible to release the babies at the end of the transect. Once we completed the transect, we set about sorting and counting the collected specimens, While transferring the live seahorses into a separate bucket, we noticed that there were a lot of minute seahorses in the bucket, and even more amazing, they were alive. We can only assume that the dead male’s pouch had relaxed enough to allow the baby seahorses to swim free. There were a total of 25 new babies.
The remarkable birth of the seahorses wasn’t the only interesting event. On the 27th November, another dead male with a large pouch was recovered. The baby seahorses in the pouch also came out, but they were not alive.
Swimming off with another generation
One of the live seahorses liberated in the estuary channel was a pregnant male. Once he had stabilized in the water, he swam into a clump of algae and we are confident that he will soon bear a new generation of Knysna Seahorses.