SEDGEFIELD NEWS - As part of the ongoing battle to ensure the survival of the endemic Knysna seahorse, a community programme headed by SANParks continues to make a difference in protecting and fighting for the tiny marine animal.

The Knysna Seahorse (Hippocampus capensis) is an endemic species to the south coast of South Africa and is found only in the Keurbooms, Knysna and Swartvlei estuaries. 

Adults reach a length of 12cm and exhibit colour variation ranging from an olive brown to a mottled dark brown/black. Observations of captive specimens demonstrate 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 fertilised 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.

Swartvlei distinction

The Swartvlei estuary population has a few unique factors. Firstly, while they belong to the same species, they have low genetic diversity compared to the Knysna and Keurbooms populations. This suggests early genetic isolation from the other two populations when sea levels began rising and separated the rivers that 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.

Rescue operation

Responding to these breaching events, SANParks has developed an efficient citizen science programme to collect both seahorses and pipefish. Volunteers register with SANParks and are added to a database. 

Once the mouth has been breached, SANParks monitors 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. Volunteers are then split into groups headed by rangers and sent to respective transects on both sides of the estuary. The focus is to collect all seahorses and pipefish, dead and alive, 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 previous rescues, it has been shown that some dried out seahorses that look dead can rehydrate and recover.

The search is focused on the exposed estuary banks, which are covered with dried and drying algal mats. Unlike most fish in the estuary, which swim to deeper water when the water level begins to drop, seahorses and pipefish, being cryptic species, remain in the algal cover which bears down on them and traps them as the water subsides.

Sizing up the collection

The most recent breaching of the mouth was on 22 November and volunteers were notified that the rescue would take place over three days from 25 to 27 November. 

One group of six that covered approximately 800m (from the transect from the Fish Eagle Green slipway to the Blue Gums at Kob Hollow) collected 145 pipe fish (four alive and 141 dead) and 63 seahorses (45 alive and 18 dead). It was most noticeable on this rescue that, on average, both the seahorses and pipefish were smaller than on previous occasions.

Gift of life

One of the seahorses recovered on the 26th was a dead male with a bulging pouch. It was placed in the bucket to see if it would be possible to release the babies at the end of the transect. Once the transect was completed, the group set about sorting and counting the collected specimens.

While transferring the live seahorses into a separate bucket, they noticed that there were a lot of minute seahorses in the bucket, and even more amazing, they were alive. The assumption is 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 27 November, another dead male with a large pouch was recovered. The baby seahorses in the pouch also came out, but they were not alive. 

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