08 December 2011
Knysna: Scores of pipefish dead
Image: Scores of dead Pipefish have been found washed up recently. Residents were speculating whether the recent sewage spill may have caused their demise.
KNYSNA NEWS - "We have been finding dead pipefish washed up on the Noetzie Beach tide-line for over a month now, lots and lots. We collected 60 without looking too hard," said a saddened Knysna resident.
These pipefish (Syngnathus Capensis) are related to the seahorse and occur in all the local estuaries and are preyed on by various fish species that enter the estuaries. The resident offered a hypothesis, "Perhaps the dredging at Ashmead or somewhere in the lagoon, or the recent pollution from sewage, is killing them, and the westerly winds then wash them onto Noetzie Beach."
Another report of dead pipefish was received from a diver who had recently dived at the Knysna Heads. "It is the sewage" he said, "I went for a scuba dive on Friday, December 2 and I saw lots [dead pipefish] at The Heads."
Professor Brian Allanson, Knysna resident and aquatic and estuarine ecologist said, "To have such a high mortality, points to a major environmental disturbance."
He explained that pipefish prefer to live within the estuary’s eelgrass beds. "If these beds are exposed to pollution, then the pipefish will be exposed to many dangers - predation or rapid lowering of dissolved oxygen linked to sewage pollution and associated toxic compounds such as dissolved ammonia."
Allanson added that he was "tempted to point a finger at the recent extreme sewage pollution of the estuary. This would have reduced dissolved oxygen and introduced ammonia."
In closing he said that the fact that the dead pipefish have been washing up for over a month, suggests that a major pollution event was the most likely cause.
André Riley, area manager for Knysna Garden Route National Park, said that although it is often difficult to pin-point the cause of high mortalities in the environment, he agreed with Allanson that "it [the dead pipefish] is linked to an environmental event. However these events are in some cases natural and/ or as a result of disturbances caused by humans."
He explained that SANParks are aware of a recent pollution incident and added, "I agree that we can assume that it could be linked to this, or as Prof Allanson states, one is ‘tempted to point a finger to recent pollution events’."
Pipefish mortality at Brenton-on-Sea
Knysna resident, Chris Gow, who wrote a Zoology Honours degree thesis on pipefish breeding behaviour using specimens collected from Swartvlei, said that he had found hundreds of dead pipefish on the Brenton-on-Sea beach.
"Seagrass beds occur in seawater which is sufficiently clear and shallow for good light penetration, but not so shallow that wave action becomes a problem. These conditions are met not far offshore from the beach at Brenton," explained Gouw. "The hundred odd pipefish we found on Brenton beach were fresh, therefore from one catastrophic event, and they were all within a stretch of less than 150 metres of strand-line." He noted that no other species had been affected.
Gouw explained in short. "Due to rough seas there has been a lot of sand movement recently, both onto and from the beaches, but equally immediately off-shore, where sand can be held in suspension or dumped at times."
He further suggested that a localised patch of seagrass became rapidly covered with sand and at the same time "this resulted in the sea becoming shallower and the pipefish could not cope with the combination of habitat loss and increased water movement. It is utterly improbable that these pipefish derived from the estuary."
Gow is of the opinion that should the death of the pipefish be due to the pollution of the estuary, "Where are all the dead seahorses? Same family, very similar habitat requirements."
In closing he added, "None of the above should divert attention from the filthy state of the estuary. We live in a topographical basin and everything ends up in the estuary if we let it."
ARTICLE: FRAN KIRSTEN, KNYSNA-PLETT HERALD JOURNALIST