Tidal pool fish deaths blamed on kelp build-up
A Fish Hoek man who swims at Glencairn tidal pool most mornings got a surprise on Wednesday last week when, instead of the usual peaceful scene, he found dead and dying fish and murky water.
"I went down to the usual spot for my swim. What immediately struck me was the absence of people, except for two familiar faces swimming,” said Andre de Villiers
As he approached, he noticed people taking photos with their phones.
“I made my way down to where they were, and upon closer inspection, I saw the fish struggling to breathe, their heads poking out of the water gasping for air.
“It became evident that there were dead and dying fish, along with struggling starfish and jellyfish. The water had also turned a murky brown.”
Gavin Finlayson, another regular swimmer, found himself diverted from his usual routine after Mr De Villiers warned him against visiting the Glencairn pool that morning.
"There's a group of us who swim at the pool most mornings. We're those dedicated early risers. Andre phoned me and warned me not to even think about swimming at Glencairn pool,” he said.
Mr Finlayson opted for Long Beach instead, but his curiosity led him to the Glencairn pool on his way back and he found City coastal management staff there when he arrived.
"I approached them and asked what the problem was, and they explained that when the tide came in, it wasn't strong enough to properly clean the pool, so it ended up filled with seaweed and lacking oxygen.“
The staff told Mr Finlayson they anticipated the pool would be safe to swim in again by the following day.
Mayoral committee member for spatial planning and environment Eddie Andrews said: "We believe this to be a eutrophication event – this is a drop of oxygen levels in the water. It usually occurs when algal or bacterial growth due to nutrient loading ‘eats’ up the oxygen.
“In this case, the tidal pool circulation is very poor. In summer, when the seas are flat, there is not great turnover of water in the pool. Add the kelp load (nutrients) in the pool with very warm temperatures and we would expect to get a eutrophic event.”
City staff had been instructed to catch and release all the live fish, collect and count the dead fish and remove kelp from the bottom of the pool, he said.
“To do so, they needed to lower the pool level and we used suction pipes to do this – this also helps with water turnover.”
Cape horse mackerel, Cape sole, mullet, bluefin gurnard, Cape stumpnose, Cape bream, klipvis, catfish, juvenile mussel cracker, blacktail, streepie, pipefish (short beak) and cuttlefish were among the rescued fish species, he said.
Such events were likely to happen at the pool once or twice during summer but it was still safe to swim there, he said.
"The pivotal issue lies in the design of the pool, which restricts water circulation and turnover during the summer months. Unfortunately, altering this design would necessitate lowering the wall height, resulting in a shallower pool,” he said.
City coastal management officials checked the pool on Thursday morning February 22.
“We found that the water levels had returned to normal, suggesting a renewal and circulation of water, and no new cases of fish deaths were observed,” said Mr Andrews.
