It took the Marine Wildlife Management team at the V&A Waterfront several days to scoop up thousands of dead mullets (a type of fish) that were floating in the harbour. For the past few weeks shoppers had to endure a strong rotting fish smell as a result.
Donald Kau, communications officer for the V&A Waterfront, says every year the Waterfront has mullets breeding in the basin, but this year the number was quite high.
He said the fish died because of lack of oxygen. “We also saw a large school of mackerel entering the harbour, and the high volume of fish obviously impacted on the levels of oxygen in the water, causing the fish to die.”
Kau says the mullets return every year to breed and their numbers are being monitored. He explained this annual occurrence does not seem to impact on its numbers too negatively as only the strong survive.
He says the reason for heightened activity of marine life in the harbour is that fish is a great source of food for the Cape Fur Seals and the Sea Gulls.
“It is also confirmed the school of mackerel was followed by a pair of Humpback Whales, so the harbour basin literally had a ‘system overload’,” Kau explained.
He reiterated that there’s no cause for concern as it is a “natural occurrence”. “The marine and harbour team sees this kind of activity every year and when it happens, they mitigate the reduced oxygen by opening the lock and flushing the bay with the water from the canals. This gets done up to six times a day.”
Renée Leeuwner, communication and media executive officer for the Two Oceans Aquarium, says because of the high ammonia levels (1 mg/litre), the aquarium has had to shut off the water supply from the harbour and will be run on a closed system (no incoming water) to protect the animals in its care.
“We have also increased the freshwater flows into our harbour basins to increase the oxygen levels in the water, which is so important for its aquatic life,” concludes Kau.