A breathtaking isopod bloom in the V&A Waterfront harbour, and around the jetties outside the Two Oceans Aquarium, made for some great photo opportunities this morning.

Photo by Ingrid Sinclair

Isopods refer to a group of crustaceans that include terrestrial and aquatic species like woodlice and rock lice. Some isopods eat decaying plant and animal matter, others graze on food particles from the water around them, a few are predators, and some are internal or external parasites.

What caused this “bloom” in the harbour is not clear, however it could be due to abundant food sources and/or optimal water temperatures. Isopods lay several hundred eggs at a time and when the conditions are right, these hatch in their numbers. When enough eggs hatch at one time a “bloom” occurs.

When isopods gather in large numbers like these, oxygen levels in the water plummet. This morning, our oxygen measurement of the water in the harbour clocked in at a very low 8% - optimal oxygen levels are at around 80%! Imagine if a million people congregated in front of the I&J Predator Exhibit – oxygen levels would be pretty low then, too. 

Some of our staff members have observed dead or dying fish in the harbour – either as a result of suffocation or from overeating.

Many fish seem to be gorging themselves, with a deadly outcome. We removed this bluefin gurnard that was eating non-stop in low-oxygen water in order to ensure its survival. It's currently with us in quarantine and will be released as soon as water conditions allow.

A bluefin gurnard had to be saved from itself. Photo by Ingrid Sinclair

Crabs were also observed climbing out of the water and resting on walls. 

Photo by Vincent Calder 

Of course nature knows no waste – birds and seals are feasting on the fish! 

While it is an incredible sight, this is not great news for us, either. Much of the water for our exhibits is from the harbour surrounding our building, and we’ve had to shut down our intake pipes. Once all the isopods die off – also as a result of oxygen deprivation – they will sink to the bottom. Then, once oxygen levels go back up (because there are fewer organisms in the water using this ozygen now) the dead isopod bodies will start decomposing like mad. This will cause an ammonia spike in the water, making the water toxic to the animals in our exhibits and so still not suitable for our use. We will be keeping the Aquarium’s life-support system on a closed system until the water quality returns to normal.

Two Oceans Aquarium CEO Michael Farquhar stepped out to assess the conditions. Photo by Ingrid Sinclair

How long it takes for the water to return to a usable condition depends on how long it takes for tidal movements to wash away some of the ammonia-heavy water and for oxygen levels to return to normal. It can take a few days, or weeks.

We will however be able to use some of the isopods for fish feed, but only once they have been frozen. We are not able to feed these to our animals in live form as they often carry parasites that can harm the animals in our exhibits.

Ispods were scooped up with a net. Photo by Ingrid Sinclair
They'll now be frozen to kill off any potential parasites before being fed to animals in our exhibits. Photo by Ingrid Sinclair
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