MMR’s Araura Marine Research station manager, Senior Fisheries Officer Richard Story, says high water temperatures, a very low tide, and little to no wind during the week of January 10 to 15 may have been the main contributors to the die-off.
Also known as “fish kills”, these events are often the first visible signs of environmental stress.
“There was a very warm and unusual weather pattern that week which may have led to such a high mortality through oxygen-depletion in the lagoon. At this stage we believe it is unlikely the die-off was caused by a toxin, but we have collected samples of the fish for further investigation,” says Story.
Sightings included one area where several hundred lagoon fish were found dead in the surface of the water and washed up onshore.
Locals claim they have not seen a fish kill like this in at least the past 15 years.
Story says MMR began conducting a rapid assessment after a local tour operator called him to report the first sighting of dead fish in the lagoon.
Assisting in this process was Natalie Prinz, a Masters student from the University of Bremen and Center for Marine Tropical Research in Germany, who is visiting Aitutaki to conduct research.
Prinz provided a rapid assessment report to MMR after carrying out surveys with Fisheries Officer Joe Kaukura in several areas of Aitutaki’s lagoon where fish were found dead.
He assessed the dead fish sightings with tide data, rain data and some water quality measurements.
“Fish kills are not unusual but (are) usually linked to extreme events: extreme runoff and hence nutrient input, leading to a phytoplankton bloom and subsequent oxygen depletion. They can occur for a number of reasons but low oxygen levels is often a factor- especially when many different species with different feeding preferences are dying,” she reported.
Prinz reported that the most likely cause of the fish kill was the combination of a spring tide, the low tide during hottest hours, rain, murky water (potentially from runoff), and the absence of wind, leading the lagoon to develop hypoxic conditions for too long, causing some species to die.
“Oxygen dissolves into in seawater from the atmosphere and is necessary for many life forms where different species need different levels of oxygen to survive. Warmer water is less dense and holds less oxygen than cold water, and salt in seawater lowers its oxygen carrying capacity too,” said Prinz.
“As the seawater heats up, less oxygen is held, stratification (the separation of water into layers) intensifies, and deeper waters lose even more oxygen. Hot air temperature can cause stratification (layering) of the water column especially when the winds die off and there is no mixing. Many tropical saltwater fish require high levels if dissolved oxygen (DO) which fluctuates in coral reefs.”
MMR is monitoring the lagoon conditions and encourages the community to report any sightings of fish kill in the future.