A couple days ago, thousands of dead fish washed ashore in Salines d’Orient, presumably dead from a hypoxic event. Hypoxia is a lowered level of dissolved oxygen in the water that can kill fish and other animals when they do not have enough oxygen to breathe. Typically, this is caused by an influx of nutrients (e.g., sewage), which causes phytoplankton blooms and increased bacterial activity. This activity reduces oxygen levels in the water, which is what kills the fish.

In this case, there is a large amount of sewage in the water, and the impact was compounded, I have been told, by blockages in the outlets that allow the pond to exchange water with the ocean. Presumably, when the flow of oxygen-rich water into the pond was stopped, the overall oxygen level dropped.

The majority of the dead fish accumulated on the western shore of the pond, which seems natural because the wind would blow the fish in that direction. Above the channel connecting Salines d’Orient to Étang de Poissons I found no dead fish. This makes me think that it is possible that the hypoxic event may not have affected the entire pond, since even with the prevailing easterly wind, there are enough irregularities in the shoreline to catch dead fish floating west.

From what I could tell, there were a minimum of six species of fish dead on the shore, as well as some crabs. There were at least some living fish in the water as well, even near the western shore of the pond. It was unclear what other organisms may have been impacted, such as mollusks, and what the overall impact will be for shorebirds and seabirds that feed on the fish and other animals of the pond.

Overall, it was a pretty gruesome display of our inability to manage our impact on the island. Having just returned from a trip to the British Virgin Islands, I haven’t had a chance to research additional details on the exact triggers of the event, but at the very least it is a clear sign of how fragile our local ecosystems are as they bear the burden of so much human impact. Below are some photos of the pond and the various fish species killed.

This entry was posted on Thursday, November 10th, 2011 at 5:20 pm and is filed under Marine Ecology, Pour les Sciences. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.