When we reported on the death of an estimated 1 million sardines in King Harbor off Redondo Beach, California last week, scientists and wildlife officials knew the fish kill was the result of natural causes.
They speculated that windy conditions or maybe a column of oxygen-poor water were to blame for the more than 35 tons of dead fish. Now, scientists are saying the sardines may have eaten toxic algae in another location that was rich with domoic acid, a powerful neurotoxin, and then became disoriented, arrived in the harbor, depleted the oxygen and suffocated.
From this story on DeepSeaNews.com:
_New data from Dr. David Caron’s lab at University of Southern California found that the gut contents of the dead sardines contained high levels of domoic acid, a powerful neurotoxin produced by the diatom (microscopic plant) Pseudo-nitzschia. Sardines feed by filtering plankton out of the water with their gills, so unlike other fish they are able to ingest toxic algae directly. Dr. Caron’s lab did not detect domoic acid inside King Harbor itself, but the fish might have eaten the toxic algae offshore, become disoriented, swum into the harbor, used up all the oxygen (a million fish use a LOT of oxygen), and died. Dr Caron wrote a very helpful explanation of what his lab is doing, what they’ve found, and what they’re working on. Here’s an excerpt regarding the role of domoic acid in the fish deaths:
_”Despite the lack of toxic algal species in the water at King Harbor during this event, analyses of the gut contents of fish collected on March 8th have tested strongly positive for domoic acid. Domoic acid is a powerful neurotoxin produced by a specific type of microalgae. The algae are strained from the water by plankton-eating fish such as sardines and anchovy, and the toxin is often found concentrated in the stomach contents of these fish during a toxic algal bloom.
“Domoic acid can cause a variety of neurological disorders, and death, of animals consuming fish contaminated with the neurotoxin. Research also indicates that domoic acid poisoning can cause abnormal swimming behavior in some fish. It is possible that high levels of domoic acid in the sardines in King Harbor may have exacerbated physiological stress of the fish brought on by oxygen depletion of the water, or may have been a contributing explanation for them congregating in the harbor at very high abundances, but this has not been confirmed.
“We believe that the fish ingested the toxin offshore (before entering the harbor) because domoic acid was not detected in the water within King Harbor on the day of the event.
“Additionally, during our 5-year study we have not observed significant concentrations of domoic acid in King Harbor. We have confirmed that plankton collected from the coastal ocean approximately 20 km southwest of Redondo Beach on March 9 had very high concentrations of domoic acid in the plankton. That finding support the idea that the fish ingested the toxin in coastal waters before entering the harbor.”_