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.