Passage Summary: People got sick from local fish because of a toxin. Now that a certain type of plankton has returned to normal levels, the city says the fish are safe to eat again.

Reasoning: Anchovies were toxic due to domoic acid; P. australis plankton levels have now dropped to normal levels.

Analysis: The missing link here is the connection between the toxin (domoic acid) and the plankton (P. australis). To resolve why a drop in plankton makes the fish safe, we need information showing that the plankton is the source of the toxin. If the plankton produces the acid that the anchovies consume, then fewer plankton means less toxin in the fish. Look for an answer that explicitly links the presence of P. australis to the presence of domoic acid in the anchovies.