Marine researchers are uncertain if the return of cetaceans to San Francisco Bay signals ecosystem health or planetary disaster; the situation for each species varies.
The presence of harbor porpoises, gray whales, humpback whales, and bottle-nosed dolphins in high-traffic waters poses risks of injury and death, amidst concerns from marine biologists.
Historical evidence suggests harbor porpoises in the bay disappeared in the 1940s due to the Navy's installment of a steel net to prevent enemy submarines, raising questions about their sudden departure.
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