Not every threat faced by young South African fur seals is as obvious as a shark or hyena. While most species of algae are not harmful, and are indeed necessary to sustain life in the sea, a few produce potent neurological poisons. These chemicals can become concentrated in algae-eating fish and crustaceans, then poison and paralyze the birds and seals that feed on them.
Red tides can also use up the oxygen in seawater, suffocating sea life.
That happens when the algae die and decompose on the bottom. Hundreds of police were called in to head off the near riots that had accompanied an earlier red tide. Lobsters on beaches can lead to riots.