Phoebe Beltran was in the homestretch of her 1,000-yard swim when she suddenly felt an intense pain as the sea lion bit her right arm.

A common neurotoxin produced by algae blooms is making California sea lions more aggressive. Here’s why it has the power to change behavior—and the danger it poses.

In recent weeks, hundreds of sea lions, dolphins and other animals have turned up in the sand dead or seriously ill, alarming rescuers and beachgoers alike.