A toxin-producing microbe turns seawater red, a phenomenon known as red tide  When the sea otter's main food source, clams, is contaminated with red tide toxins, sea otters do not forage in those areas
One explanation for this behavior of sea otters is that sea otters taste a few clams before they officially forage in a place
Sea otters also don't eat clams from places where red tide-affected areas of water are artificially dyed reddish-brown.