Ciguatoxin which comes from marine algae can cause ciguatera poisoning when some tropical saltwater reef fish that contain this toxin are eaten. This problem rarely affects consumers of commercially caught fish since commercial fishermen avoid areas known to contain ciguatoxic fish. Recreational fishermen who aren't familiar with local fishing areas are more likely to encounter toxic fish. Over 90 percent of the reported cases of ciguatoxin poisoning in the U.S. from 1978-87 occurred in Puerto Rico, Guam, the Virgin Islands and Hawaii. Since this toxin is not destroyed by cooking, visitors to tropical areas should only patronize reputable dealers or restaurants and be prudent about the recreationally caught reef fish that they eat.
Algae blooms can also produce several types of shellfish toxins.
Shellfish harvesting waters are regularly monitored and tested for algae that produce these toxins, and affected waters are closed to shellfish harvesting when blooms occur. This program has been effective and illness is rare. To further minimize risks, individuals who harvest their own shellfish should check with local authorities and heed all warnings regarding shellfish closures. One type of toxin, scombrotoxin, is caused by improper fish handling rather than naturally occurring marine algae. Scombroid poisoning which is the illness caused by eating fish that contain this toxin isn't severe for most people but can be uncomfortable.