The shipping industry has also been blamed for increase worldwide distribution of organisms causing red tides. An overabundance of metabolically-active microorganisms can cause 'dead zones' by depleting the water of oxygen. Fish and other macroscopic organisms will either die or avoid these dead zones, and therefore, can have an adverse economic impact on fishermen and the seafood industry. In some cases, though, the fish kills are the result of toxins produced by dinoflagellates. An estimated 10-50,000 people are affected annually.
Shellfish poisoning is another common manifestation resulting from red tides.
The shellfish, which are less sensitive to the toxins, will ingest the dinoflagellates and retain the toxins. Temporary amnesia has also been associated with eating contaminated shellfish. Developed countries typically monitor shellfish beds and other resources for dinoflagellates. SP Toxin Comments Gymnodinium breve neurotoxic brevetoxin Known for fish kills in Gulf of Mexico. The toxin blocks Na-channels.