I read an interesting study on it back when my grandfather had it. The findings were that it was generally only found in areas with resorts or large populations near the coast. The study also showed a massive increase in the number of cases in areas where the population, and inherent industry and infrastructure; had significantly increased. They were doing further studies to find if it was linked with waste going into the waters from the population causing significantly higher quantities of the benthic microalgae and cyanobacteria, which is where the toxin actually comes from.
Simply, the small fish eat the algae and the big fish eat the little fish. The toxin is cumulative, so the more little fish the big fish eat over time, the more of the toxin they 'hold'. The older the fish, the worse the contamination will be.
Long story short - takes pics of the big fish and let them go.
Ben.