Red tides are caused by an overgrowth of a microscopic algal species called Karenia brevis. K. brevis is a naturally-occurring alga in the Gulf of Mexico that can discolor the water and kill fish and other marine life.

Filter-feeding shellfish such as oysters, whelks, clams and mussels concentrate K. brevis in their tissues, which can result in Neurotoxic Shellfish Poisoning should a person eat the affected oyster. NSP is not a fatal shellfish poisoning and people generally recover in 2-3 days.

Crustaceans (shrimp and crabs) do not accumulate brevetoxin like filter-feeders do and are safe to eat during red tides.

Fish that look and act healthy (no lesions, fight the hook energetically) are generally considered safe to eat. Brevetoxin accumulates in the organs of fish and not in the muscle tissue.

Coastal Fisheries staff monitor the event by collecting water and assessing fish kills to determine areal extent and numbers and species of fish and wildlife impacted affected.

The first know red tide in Texas was in 1935. Red tides have probably occurred before, but no one knew what they were. The Spaniard’s noted discolored water and dead fish in 1530, but scientists did not know red tides were caused by dinoflagellates until 1947.

In 1935, the first known red tide occurred in Texas off Mustang and Padre Islands. The associated fish kill was estimated over 4 million kg (8.8 million lbs) of fish.

Over the next 50 years, Texas experienced approximately one red tide every decade, with one exception in the 1970s (1935, 1948, 1955, 1972, 1976, 1986.) Since the 1986 bloom in Texas, red tides have occurred at greater frequencies with four events in 1990’s and six events in 2000.

The intensity and extent of the red tide associated fish kills have been variable. Texas’ most severe red tides events occurred in 1986 and 1997. Estimates were over 20 million fish killed in each year, with less than one per cent being recreational and/or commercial species. Other events had lower impacts with 2000 and 2009 red tide events reporting 2-3 million fish/event killed.

The majority of past red tide events in Texas, began offshore from late August/early September and lasted into November. Associated bay (inshore) impacts sometimes followed the onset of offshore blooms occurring into January of the next year.

Rhonda Cummins
Coastal & Marine Resource Advisor