by Gundy Gunderson
Red Tide is a natural ocean phenomenon that occurs within the warm and temperate waters of the world. Some places are notorious for their red tides such as the waters off the Malabar Coast of India or near Dakar on the west coast of Africa. Most of the world’s oceans experience red tide in some shape or form. Common along the California and Northern Baja coast especially south of point Conception, red tides occur with considerable frequency and with varying degrees of intensity. West coast anglers have, through the years, become very familiar with the occurrence as it can make fishing conditions tough, if not impossible.
The phenomenon of red tide has been around since ancient times. Some biblical scholars believe the seventh chapter of Exodus mentions red tide when the river Nile turns to “a river of blood” where many fish were killed and the people could not drink the water. References to red tide are also found in Homer’s Iliad, the works of Tacitus and in the logs of captains and navigators going back to the 16th century. Spanish explorers in the New World named the Gulf of California the Vermilion Sea because of the red tides that were so common in the long slender body of water.
The phenomenon of red tide is caused by microscopic organisms in the plankton. Plankton are the drifting or weakly swimming sea life that are the foundation of the ocean food chain. Specifically, dinoflagellates, a microscopic marine life form with two long slender appendages, are the organisms responsible for red tide. Considered by some scientists to be plant and others animal, there are from 300 to 400 species known to exist.
When conditions are just right, these dinoflagellates reproduce at an accelerated rate creating a red tide “bloom”. During a bloom as many as 50 million of these tiny organisms may occur in a liter of water. Typical off-color or brown ocean water will harbor only about a half to one million organisms per liter. The color of the water can vary from yellow to brown to a deep terracotta red.
Curiously, an inland sea in California has, over the years, served as a sort of laboratory for the study of red tide blooms. The Salton Sea is a large shallow inland sea in the eastern California desert. The lake was filled with fresh water from a broken levee off the Colorado River. The water pooled for nearly a year in an ancient salt lake basin turning the body of water saline.
The salinity was similar to the Sea of Cortez so California Department of Fish and Game Biologists planted several species from the Cortez and a few thrived. Orangemouth Corvina, Gulf Croaker and Sargo were later joined by Talapia to create a robust food chain.
It is not known how the dinoflagellates got into the sea but it soon became apparent that the salt-water sea was capable of creating red tides. Scientists believe the intense heat of the desert sun combined with the nutrient rich agricultural runoff that drains into the sea from the Imperial Valley fueled these massive blooms that killed thousands and thousands of fish. In fact, much of the shoreline of the sea is composed of fish vertebrae instead of sand. In the warmer months, the blooms would occur so rapidly that the fish could not escape the areas quickly enough and whole stretches of water would be littered with dead floating fish. Although a prolific fishery, the balance was knife edged and red tide blooms and fish kills became a part of the lake ecology. The blooms never adversely affected the fishery as it once had the highest catch rates of any body of water in California. The lake record orangemouth corvina was over 40 pounds. Eventually, curtailed fresh water runoff and evaporation caused the lake salinity levels to rise, crashing the fishery.
Scientists believe that the conditions favorable for the bloom of red tide are many but sun power combined with nutrient rich water seem to play a role in supercharging reproduction in the dinoflagellates.
Red tide begins to show in streaks and patches, looking a little like a few drops of red food coloring in a glass of water. Then, within days, the water will turn a solid brown or terracotta red and only the edges of the bloom will be streaky.
Normally the red tide occurs along the coastal strip rarely occurring more than about four miles out. It is believed upwelled nutrients along coastal canyons fuel the blooms. Additionally, coastal waters are richer in man-created nutrients from agricultural and urban runoff that can combine with natural nutrients to intensify blooms.
Because, sun power often provides the energy for these red tide blooms, the red tide is usually confined to the top of the water column seldom being found below 60 feet.
Red tide blooms can kill fish in a couple of different ways depending on the characteristics of the individual dinoflagellates. Off Southern California, severe fish kills are rare however fish kills of fish and invertebrate have been reported as far back as the 18oo’s. West Coast red tide fish kills are believed to be caused by suffocation due to oxygen depletion. During intense blooms, dissolved oxygen levels are reduced to nearly zero during the die-offs of plankton, which terminates the bloom. Compounding the die off, dead and decomposing fish allow bacteria to flourish and further deplete oxygen and toxify the water. Fish and invertebrate probably die from suffocation because none of the dinoflagellate are toxic by themselves. With that said some species are more associated with kills then other. Off California, Gonyaulax polyedra has acquired that reputation.
The other way red tides will kill fish and invertebrate is by producing potent chemical toxins. This way is more common but not limited to the East Coast. When the bloom is severe, fish die rapidly from the neurotoxic effects of the red tide which enter their bloodstream through the gills. Off the Florida coast there is an unarmored dinoflagellate that is highly toxic to fish called gymnodinium breve. Because the fish die so quickly, these toxins do not have time to build up in their tissue. Fish exposed to lower (sub-lethal) concentrations, however, may accumulate these toxins in their body. New evidence from current research suggests that such bioaccumulation in fish eaten by dolphins may have been a major factor in the deaths of more than 700 of these marine mammals in 1987.
The last severe fish kill off southern California was 1964 was believed to be caused by the dinoflagellate Gonyaulax polyedra. Large number of fish and invertebrate were killed. Fifty ton of anchovies died in Santa Cruz Harbor. Anchovies, mussel and barnacles were killed in Marina Del Rey, King Harbor and LA Harbor. One hundred and seventy miles off the Northern Baja coast was smothered in the tide. There were even some instances of squid and abalone die offs.
Some of the most intense blooms occur in sheltered waters. With limited circulation, the blooms intensify and quickly trap a variety of fish and invertebrate.
Although, there are other types of marine life that are luminescent, the bluish green bioluminescent light we see in the glow of the prop wash or crashing waves at night, is due principally to the red tide dinoflagellates. The process is not fully understood but scientists believe dinoflagellates are stimulated to bioluminesce by the deformation of the cell membrane caused by a shear force which is the movement of water over their surfaces.
The phenomenon of bioluminescent has been put to use in the commercial sardine anchovy and mackerel fisheries in California. When operating at night, the purse seine fisherman locate fish schools by the “ fire “ in the water caused by the fish swimming through millions of dinoflagellate stimulating them to luminesce. Red tide blooms can pose a serious problem for public heath through shellfish contamination. Bivalve shellfish, especially oysters, clams and mussels, can accumulate enough toxins to become toxic to humans.
Shrimp, crab, scallops and lobsters in red tides are safe to harvest and eat because toxins do not accumulate in the meaty or hard tissue that most folks consume. Eating the liver, organs, or other soft tissue of shellfish, however, is not recommended. The fillet of a freshly caught finfish in red tides is safe to eat, provided the fish behave normally. With that said, there has been no evidence of harmful effects in humans from red tide contaminated fish. Again, however, it is not a good idea to eat liver, organs or other soft tissues.
Oysters, clams, mussels, mollusks and whelks are unsafe to harvest & eat since they may accumulate red tide toxins in their tissues. This remains effective until local authorities determine that the waters are clear of red tide & shellfish are free of red tide toxins, which may take several weeks after a red tide bloom is over. California places a quarantine on mussels during the warm water months of the year when blooms are more common, usually May 1st thru October 31.
Physically, red tide has little effect on humans who come in contact with the water, however, some people have reported skin irritation after swimming.
Although red tide episodes are not the prettiest thing the ocean has to offer, they are an important part of boom and bust cycles that define the vast and mysterious sea.