Harmful and toxic algae - Health Management in Aquaculture

Harmful and toxic algae

Microscopic algae are important food for filter-feeding bivalves (oysters, mus-sels, scallops and clams) and for the larvae of commercially important crustaceans and fishes. Proliferation (algal blooms) of this algae up to a million cells/ l, or cells/ml, is beneficial to aquaculture and wild fisheries operations. In some situations, however, algal blooms can cause severe economic losses to aquaculture, fisheries and tourism, and have major impacts on health and en-vironment. There are 5,000 species of marine phytoplankton (Sournia et al. 1991). Some 300 species can, at times, occur in such high density that they discolor the surface of the sea (red tides). Only 40 species have the capacity to produce potent toxins that can find their way to fish and shellfish and, eventu-ally, to humans.

The first written reference to harmful algal bloom appears in the Bible (1,000 years B.C). In Exodus 7:20-21 is written that at that time in Egypt all the waters in the river turned into blood, and all the fish in the river died, and the river stank, and the Egyptians could not drink the water in the river. In this case, a non-toxic bloom-forming alga became so densely concentrated that it gener-ated anoxic conditions resulting in indiscriminate kills of both fish and inverte-brates. Oxygen depletion developed due to high respiration by the algae (at night or in dim light during the day), but most probably, bacterial respiration during decay of the bloom caused it.

One of the first recorded fatal cases of human poisoning after eating shellfish contaminated with dinoflagellate toxins was in 1793 in Poison Cove in British Columbia. The seawater became phosphorescent due to dinoflagellate blooms. The causative alkaloid toxins, now called paralytic shellfish poisons (PSP) are so potent that about 500 micrograms of toxins, which can easily accumulate in just one serving of shellfish (100 gram), could be fatal to humans.

On a global scale, close to 2,000 cases of human poisoning (15% mortality) through fish or shellfish consumption are reported each year and, if not con-trolled, the economic damage through reduced local consumption and reduced export of seafood products can be considerable. Whales and porpoises can also become victims when they receive toxins through the food chain via contami-nated zooplankton or fish. Poisoning of manatees by dinoflagellate brevetoxins contained in salps attached to seagrass, and of pelicans by diatom domoic acid contained in anchovies have also been reported.

Harmful algal bloom has become apparent only as a result of increased interest in intensive aquaculture systems for finfish. Some algal species can seriously damage fish either mechanically or through production of hemolytic sub- stances. While wild fish stocks have the freedom to swim away from problem areas, caged fish appears to be extremely vulnerable to such noxious algal blooms.

In the Philippines, red tide was reported in 1908 in Manila Bay as due to Peridinium blooms. Thereafter, minor nontoxic red tide outbreaks became al-most an annual event in Manila Bay, particularly in the Cavite Area. It was not until June 1983 that the first outbreak of a toxic red tide caused by Pyrodiniumbahamense var. compressa occurred in Samar, Central Philippines.

In 1987, the presence of dinoflagellate blooms caused by Pyrodinium baha-mense var was detected in the coastal waters of Masinloc, Zambales, extendingfrom Subic to Santa Cruz. Almost simultaneously at that time, the toxic red tide recurred in Samar. Another case of paralytic shellfish poisoning (PSP) was re-ported on August 19, 1988 in Orion, Bataan, followed by another 28 cases in Limay within a four-day period.