Red tide is an occasional natural phenomenon in Sabah where microorganisms (dinoflagellates) which are naturally living in the sea undergo a population explosion. The numbers become so large and dense that sometimes they impart a brownish-red colour to the sea. The micro-organisms usually are not very numerous in the seas and thus do not represent a health threat. Only when they become many and are eaten in large numbers by filter-feeding sea organisms such as oysters, mussels and other bivalves that render these shellfish toxic because of accumulation in their guts. Some fishes, which eats these organisms or other larger sea life (which originally eat this dinoflagellates), can become toxic since there will be an accumulation of these organisms in their guts.
Above: Two types of clams that can become toxic due to Red Tide
This become a public heath problem when people eat these toxic clams and suffer from Paralytic Shellfish Poisoning (PSP). In addition, these dinoflagellated organisms become so many in the sea that they deplete the oxygen in the water causing fish kills by means of suffocation. This happened in Sabah in 2003 and 2004 when the dinoflagellate – Cochlodinium polykrikoides populations exploded in numbers off the West Coast of Sabah. Such was the effect of the suffocating numbers that millions Ringgits worth of cultured marine fish in cages were killed and lost.
Brown patches of seawater (Red Tide) Likas Bay. March, 2009
Through the years in Sabah there have been several instances of people getting sick as well as number of fatalities cause by the ingestion of a group of compounds known generally as marine toxins, including saxitoxins which cause PSP.
PSP happens when a person ingests shellfish contaminated by a toxin produced by planktonic algae (dinoflagellates, in most cases) upon which the shellfish feed. The toxins are accumulated and sometimes metabolised by the shellfish. Ciguatera is a form of human poisoning caused by the consumption of subtropical and tropical marine finfish (reef fish such as barracuda, grouper, and snapper) which have accumulated naturally occurring toxins through their diet. Tetraodon Fish Poisoning is generally known as being poisoned by eating pufferfishes. It is one of the most violent intoxications from marine species. The gonads, liver, intestines, and skin of pufferfish can contain levels of tetrodotoxin sufficient to produce rapid and violent death. (Download a document about Tetraodon Fish Poisoning.
Pufferfish is the source for Tetraodon Fish Poisoning
Lutjanus bohar (Ikan merah bohar), a snapper which
has been known to cause Ciguatera poisoning at Kota Kinabalu
Red Tide first occurred in 1976 in Sabah and has been a fairly annual occurrence, although serious outbreaks has occurred only a few number of times. The Red Tide Monitoring System is a system put in effect by the Department of Fisheries, Sabah all year round to determine the occurrence of red tide, and especially the presence of PSP-toxic aquatic sealife. This system involves the collection of PSP-prone species of mollusc and fish and analysed at the Likas Fisheries Research Center. In accordance with this monitoring system, the location and species of the sample sealife are recorded. In addition, seawater samples are taken from the same location which are then analysed to determine the concentration of dinoflagellates and diatoms.
Checking Red Tide-causative organisms in the lab
The method by which the levels of toxins are determined from the sample shellfish and bivalves is the Standard Mouse Bioassay. In this test, mice (mice which are cultured specially for Assay) are injected with the toxin obtained from the clams. The assay is standardized using purified saxitoxin. Results are given as micrograms of saxitoxin equivalents per 100 grams of shellfish meats (µg/100g).
Cochlodinium polykrikoides, a Red Tide-causative dinoflagellate
Three motile cells of Pyrodinium bahamense var. compressum in chain form
In conjunction with the Red Tide Monitoring Program is the Public Safety Program. The basic aim of this program is to prevent the threat of PSP poisoning from actually affecting the local population. This is done by informing the public (through the mass media and direct contacts) not to collect or fish, sell or eat affected species of seafood (read a sample Press Release). This activity is carried out by the Department of Fisheries closely in conjunction with the Health Department. The actual activities include warnings and information dissemination regarding the dangers of Red Tide as well as stopping and preventing people from selling these hazardous seafood working under the ambit of the Food Act. As harmful algal blooms are also a threat to aquaculture species, the Department also give advisory to fish farmers and other aquaculture operators.
A newspaper Red Tide Alert on 24th March, 2012
Download a primer on Red Tide below ( PDF File 3.59 MB).