Red Tides

Since about half a century ago, red tide has been occurring in many coastal places of Japan, such as Tokyo Bay, Ise Bay, Osaka Bay, and Ariake Sea. Red tide is algal accumulation that could be a result of eutrophication in bays and lakes. Eutrophication is the enrichment of an ecosystem with chemical nutrients, typically compounds containing nitrogen, phosphorus, or both.

“The History of Great Japan” edited more than 300 years ago, described 16 cases of red tide, seven in freshwater and the rest in the marine environment. The oldest was an occurrence in 731 AD. A case that occurred in 1234 AD was reported to cause fish mass mortality and human fatalities after eating fishes.”

Two types of harmful algal blooms (HABs) are known in Japan. The first one is a noxious algal bloom associated with the mass mortality of marine organisms, especially fish in aquaculture cages and shellfish hanging from rafts. Most of the noxious blooms cause water discoloration, i.e. red tides, but less than 20% of the red tides,as the term is used here, cause harmful effects.

The second type of HAB is a toxic algal bloom causing contamination of shellfish, either PSP (paralytic shellfish poisoning) or DSP (diarrhetic shellfish poisoning). The amount of toxin in shellfish and the cell number of toxic dinoflagellates are monitored regularly by local governments. Closures of aquaculture areas for the harvesting and marketing of shellfish occur depending on the amount of toxin, not the toxic plankton concentration. Therefore not all blooms of toxic plankton are recorded as HAB occurrences. Toxic plankton blooms with toxin contamination lower than the permitted level are not recorded as HAB occurrences.

In some fishing villages in northern Japan, warning about possible toxin contamination might be passed on from ancestors and known among elderly local fishermen. There are several traditional folk tales such as “Do not eat shellfish during snow water runoff into the sea”, i.e. it warns that shellfish may become inedible in early spring. After modern aquaculture developed, fishermen tried to sell their products year round but closure of marketing happens often in the spring. Thus we notice the wisdom from the experience that led to the folk tale.

At the same time, dams have been constructed in Japan on rivers that flow into the bays where red tide has been occurring. The correlation between red tide occurrence and dam construction in Japan was researched using the data of many government organizations. The results indicate that the construction of dams influences the occurrences of red tide. Because it seemed that small particles flowed from dams contain nutrients that stimulate the growth of algae.

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