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Phantom Algae Kill Fish

A worldwide increase in toxic small plants (phytoplankton), including algal bloom, over the past 20 years has coincided with increasing reports of fish deaths in many parts of the world, says Dr Ravi Gooneratne, a toxicologist in the Animal and Veterinary Sciences Group at Lincoln University.

There are US reports of a mysterious toxic "alga-like" organism that requires live fish to become active. It is now thought to play a major role in the annual die-off of fish on the Atlantic coast and is also blamed for massive fish deaths off the North Carolina coast and in Chesapeake Bay.

Substantial research efforts are currently under way to discover whether this alga may provide the answer to the mystery of millions of fish deaths. Scientists state that it rests as a cyst on the bottom of the sea until stimulated by the presence of live fish. The alga is so resistant that it can survive in extreme conditions such as strong acids or drying in the sun for over 35 days.

This "phantom" alga is different to the algal bloom that was responsible for the recent shellfish poisoning in New Zealand. In New Zealand, the toxic algae that shellfish feed on is not toxic to the shellfish but is toxic to humans who eat the shellfish. In contrast, the toxins produced by the phantom algae cause nervous disorders in fish and shellfish, resulting in death within a few hours of exposure. The toxins induce disorientation and lethargy in fish, and the alga then feeds on parts of the dying fish.

This bizarre occurrence of algae actually hunting down a fish to eat is new to scientists, and can be compared to grass feeding on sheep. The alga is still too new to have a name and scientists do not yet know whether it is a plant or an animal.

Identification is difficult and poses problems for scientists, because within a short period after a fish kill, algae suddenly decline in numbers. The algae then rapidly transform to a resting, non-toxic state and settle to the bottom of the water. With well-timed sampling these algae may be discovered at the scene of the crime.

New Zealand officials have been baffled by our recent shellfish episode and the large number of fish deaths in the last few months. This is because New Zealand has never had to respond to anything like this before, according to Gooneratne.

He says that New Zealand needs toxicology training with a good science base to produce specialists who can identify and respond quickly to such problems.