Crayfish disease and related problems in Australia

“White tail” or “Porcelain disease” is the most documented of crayfish diseases. Caused by the microscopic, single celled microsporidian Thelohania, it can be identified in its later stages by the flesh on the underside of the tail turning white or porcelain coloured, and the animals’ walking legs becoming splayed and ridged. This disease is invariably fatal. It appears to be transmitted by cannibalism. Although “White tail” does not affect humans, it does affect the appearance of the crayfish and therefore marketability of the animal.

Temnocephalid flatworms are commensals that attach themselves to the soft underside of freshwater crayfish via a suction pad located at one end of the worm. They feed on particles of food that crayfish stir up when feeding. While not actively harmful, if present in large numbers due to plentiful food, they and their eggs can restrict water flow through the crayfish’s’ gills, causing asphyxiation.

Epistylis causes similar respiratory issues if present in large numbers, but is otherwise benign. A stalked protozoan, it grows on external surfaces of fish and crustaceans. Their presence in large numbers suggests a high organic loading, low oxygen levels and poor water quality generally.

Stress seems to be the primary contributing factor in outbreaks of disease in farmed crayfish populations. It weakens the animal’s defences against disease and cannibalism, inhibits growth and decreases survival rates. Issues directly related to the stress levels of crayfish include water temperature, sunlight, competition, cannibalism and handling. In water temperature outside of a comfortable growing range, crayfish are susceptible to stress. A related issue is direct sunlight, which can damage crayfish’s eyes and cause direct heat stress. Competition for food and shelter is a primary cause of stress for crayfish. If stocking rates are too high, crayfish will attempt to leave there ponds and seek better environments. Also, cannibalism can result from stressed living conditions created by lack of food and shelter. Poor handling methods can cause stress and injury. Handling of stock should be minimised and carefully planned. Animals stressed in this way can exhibit a diminished growth rate and increased mortality rate.

Over all, the crayfish is a very hardy species, and is able to withstand disease and many extremes in its environment. By understanding its weaknesses and making careful allowances in these areas, Australian crayfish can be cultured very successfully.