Publication Abstract

Spot the difference: Mechanical control of Ichthyophthirius multifiliis in raceways.

N.G.H. Taylor*, S.M. Picon Camacho, R. Bawden, A.P. Shinn

Ichthyophthirius multifiliis, more commonly known as ‘white spot’ is recognised as one of the most pathogenic diseases of wild and cultured freshwater fish. In trout hatchery systems infections can quickly establish and result in high mortality if left unregulated. Current control centres around the use of regular bath treatments with formalin, however, treatments are not always effective and are labour intensive. Environmentally safe, non-chemical alternatives are required. This paper describes the proformance of a mechanical system ("SystemIch") developed to remove cysts from commercial trout raceways. The system consists of two parts: a specially designed suction head connected to a pump that is used to vacuum the bottom of hatchery raceways, and a low-adhesion polymer raceway lining. 

Following tests on a series of potential linings, the most efficacious product in preventing settlement of cysts was used to line three raceways each measuring 6m (length) × 1m (width) × 1m (depth) in a commercial trout hatchery. Over a period of three months, lined raceways were vacuumed on a daily basis. The three control raceways were maintained according to the farms normal husbandry procedures. Twenty fish were sampled from each raceway at fortnightly intervals, and the abundance of the parasite in each raceway determined. The mechanical system led to a 99.4% (p<0.0001) reduction in the abundance of the parasite in the test raceways, with a mean abundance of 320 parasites per fish in control raceways compared with 2 parasites per fish in the test raceways at the end of the trial. Additionally fish survival was significantly higher in test raceways over control, with 98.5% of the stock surviving in the test raceways compared with only 93% in the control by trial end.

This mechanical system has been shown to effectively reduce the burden of I. multifiliis and its attributed mortality. Additional benefits are likely to be improved water quality, more efficient raceway cleaning and reduced chemical use. Further details are available at: http://www.pisces-aqua.co.uk/whitespot.htm