This paper describes an approach to demonstrate freedom of individual rainbow trout farms from Gyrodactylus salaris Malmberg, 1957. The infection status of individual farms is relevant should G. salaris be introduced into a country or zone previously known to be free of the parasite. Trade from farms where G. salaris may have been introduced would be restricted until freedom had been demonstrated. Cage, fish and parasite sample sizes were calculated based on the minimum detectable prevalence (P*), test characteristics, population size, and Type I and II errors. Between 5 and 23 cages per farm would need to be sampled to demonstrate freedom at a cage level P* of 10%. The number of fish sampled per cage depended mainly on the test sensitivity (probability of correctly identifying an infected fish). Assuming a test sensitivity of 99% at the fish level, 59 fish per cage are needed (P* = 5%). Since G. salaris may exist in mixed infection with G. derjavini, testing a sample of gyrodactylid parasites may not result in the parasite being detected when present. Test sensitivity at the fish level depends on the number of gyrodactylids on the fish, the proportion of which are G. salaris and the number examined. Assuming a P* of 5% (i.e. G. salaris are at least 5% of the gyrodactylid population), between 20 and 73 parasites per fish would need to be sampled (depending on abundance) to maintain the Type I error at 0.01 (thus a fish level test sensitivity of 99%). This work identifies the critical information, and further research, needed to assess freedom from G. salaris with a known level of confidence; this is essential to provide a sound scientific basis for decision-making about disease control measures.
KEY WORDS: Gyrodactylus salaris · Rainbow trout · Surveys · Disease freedom · Contingency planning
Resale or republication not permitted without written consent of the publisherDis Aquat Org 79: [47][48][49][50][51][52][53][54][55][56] 2008 1990, Bakke et al. 1990b, 1992a,b, Jansen & Bakke 1995, Soleng & Bakke 2001 and indefinitely in rainbow trout Oncorhynchus mykiss (RBT) populations (albeit at a low prevalence, low abundance and causing no signs of infection) . G. salaris has been found on farmed RBT in Denmark, Sweden and Finland (see Bakke et al. 2007 for details). A RBTadapted strain of G. salaris which is not pathogenic for salmon and can achieve infection rates of several hundred parasites per fish in RBT has been found in Denmark (Jørgensen et al. 2007). Recently, a strain of G. salaris has been found on Arctic charr, which has not affected Atlantic salmon in the same river. Under experimental conditions, salmon are found to be either resistant or only slightly susceptible (Olstad et al. 2007); this is the first published report of a variation in the pathogenicity of Norwegian populations of G. salaris.A limited amount of experimental work has demonstrated that Atlantic salmon in the British Isles are susceptible to Gyrodactylus salaris (Bakke & Mackenzie 1993). Other populations in Europe are also...