A personal-interview survey was conducted to determine the frequency of and reasons for chemotherapeutic treatments on Ontario trout farms during 1990. Sixty-two fanners, producing 91% of Ontario's farmed trout, participated in the survey. Farmers had most often received advice on treatments from other farmers. The number of treatment regimes administered during a production cycle ranged from zero (5% of farms) to more than nine; the median on user farms was 2.3 treatment regimes. Chloramine-T and formalin were the most commonly used chemicals (they were used on 66 and 53% of the farms, respectively). Preventive treatments accounted for 67% of all treatment regime initiations; initial therapeutic treatments, primarily for gill diseases, accounted for 22% and repeat treatments accounted for 11%. At least some 2-10-cm fish were treated on most farms. Fish in increasingly larger size ranges were treated for therapeutic purposes on progressively fewer farms; prophylactic treatment, however, appeared to be independent of the size of treated fish. Farmers who used prophylactic treatments tended to treat fish much more frequently. There was, however, large variability among farms in the frequency of prophylactic treatment regimes, indicating that farm-specific factors may strongly influence the perceived need for chemotherapeutic prophylaxis. The relatively common occurrence of treatment failure indicates that at least some therapeutic treatments may be applied incorrectly.The impending crisis in the registration offish-ed (Meyer 1989). A necessary first step is to asery chemicals predicted over a decade ago by certain which nonregistered drugs practicing fish Meyer and Schnick (1978) has finally come to pass, farmers use presently. At the same time, extension Regulatory agencies are becoming increasingly re-efforts must be intensified to help fish farmers destrictive toward the use of nonregistered products crease their use of chemotherapeutants. In particin aquaculture, and fish farmers and fisheries sci-ular, farmers require more information on which entists face a dwindling selection of drug-based chemical treatments they can avoid, as well as disease control options. At present, only a few information on ways they can increase the efficacy therapeutants are registered in North America, of the treatments they do administer. A prereq-Pressures from consumer and environmental uisite to such extension programs is an evaluation groups for safe food and water supplies further of the current knowledge base and practices of fish exacerbate this shortage.farmers (Blackburn 1984).
Registration-oriented research on chemicalThis report presents the results of a personalcompounds useful in aquaculture is urgently need-interview survey conducted in 1990-1991 to de-
A mail survey of Canadian fish diagnosticians was conducted to determine how frequently samples of asymptomatic fish yielded an estimated apparent prevalence (ap) of greater than zero but less than 2% or 5% for infectious pancreatic necrosis virus (IPNV). infectious hemalopoielic necrosis virus (IHNV), Aerornonas sulmonicida, or Ceratomyxa shasta. The relationship of host-specific factors to the frequency of samples with low ap (^5%) was also investigated. Fourteen percent of IPNV-positive samples and 26% of A. salmonicida-posilivc samples had an ap of less than 5%, with Atlantic salmon and rainbow trout, respectively, having the highest frequency of samples with low ap. Forty-seven percent of IHNV-positive samples and 35% of C..v/ia.v/a-posiiive samples had ap values of less than 5%; no factors were significantly associated with the ap of either pathogen. The results are used to discuss the probability of misclassifying the infection status of a population of fish. This probability is influenced by the sample size and the apparent prevalence, which is a function of the true prevalence and the sensitivity and specificity of the diagnostic test. Formulae for estimating the probability of correct classification of a population's infection status are reviewed.
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