Evaluating abnormal mortality as an indicator of disease presence in the Atlantic salmon industry using the receiver operating characteristic (ROC)

Aquaculture faces many threats, including diseases, of which some are notifiable under current UK regulation, e.g. infectious salmon anaemia (ISA) and infectious haematopoietic necrosis (IHN). Abnormal mortality is one possible indicator of the presence of infectious disease on a site that may be used, by the regulator, as a surveillance alert that allows them to identify possible notifiable diseases and to activate measures of control to reduce the risk of spreading those diseases. Therefore, mortality records at the farm level may be a useful indicator for regulatory surveillance purposes in order to identify potential disease outbreaks. In the UK, regulators and producers have discussed abnormal rates of mortality that may be considered as a trigger to notify the official regulator. In our study, the receiver operating characteristic (ROC) approach was used on mortality data from production cycles of a site production database of marine Atlantic salmon belonging to a single company. The usefulness of these data in helping the detection of infectious diseases was determined using measures of sensitivity and specificity. For fish under 750 g, the abnormal rates of mortality did not provide a strong indication of the presence of disease. The area under the curve (0 ≤ AUC ≤ 1) values were generally low with the exception of cardiomyopathy syndrome (CMS) that showed AUC = 0.77 for weekly mortality and AUC = 0.73 for five-week rolling mortality. However, abnormal levels of mortality for fish with weight over 750 g provided a strong indication of the presence of disease with the exception of both suspected and confirmed IPN. The probabilities of triggering official notification were low since mortality events over the percentages proposed happened infrequently. The most efficient trigger will be for weekly mortality (1%) for fish with weight over 750 g since abnormal mortalities in such large fish are more likely to be associated with the presence of disease.

► Large fish production database analysed using receiver operating characteristic.
► Mortality thresholds can provide evidence of infectious disease in salmon of > 750 g.
► Thresholds provide little evidence of infectious disease in smaller fish.
► Mortality reporting for larger salmon is demonstrated quantifiably to aid surveillance.