AN EVALUATION OF SURFACE MORIBUND SALMON AS INDICATORS OF SEAPEN DISEASE STATUS

This observational study utilized modified mark and recapture techniqu es to examine the reliability of moribund fish caught at the surface o f the water as indicators of the disease status of commercial salmon f arms in British Columbia, Canada. The results indicate that the vast m ajority of fish captured at the surface are truly moribund, but that t he pattern of disease in these fish can be significantly different fro m that seen in moribund fish not accessible to capture from the surfac e. In some cases, estimates of the proportional mortality attributed t o specific disease categories differed by 30% between surface-caught f ish and dead fish. The small number of fish caught at the surface make s it difficult to detect disease of low prevalence. In addition, the l ow proportion of total mortalities accessible to surface capture prior to their death makes extrapolation of disease data derived from surfa ce samples to the entire pen tenuous. We conclude that samples of surf ace-catchable fish do not necessarily reveal the true disease pattern of a seapen and that future studies should be directed towards develop ing additional methods of disease monitoring and investigation on seaf arms such as mortality surveys and behavioural observations.