Scottish Marine and Freshwater Science Volume 4 Number 3: Epidemiology and Control of an Outbreak of Viral Haemorrhagic Septicaemia in Wrasse Around Shetland Commencing 2012

8 Removal of Wrasse from Sites

Wrasse at the first-reported site were euthanased, double-bagged and sent for incineration at the Lerwick waste management facility on 19 December 2012. The additional five VHS positive farms initiated a wrasse removal programme commencing 20-24 December 2012 inclusive. Stocked wrasse were captured using creels, raising the dead-basket, hand-nets and divers over a period of several weeks. Captured wrasse were euthanased and either incinerated at farm shore-bases with ash sent to landfill (farms on the west coast of Shetland mainland) or double-bagged and sent for incineration at the Lerwick waste management facility (the farm on the east coast of Shetland mainland). This progressive approach to removing wrasse was adopted to avoid unnecessary stress to Atlantic salmon stocks which are not listed as susceptible to VHS under EC Council Directive 2006/88/ EC.

All wrasse are known to have been euthanased at the first-reported site. Two of the farms subsequently harvested their Atlantic salmon stocks during February and March 2013 with processing taking place at a plant located in south-west Shetland mainland in the same locality as three of the VHS positive sites. Three of the six sites have, therefore, been cleared of stocked wrasse.

Three farms remain in production with expected harvest dates of autumn 2013 and summer 2014. Records for these farms, covering a period of between eight to 11 weeks from the start of the wrasse removal programme, indicate that approximately 40,000 of an approximate 42,000 wrasse stocked onto the farms (95%) were either captured or described as mortalities. This estimate does not take into account undetected mortality prior to the start of the removal programme. Wrasse removal continues as wrasse are observed and the dead-basket is raised, and will continue, albeit with a reduced effort, until the farms are harvested and become fallow.

An alternative approach to evaluating wrasse removal is to fit a gamma distribution to the number captured each day and use this to estimate the proportion of wrasse removed by the end of the programme. Such an analysis involves assumptions including an equal effort to catch wrasse throughout the removal programme, but overcomes the problem of undetected mortality.

The results of such an analysis (Table 2) indicate that, overall, 99% of wrasse were removed. Output from the transport-epidemiology model ( Appendix 1) indicates a reduction in the area around sites that the burden of VHSV exceeds the assumed minimum infectious dose.

Table 2. Removal of wrasse on three farms currently under production

^α including recorded mortality prior to the removal programme but not accounting for undetected mortality;
^β assuming an equal effort to capture wrasse throughout the removal process.