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

11 A 'History' of the VHS Positive Wrasse Populations

11.1 Capture

Free-ranging wild wrasse were caught by a single fisherman (hereafter referred to as 'the fisherman') contracted by the fish-farm client using a boat dedicated to this purpose. Wrasse were captured not less than 5 km from any active farm within an area north of the Ardnamurchan peninsula through to the east coast of the Isle of Eigg and south of Arisaig (hereafter referred to as the Sound of Arisaig) from June to October 2012 inclusive. Wrasse from this area were eventually stocked onto sites around Shetland mainland and the west coast of the Scottish mainland.

Wrasse were captured in baited creels deployed daily over approximately 500 m ^[2] . Bait consisted of unpasteurised crustaceans caught within the area fished. Creels were emptied daily in the early morning and moved, without drying or disinfection, to another location within the same area. Species other than wrasse, described as cuddies (juvenile gadoids), conger eels and scorpion fish were also caught, but not in substantial numbers. These, together with wrasse which were regarded as being too small, were returned to sea during a sorting process.

Fish were sorted by species on the boat at the location of capture on a table connected by chutes to four sorting tanks. The wrasse from the sorting tanks were then counted into one of four on-board holding tanks. The water used for the tanks was from the area of catch and was not disinfected. Occasional mortalities (up to two) occurred during the approximate eight hours that the wrasse were on the boat. Tanks on the boat were occasionally steam cleaned.

11.2 Transport to the Temporary Holding Facility

Wild-caught wrasse destined for Shetland were transferred by road to the temporary holding facility in a transport tank on a pick-up by the fisherman (Figure 5). The transport tank and vehicle were steam cleaned after each delivery. The water used for the transport tank was from the point of loading and was not disinfected.

The same transport tank and vehicle were used to deliver wild-caught wrasse destined for sites out-with Shetland to the same temporary holding site. The transport tank and vehicle were also, on occasion, used to deliver wild-caught wrasse direct to farms rather than the temporary holding facility. The transport tank and vehicle were not used for any other type of delivery over this time.

Figure 5. Movements of wild-caught wrasse stocked onto sites

11.3 Temporary Holding

The temporary holding facility is owned and operated by a business independent of the fisherman and fish-farm client. The temporary holding facility was contracted by the fish-farm client to hold, sequentially, four batches of 26,000 wild-caught wrasse prior to transport to Shetland for stocking on farms during 2012. The temporary holding facility also stocks broodstock and hatchery-reared wrasse and these were not supplied to Shetland.

Wild-caught wrasse intended for Shetland were brought onto the site on at least 26 occasions between 10 July and 21 October 2012 and held in up to five tanks located in three separate housed areas of the facility for varying periods of time. Incoming stock was added to existing stock with some splitting and merging of stock intended for Shetland.

The first two batches of wrasse for Shetland were subject to routine laboratory tests for VHS by a commercial provider during August 2012. Samples from a total of 120 individuals (60 for each batch) comprising mixed species of wild-caught wrasse were tested in pooled groups of five individuals using virus isolation with ELISA confirmation ^[40] . All pools tested negative for VHS.

In addition wrasse destined for farms operated by the fish-farm client out-with Shetland also passed through the temporary holding facility. At least three of the tanks, located across two areas on the facility, were used for these subsequent to their use for wrasse transported to Shetland.

Sea-water for the facility originates from an inlet located 0.8 km away from the outlet. Water supplying the site passes through a header-tank with removal of particulate matter through a sand-filter and disinfection by ultraviolet; there is no flow of water from tank-to-tank. All tanks were cleaned and disinfected on being emptied. While there was occasional shared use of some equipment such as hand nets and carrying bins, all but personal protective equipment was disinfected prior to moving. Footbaths were also present between areas of the site. Feed for wrasse destined for Shetland comprised commercial dry pellets.

In general the temporary holding facility attempted to keep stock for each of the four batches intended for Shetland as distinct populations with no carry-over of stock from batch to batch. In practice some carry-over of small numbers of fish may have occurred. Likewise, an attempt was made to keep stock intended for Shetland and elsewhere distinct with no carry over. While facility records are consistent with this, some carry over may have occurred on some occasions, although this is not certain.

11.4 Transport from the Temporary Holding Facility to Shetland

Wild-caught wrasse were transported from the temporary holding facility to five sites around Shetland mainland in nine shipments between 20 August and 31 October 2012 inclusive (Figure 5). Seven shipments were by one transport business (Tr _a) and one shipment each by another two businesses (Tr _b and Tr _c). The transport businesses were owned and operated independently of the fisherman, temporary holding facility and fish-farm client. Shipments involved disinfection of the tank, trailer and truck prior to entering the temporary holding facility, the loading of wrasse using disinfected water, transport by road and ferry to Lerwick, and transport by road to the shore-base of the destination sites. The wrasse from eight shipments were stocked onto single sites, and the wrasse from one shipment stocked onto two sites in the same locality sharing a shore-base.

The five sites received either one or two shipments by Tr _a. This transport business is a common link between VHS positive sites. The same trailer and tank was used for the deliveries although the truck itself was different. The trailer, tank and truck were disinfected prior to entering the temporary holding facility and following delivery. The trailer and tank were dedicated to the transport of wrasse and before commencing these deliveries had not been used since 2011. The transport business also made deliveries of wrasse from the temporary holding facility to multiple VHS negative sites out-with Shetland (Figure 5) and these were interspersed with deliveries to Shetland.

Two sites received a delivery by Tr _b. This transport business also made a delivery of wrasse to a VHS negative site out-with Shetland subsequent to this. The third transport business (Tr _c) did not make any subsequent wrasse deliveries.

11.5 Movements Within Shetland

One movement of wrasse within Shetland occurred (Figure 5). This involved the transfer of 1800 wrasse on 16 November 2012 from a farm which had received wrasse from the temporary holding facility on 6 September 2012 to a nearby farm sharing the same shore-base which had not (and did not) receive a delivery from the temporary holding facility. The movement was carried out by fish-farm staff.