Evidence of tench rhabdovirus detected

in seven English rivers

The Environment Agency has detected antibodies to tench rhabdovirus in fish in seven rivers in England, indicating that the fish have been exposed to tench rhabdovirus during their lifetimes.

While these findings show that the disease is more widespread than previously thought, no fish mortalities have been reported since 1999 and so the virus may not be a serious risk to fish populations.

This virus was previously believed not to be widespread in the wild, having only been detected twice in England and Wales. In 1999, it caused fish mortalities in five waters which had all been supplied with bream from a single source in Northern Ireland and in 2004 it was found at our Calverton Fish Farm. Both Calverton and Leyland Fish Farm were cleared and disinfected as a precaution.

Restocking of both farms has started. To prevent re-infection, we have been testing fish for exposure to tench rhabdovirus from rivers and stillwaters before using them as broodstock.

Using non-lethal blood tests, developed by the Centre for Environment, Fisheries and Aquaculture Science (CEFAS) as a result of the investigation at Calverton,  our  fisheries scientists have been screening potential broodstock from thirteen rivers and eight stillwaters across England. They found evidence that the virus may have been, or may still be present in, seven of the thirteen rivers. Those with positive results are:

·         River Trent in Nottinghamshire

·         River Teme in Worcestershire

·         River Nidd in Yorkshire

·         River Wharfe in Yorkshire

·         River Witham in Lincolnshire

·         River Douglas in Lancashire

·         River Wye in Derbyshire

Not every test undertaken, however, has proved positive. Samples taken from the Ribble and the Pennington Flash in Lancashire, the Cam in Cambridgeshire, the Test in Hampshire, the Bain in Lincolnshire and eight stillwaters in northern England have all tested negative. The Dover Beck, which is immediately downstream of Calverton Fish Farm, has also tested negative.

Our Senior Fish Health Scientist, Nigel Hewlett, says: “We do not know how serious these latest findings are.

‘They do suggest that the virus may be more widespread than we originally thought, but, as it does not appear to have caused any fish mortalities in the wild since 1999, it may not prove to be a serious risk to fish populations.’

Where tench rhabdovirus outbreaks have been encountered in the past, they have resulted in acute and significant fish kills, albeit these have been in exceptional circumstances which may have rendered the fish more susceptible to the disease. No such incidents have been recorded from any of the rivers recently tested.

Our Head of Fisheries, Dafydd Evans, says: ‘All these rivers are extremely valuable and popular fisheries, and we have no evidence that the virus has had an impact. More work on the disease is needed, however, and we will be working closely with CEFAS to get a better understanding of its distribution, what impact it can have on wild fish stocks and what risk this poses to fisheries.

‘We do not intend to impose any immediate restrictions on fish movements, but we would advise fishery owners and anglers to take the usual precautions – always take care when buying fish for restocking.’

·         Tench Rhabdovirus is a viral disease of a range of freshwater fish. It is not notifiable under EU Fish Health legislation. Little is known about the disease, but when it has been detected in England and Northern Ireland, it has been associated with fish mortalities in fish farms and fisheries.

·         Discovery of the antibody to a virus can only tell us that the fish has been exposed to the virus at some point.  It does not tell us if the virus is still present.

·         Outbreaks of Tench Rhabdovirus are very rare. It has only been encountered in England and Wales on 2 occasions – once when it caused serious mortalities in 5 fisheries supplied with bream from Northern Ireland and once at Calverton Fish Farm. In these cases there were exceptional stressors on the fish, which are thought to have been  significant contributing factors.

·         Calverton and Leyland Fish Farms produce around 500,000 fish per annum and are the sole source of coarse fish used to restock our recovering rivers.  Calverton also takes a leading role in spawning threatened species and in supporting research projects such as the current investigations on endocrine disrupting substances.

·         Calverton Fish Farm, Nottinghamshire, operated by the EA, is a major source of river coarse fish in the UK. It is one of only two of our coarse fish farms, the other being in Leyland, Lancashire. Calverton was established during the late 1930s, producing mainly trout until 1986 when it was converted into a dedicated river coarse fish production unit. Since then, over four million fish from Calverton have been released into waters the length and breadth of England and Wales.