Virus devastates trout fry

Press, Volume CXVI, Issue 34130, 17 April 1976, Page 20

Virus devastates trout fry

(By

BRIAN CLARK)

Anglers in Britain, whose sport is just recovering from the worst salmon disease in history, are watching with morbid fascination the career of a virus which has begun to kill trout fry by the million in East Anglia.

Their interest — and the interest ef biologists — centres on three things: the threat that the virus poses to trout fishing in the short term, the fears of what it conceivably could do the vast, coarse fisheries of the Midlands in the medium term, and the fact that the way it has been allowed to spread provides a chilling comment on the ability to contain the even worse fish diseases which are queueing up on the other side of the Channel. The virus was first discovered at a fish farm on the River Nar last year; and it was identified by biologists, who have never been known for their poetry, as Infectious Pancreatic Necrosis.

IPN does not affect adult fish but is carried by' them. Adults of both sexes shed virus particles into the water; and hen fish pass them on in the membranes of the eggs they spawn. The fry are then killed by the virus before they can mature. SWIFT SPREAD From the first trout farm, the virus swiftly spread to a second fishery, about two miles downstream on the Nar; and since then, through supplies of stock fish, it has physically been transported to three other fish farms in the region. Fish from the same nucleus of two have additionally been released in

more than 60 other fisheries I in East. Anglia and the Midlands; and a sixth fish farm, has somehow contracted the disease independently of the 'Others. The Government has now imposed a clampdown on fish movement in the area; but the ban could well be a posthumous one in the most literal sense. Five of the six farms are on rivers; and two are on streams which could con-, laminate all the major river systems of East- Anglia and the Midlands. Directly and indirectly exposed to the disease over the last few months, therefore, have been the Yare and its tributaries, the Great Ouse and its tributaries, the vast Nene and Welland systems, and the complex canai network which covers the Eastern Midlands. DISTURBING ASPECT

One of the most disturbing aspects of the outbreak is that no one as yet knows just how far down these waterways the disease actually has spread, because the physical resources to trap and examine fish from them do not exist.

Perhaps even more disturbing is the realisation of a danger I predicted last year, before the present crisis had become known. One of the waters now exposed to IPN in the Great Ouse system has water channelled from it into the headwaters of the Stour and Blackwater systems in Essex. This Ely Ouse “water transfer” is one of the prototypes for a series of . canals which will link many of the country’s water svstems to one another in an attempt to redistribute the national rainfall. As a result, enormous areas of waterway have been exposed to disease in the current

outbreak which otherwise might never have been. Should present research show that IPN could kill coarse fry — or if some virus with a more catholic taste than trout were piggybacked about the country in this way — larger schemes which eventually the impact could deal coarse fishing a blow from which it would take decades to recover. RAINBOW FRY

Regardless of whether IPN kills coarse fish, however, its lethal effect upon trout fry has already been established beyond doubt. Several million rainbow-trout fry — the most vulnerable — have already been lost in the six farms known to have been infected so far; and if the virus were to reach other breeding centres on a significant scale, future sport could be threatened as inroads are made into the fish now being reared for stocking in one and two years’ time. If more fish farms were to be infected, bans on fish movements would also become more widespread; and the effect would be that the waters which anglers fish would be starved of trout. More immediately still there are now real fears for the ! remaining native brown i trout which breed naturally j in lhe upper reaches of most : ] of the rivers affected. ; I For both fishermen and , biologists, however, the most chilling aspect of the ' IPN affair remains the way the virus, before its discovery, has been moved about the country by man himself; helping to achieve in hours what nature on her own might never have achieved in years.