A STEADY stream of fish surfaced from the waters of Lake Erie last week as biologists aboard the research vessel Esox hauled in a gill net laden with hundreds of perch, bass, carp and freshwater drum. The bountiful sample, taken off Kelleys Island near the US-Canada border that bisects the lake, comes as a relief to Jeff Tyson, an Ohio fisheries biologist from nearby Sandusky (see Map). Yet Tyson knows a killer is lurking.
This time last year dead fish inundated the shoreline in such numbers that Tyson found it difficult to get his boat into the water. Many had their eyes popping out of their sockets and were haemorrhaging blood from open sores. The cause was viral haemorrhagic septicaemia or VHS, an Ebola-like disease common in Europe that now threatens to devastate fish populations across North America.
VHS first appeared in the Great Lakes in 2003. Last year it killed tens of thousands of fish in mass die-offs, and is capable of infecting dozens of species, including salmon, trout, walleye and perch. This spring the mortality rate appears to be on the rise again as warming waters in the Great Lakes enter the 4 to 15 掳C window when viral reproduction is at its height and fish are most stressed by hormonal changes associated with reproduction. Though the virus poses no threat to humans, scientists and local officials agree it could devastate the lakes鈥� $4 billion sport and commercial fishing industry.
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鈥淚n terms of fish health, it would be hard for me to imagine anything else that could be any worse than this,鈥� says James Winton, the chief of fish health at the US Geological Survey in Seattle, Washington. Winton has been tracking the disease since it was first detected in salmon in the Pacific north-west in 1988. At that time, some biologists proposed such draconian measures as killing all fish in rivers where infected salmon spawned, but the particular strain of virus proved to be less virulent than many feared.
鈥淚n terms of fish health, it would be hard for me to imagine anything that could be worse than this鈥�
However, the scale and speed of the spread of the Great Lakes strain is a different story, says Winton. 鈥淚 think some populations will grow resistant to the virus but I don鈥檛 think they will ever recover to pre-virus numbers.鈥�
American and Canadian officials have tried to contain the virus by restricting the export of live bait fish out of areas known to harbour infection, and by urging sport fishers to wash their boats when moving between lakes, but it may already be too late. On 9 May, VHS-infected fish were found in an inland lake in the state of Wisconsin, suggesting the virus has spread through Lake Michigan and may be closing in on the Mississippi river just to the west.
The new strain of VHS found in the Great Lakes is most closely related to a saltwater form of the virus, which suggests it may have been introduced via ballast water from ocean-going ships, a common entry point for invasive species.
So far, the federal governments on both sides of the border have failed to pass laws that would force ships to sterilise their ballast water 鈥� a measure strongly opposed by the shipping industry. Tired of waiting, Michigan state passed its own law in January requiring all ocean-going ships to treat their ballast tanks before discharging. Several other Great Lake states are now considering similar measures.
That may prevent future invasions, says Tyson, but it is unlikely to control VHS now that the virus has a foothold. He notes that when VHS turns up in aquaculture ponds in Europe, the only way to get rid of the disease is to drain the contaminated waters. 鈥淵ou can鈥檛 do that with the Great Lakes,鈥� Tyson says. 鈥淲e are going to have to live with it, and hopefully the fish can too.
