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Lobster and Salmon Threatened on Northeastern U.S.-Canada Seaboard

Conditions during October proved lethal, albeit for separate reasons, for lobster and salmon populations along the northeastern U.S.-Canadian seaboard. With mortality rates for those species ranging from 30-50% in some regions, the economic consequences for the fishing and lobstering industries are potentially significant--to say nothing of what seems to be happening to this important marine ecosystem. Although damage was particularly bad along Long Island Sound between New York and Connecticut, where perhaps as many as one million lobsters were killed, the cause behind this massive die-off remains elusive.

``No evidence of bacterial or parasitic infection was seen,'' says Richard Robohm of the Northeast Fisheries Science Center, who is coordinating an investigation into what is causing these kills. ``Initial results suggest that the lobster mortalities are not the result of bacterial infection,'' he says. For instance, although Aerococcus viridans devastated lobster populations in the Sound in 1991 and 1992, there was no evidence of this bacterial pathogen in the hemolymph and hepatopancreas tissues obtained in 1999 from dead lobsters. Two other bacterial morphotypes, one a Vibrio species, that were isolated from such tissue and suspected of causing disease, in fact produced no ill effects when tested in healthy lobsters.

Meanwhile, two other possible explanations for the lobster die-off are still under consideration. One is that a virus was responsible. While not detected in previous die-offs, afflicted lobsters will be examined for shrimp virus in the next few months by Don Lightner of the University of Arizona.

The other explanation being considered for the die-offs is that nonpathogenic microbial blooms that followed local coastal flooding during Hurricane Floyd depleted oxygen `levels in the Sound--in effect, suffocating the lobsters. ``The timing [between the hurricane and the die-off] is about right'' for this explanation to make sense, but other information will be needed for this hypothesis to hold up, according to Bob Steneck of the University of Maine. The species specificity of the mortalities argues against such a broad cause as oxygen depletion.

Also in October, but several hundred miles to the northeast, an epidemic of infectious salmon anemia (ISA) reappeared in waters near New Brunswick, Canada, a mere 3 years after devastating commercial penned salmon stocks throughout the region. This disease was first noted in Norway in 1984, and, since then, has occurred not only in New Brunswick but also in Nova Scotia and Scotland in 1998. The incurable orthomyxoviral infection, whose symptoms include lethargy and organ hemorrhaging, has a mortality rate as high as 50%.

In October 1999, additional infections were detected in salmon that escaped from pens and also in wild salmon as they were migrating to breeding sites in at least one local river. The escaped salmon came from Passamaquoddy Bay, which is the heart of New Brunswick's aquaculture industry and only 3 miles from Maine salmon pens. A few of these wild fish subsequently died of the disease, and others remain infected, according to Fred Whorisky of the Atlantic Salmon Federation (ASF). Salmon recovered from several rivers are being tested to establish the geographical extent of the infection.

How the orthomyxovirus spreads among salmon remains elusive. ``Horizontal transmission in seawater has been demonstrated,'' says Fred Kibenge, an ISA researcher from the University of Prince Edward Island (UPEI). ``Both rainbow and sea trout can be experimentally infected and become asymptomatic carriers,'' adds ASF's Whorisky. ``There must be wild hosts, but we don't know what they are yet.'' He speculates that sea lice, which can harbor the virus and ``stress the fish,'' may transmit the disease. These incidents leave him pessimistic about the future for commercial and wild salmon in New Brunswick. ``The disease has struck at the worst possible place and time,'' he says. Salmon in the region are ``in danger of extinction,'' he adds. ``Finding enough fish with natural resistance to rebuild from is going to be very difficult.''

Brian Hoyle
Brian Hoyle is a science writer based in Bedford, Nova Scotia, Canada.