General · 22nd February 2013
A lethal fungus has spread around the world, killing frogs at a rate 40,000 times faster than at any time since these amphibian species formed some 360 million years ago. Habitat loss is a factor, too, as wetlands are drained and forests are cut. But the most lethal and uncontainable enemy of frogs — and salamanders, too — is a single-celled fungus called Batrachochytrium dentrobatidis (Bd), a very strange killer since it belongs to a family of fungi that has long co-existed with frogs and has been relatively harmless. What happened to make it lethal is a mystery biologists set out to explain (NewScientist, July 7/12).
At first they suspected that climate change might be creating the ideal conditions for the fungus to flourish. Another candidate was pollutants. But the definitive answer came when researchers sequenced the genome and discovered that samples of the lethal Bd collected from everywhere in the world were essentially identical. Dr. Matthew Fisher, an epidemiologist from Imperial College London, calls this variant “the global panzootic lineage”. Since it doesn't survive in salt water and it has no airborne stage, it had to be getting from continent to continent with the help of people.
Two species of frogs have been traded internationally for decades. One is the African clawed frog (Xenopus laevis), used for research purposes, and the other is the North America bullfrog (Rana catesbeiana), used for meat. Both species are relatively resistant to Bd so they could carry it undetected to wild frogs — this vector was confirmed when the first two outbreaks of Bd in wild frogs were detected in a site downstream from a bullfrog farm in the Philippines and in a newly established bullfrog population in a lake in the United Kingdom.
The other factor contributing to Bd's virulence is the crowded conditions in which captive frogs are bred. In a wild environment, “natural selection tends to make diseases less virulent, because pathogens that rapidly kill their hosts have less chance of spreading. In crowded conditions, however, evolution favours the nasty” (Ibid.). Somewhere in a frog farm, two related species of Bd combined to form a new and lethal variant. It was then distributed around the world with the farmed frogs. In other words, the Bd that is killing hapless wild frogs everywhere on Earth is “our own Frankenstein monster” (Ibid.).
This scenario should be familiar because it corresponds exactly to net-pen salmon farming in BC's West Coast where viruses have been brewing for years in crowded “feedlot” conditions. The prospect that these farms could import and then breed a lethal variant virus which could subsequently escape to wild salmon has been haunting Alexandra Morton since her studies first found the same viral diseases in both farmed Atlantic salmon and native salmon stocks.
Morton worries that all the conditions are in place for a wild salmon catastrophe. Eggs that salmon farms import from around the world arrive with exotic diseases. Viruses flourish amid the hundreds of thousands of fish that are confined in individual net-pens, a threat accentuated by the fact that viral diseases are known to exchange genetic material to create new strains. Pesticides, parasites, feces and diseases pass unobstructed through the net-pens into the surrounding marine ecosystem. And the industry has further increased the risk by choosing to locate many of their salmon farms along the migration routes of the wild fish.
Morton's concerns are credible. Although motivated by a passion to protect wild salmon and the entire West Coast ecology they support, she nonetheless thinks like a scientist. Her arguments are rational, her studies are empirical, her gathering of data is rigorous, and her fears are justified. They are also shared by almost everyone who is free from the economic leverage purveyed by the salmon farming industry.
As Morton points out in her electronic newsletter, when the salmon farming industry first wanted to import Atlantic salmon eggs to the West Coast in the 1980s, the proposal was widely opposed by “the Steelhead Society of British Columbia, United Fishermen and Allied Workers Union, BC Ministry of Environment, even members of the federal fisheries salmon transplant committee, and the Director General of Fisheries and Oceans, Pacific Region... . They all cited concern that exotic diseases would accompany these shipments.” For this reason, in 1986, Dr. Dave Narver, Director of the BC Ministry of Environment, warned that the “introduction of exotic races of salmonids into British Columbia is probably the most critical issue ever to face the maintenance of wild salmon stocks.” In 1990, Pat Chamut, Director General of the Department of Fisheries and Oceans, warned that the “continued large-scale introductions [of salmon eggs] from areas of the world including Washington State, Scotland, Norway and even eastern Canada would eventually result in the introduction of exotic disease agents of which the potential impact on both cultured and wild salmonids in B.C. could be both biologically damaging to the resources and economically devastating to its user groups.”
Morton believes that the salmon farming industry, in conjunction with sympathetic government agencies, have set in place the conditions that could unleash a viral catastrophe in BC's wild salmon populations. For her, the ingredients for crisis are in place and the waiting is agonizing.
The crisis of frogs and fishes is analogous. History repeatedly reminds us that our ignorance has a propensity to combine with our venality to create disasters. Frogs all over the planet are dying in massive numbers because we were instrumental in concocting a Bd “monster”. The possibility exists that we are about to inflict the same fate on our beloved wild salmon.