Early Mortality Syndrome in Salmonids

Early mortality syndrome (EMS) is an international problem that affects various species of salmon and trout and can cause catastrophic losses of very young fish. In the Great Lakes Basin, for example, salmonids have suffered a post-hatch mortality of up to 90 per cent, depending on the year, species and location.

The exact cause of the syndrome is not well understood, but scientists at Environment Canada's National Water Research Institute (NWRI) are working with fishery and resource managers in Canada and the United States to study the interactions among contaminants, thiamine deficiency and antioxidant vitamins—work they believe will shed new light on the problem.

Symptoms of EMS appear between hatching and first feeding, and include loss of equilibrium, lethargy, swimming in a spiral pattern, hyper-excitability, hemorrhaging and death. The species affected include coho and chinook salmon, and rainbow (steelhead), brown and lake trout. Atlantic salmon in the Finger Lakes of New York State and the Baltic Sea experience similar early life-stage mortality syndromes, called the Cayuga Syndrome and M74 respectively.

Thiamine deficiency in eggs is a common link among these three syndromes, and treatments of thiamine on eggs or fry have been shown to enhance survival and reverse their effects. Although the cause of this deficiency is not known, it appears to result from thiamine-degrading enzymes in the salmonids' diets. In the Great Lakes, certain salmonids feed on non-native fish species, such as alewife and smelt. Quantities of thiamine-degrading enzymes in these species have been documented at up to a hundred times that of native species, and implicate them as a likely cause in the development of thiamine deficiency.

The fact that EMS and M74 are more common in contaminated ecosystems points to the possibility of thiamine- or thiaminase-contaminant interactions. Contaminants may increase the thiamine requirements of salmonids, or the effects of contaminants may only show up when the environmental availability of thiamine is low. In Finland, researchers discovered higher concentrations of dioxin-like contaminants called planar halogenated hydrocarbons (PHHs) in the muscles of female salmon that had M74 appear in their offspring.

So far, no link between any particular contaminant and EMS has been established in North American studies, but research to date has been short-term. Longer-term investigations are needed to determine whether salmon stock with elevated PHHs and low thiamine levels experience an increase in the syndrome's occurrence.

Another line of inquiry is a possible link between the thiamine deficiency and antioxidant vitamins. In a Lake Ontario study, researchers analyzed female lake trout for antioxidants and found that adults whose offspring developed EMS had lower vitamin E (an antioxidant) levels compared to those whose offspring did not. More work is needed now to clarify the link between thiamine deficiency and other factors like antioxidant vitamins.

NWRI scientists and their partners in Fisheries and Ocean Canada, the United States Geological Survey, the Michigan and Wisconsin departments of Natural Resources, the United States Fish and Wildlife Service and the Chippewa-Ottawa Treaty Fishery Management Authority are undertaking a broad range of activities as part of their research. These include quantifying the thiamine and thiaminase in the food web, determining the extent of the syndrome in salmonids, improving procedures for thiamine therapy, developing a laboratory model, and testing a large range of chemicals and contaminants to see if they act synergistically with thiamine deficiency.

Other areas to be explored are the long-term effects on fish surviving EMS, whether other species that consume alewife and smelt experience reproductive difficulties related to thiamine deficiency, and whether blue-green algae may act as a source of thiaminase. The findings will help to protect self-sustaining fish populations and to rehabilitate degraded populations of native species in the Great Lakes.

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