Introduction and Overview of Early Life Stage Mortality
D. C. Honeyfield, J. D. Fitzsimons, S. B. Brown, S. V. Marcquenski, and G. McDonald
Early mortality syndrome (EMS) is the term now widely used to describe mortality affecting early life stages of various salmonid species in the Laurentian Great Lakes, particularly in Lakes Michigan and Ontario and, to a lesser extent, in Lakes Huron and Erie. The species affected include coho salmon Oncorhynchus kisutch, chinook salmon O. tshawytscha, steelhead or rainbow trout O. mykiss, brown trout Salmo trutta, and lake trout Salvelinus namaycush. The mortality rates are in excess of those attributable to poor fertilization, overripening of eggs, rearing environment, husbandry, or infectious disease. The clinical signs of EMS include loss of equilibrium, swimming in a spiral pattern, lethargy, hyperexcitability, and hemorrhage before death. For the most part, the symptoms develop just before and at first feeding. Early mortality syndrome in coho salmon and other salmonids was present in hatcheries in the 1960s, with mortality rates around 20% or less, and was compensated for by increasing the egg take at spawning (Marcquenski and Brown 1997). However, beginning in the early 1990s, mortality rates attributable to EMS in feral coho salmon rose dramatically in Lake Michigan hatcheries, to 60–90% (Figure 1; J. Hnath and M. Wolgamood, Michigan Department of Natural Resources, Fish Health Laboratory, personal communication). Similar syndromes have been reported for Atlantic salmon Salmo salar from the Finger Lakes in New York State (termed Cayuga syndrome; Fisher et al. 1995) and for Atlantic salmon in the Baltic Sea (termed M74 for the year in which it was first reported; Johansson et al. 1995). In recent years, mounting losses of Baltic salmon offspring to M74 (Figure 2; H. Börjeson, Swedish Salmon Research Institute, personal communication) have paralleled those for coho salmon (Figure 1). These mortality syndromes are entirely confined to eggs collected from feral broodstock in all affected regions. Eggs derived from broodstock maintained on commercial feeds do not exhibit these mortalities.
A common connection among the mortality syndromes (EMS, Cayuga syndrome, and M74) is that eggs of affected stocks have very low thiamine levels and that sac fry mortality can be dramatically reduced by therapeutic thiamine treatments of eggs or sac fry. Until new information is forthcoming, we consider these three early life stage mortality syndromes to be synonymous in that they result from a thiamine deficiency. Nevertheless, there may be aspects of early life stage mortality that are specific to a species, and there may be more than one possible mechanism by which the thiamine deficiency develops.