Efficacy of Thiamine, Astaxanthin, β-Carotene, and Thyroxine Treatments in Reducing Early Mortality Syndrome in Lake Michigan Salmonid Embryos
M. W. Hornung, L. Miller, R. E. Peterson, S. Marcquenski, and S. B. Brown
Abstract.—Lake Michigan Skamania strain steelhead Oncorhynchus mykiss and coho salmon O. kisutch fry exhibit an early mortality syndrome (EMS) in which death is preceded by loss of equilibrium, inability or lack of feeding, and general lethargy. It was hypothesized that decreased egg concentrations of carotenoid pigments, thyroxine, or thiamine contributed to this syndrome. Thiamine analyses of Lake Michigan coho salmon eggs from individual family groups exhibited 16–97% EMS if egg total thiamine concentrations were less than 0.9 nmol/g or egg free thiamine concentrations were less than 0.3 nmol/g. In eggs with total or free thiamine concentrations greater than 0.9 or 0.3 nmol/g, respectively, the range of EMS in fry was 5–12%. Immersion of steelhead and coho salmon eggs in 1.4 mM or greater thiamine hydrochloride significantly decreased EMS compared with controls. Immersion of coho salmon eggs in 2.8 mM thiamine increased the mean concentration of free thiamine in the eggs to 1.0 nmol/g, compared with 0.26 nmol/g in controls. Injection of either β-carotene or astaxanthin (0.86 or 8.6 µg/g, respectively) in steelhead eggs did not significantly reduce the occurrence of EMS. Early mortality syndrome was not decreased in steelhead after immersion of eggs in 2 mg/L thyroxine, but it was significantly decreased in steelhead sac fry immersed in 2 mg/L thyroxine. These results suggest that low egg thiamine is a predisposing factor for EMS; however, other factors that are variable among individual coho salmon females may influence the occurrence of EMS-related mortality. Whether the addition of exogenous thiamine corrects a thiamine deficiency or protects fry from developing EMS through some other mechanism is currently unknown.