However, it is possible that such changes may affect long term fitness. In several instances, we observed degradation of the internal organs and musculature of the gut cavity in the fish that died. In addition, we observed external lesions on,5 fish. In the latter individuals,Dasatinib the body wall was apparently ‘‘dissolved’’. This is consistent with observations in other vertebrates that have shown tissue degradation in the gut. We speculate that the degradation was caused by the formation of sulphurous acid. Sulphurous acid is formed by the equilibrium reaction of sulpher dioxide and water. Taken together, these results suggest that the fish may have died from multiple causes, including tissue breakdown, neurotoxicity, inhibition of enzyme activity, or oxidative stress. Within the body, sulfite is oxidized to the sulfate ion by sulfite oxidase. We hypothesize that the variability in susceptibility among species and lifestages was caused, in part, by differences in the expression of this enzyme. Likewise, up regulation of sulfite oxidase may explain the decrease in mortality after,EX 527 exposure. Interestingly, in some instances we observed mortality after a single feeding of the cured eggs. Of these, 4 fish had a single egg in their gut and 13 had between 2–5 eggs in their guts. This suggests that some fish are particularly sensitive to the negative effects of some cures. Furthermore, it is not unrealistic to expect that wild juveniles would consume 1–5 eggs one or more times during their residence in freshwater. In addition to sodium sulfite, some cures may also contain other sulfites such as sodium metabisulfite or sodium bisulfite. The available toxicity data suggests that other forms of sulfite may be equally as toxic to fish. For example, the average LC50 for sodium sulfite is 660 mg/l in the western mosquitofish whereas the average LC50 for sodium bisulfite is 240 mg/L. We were not able to find any data for salmonids. Interestingly, sodium nitrite, another ingredient in some cures, appears to be more toxic than either of the sulfites. Given the available data, we would urge caution in using any form of sulfite or nitrite without further testing to determine the effects on juvenile fish. Because the effect appears to be due to the breakdown of chemicals contained within the cured eggs while in the gut, a number of factors may influence the effect in the wild.