The Effect of Acidification on the Survival of American Eel
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The geographic range of the panmictic American eel (Anguilla rostrata) has contracted in recent years because of the pronounced decline in recruitment of glass eels and elvers to the Laurentian Basin. In consequence, the American eel was assessed in 2006 as a species of special concern by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC). The evident sensitivity of American eel status to elver recruitment highlights the importance of understanding both the mechanisms contributing to the delivery of glass eels from the Sargasso Sea to continental waters and mortality following their recruitment to coastal and inland waterways. The potential for variability in environmental quality at localized geographic scales to affect American eel productivity and hence the status of the species and the fisheries it supports is not fully understood. The Atlantic coast of Nova Scotia is an ideal location to examine the relationship between water quality and American eel productivity. Within Nova Scotia there is wide natural variation in freshwater pH, which has been further increased in recent years by the effects of acid precipitation. This variation occurs over a small geographic range of several hundred kilometres that overlaps an area of high elver influx. As low environmental pH is known to adversely affect aquatic ecosystems, it has been identified as a possible threat to elver survival. In this study, the effect of low pH on elver survival was examined in both laboratory and field based trials using wild glass eel/elvers that were captured upon entry to fresh water. Trials examined the mortality rate of elvers at pH levels within the range of 4.0 - 7.0 over a 10 day period. The relationship between elver development and mortality at low pH was also examined through pigmentation analysis. Laboratory and field based studies resulted in zero mortality among elvers in natural and artificial acidic environments with pH levels as low as 4.0., thus indicating that the American eel is fully acid tolerant upon initial migration into fresh water. Sub-lethal effects of acidification were explored by examining the hematological parameters of river resident yellow-phase American eels exposed to varying levels of acidity in the laboratory. The level of acidification proved not to be a factor in determining both hematocrit and blood plasma osmolarity levels, as there were no significant differences in these variables between eels exposed to acidic conditions and those exposed to control neutral pH conditions. These results suggest that through the use of a highly effective mechanism for regulating blood ion concentration, the eel is able to tolerate low pH conditions.