Habitat Use of American Eels in the Saint Lawrence River Inferred from Multi-Element Otolith Line Scans

Although considerable evidence has demonstrated that catadromy is a facultative behavior for the American eel, this question has yet to be investigated within a large hydrographic system. In an attempt to retrace habitat use retrospectively, LA ICP-MS was used to obtain multi-element line scans from the otoliths of 110 yellow-stage and silver eels sampled at various locations throughout the vast Saint Lawrence River Lake Ontario (SLRLO) system. Elemental profiles for ⁸⁸Sr, ¹³⁸Ba, ⁵⁵Mn and ²⁴Mg enabled us to quantitatively distinguish three chemical signatures that might correspond to distinct habitats within the SLRLO. Elevated strontium and low barium levels suggest that one of these signatures corresponds to brackish estuarine habitats while the other two represent habitats within freshwater. Analysis of dissolved element concentrations from water samples supports the interpretation that the latter two correspond to tributary and main stem river signatures. Most (78%) of the observed switches between these three habitats occurred within the first four years after the elver check, providing evidence that eels are more likely to settle in one habitat as they grow older. The patterns of habitat use and movements inferred from this study have important implications for the management and conservation of the species.