Migration of Silver American Eels through a Macrotidal Estuary and Bay
Rodney G. Bradford, Jonathan W. Carr, Fred H. Page, and Fred Whoriskey
Abstract.—The horizontal and vertical movements of 20 silver American eels Anguilla rostrata migrating through a macrotidal estuary and bay were tracked with hydroacoustic telemetry. Eels initiated their seaward migrations around or shortly after sunset, mostly migrated at night, exhibited no bias for migration on ebb versus flood tides, and swam both with and against tidal currents with little preference for depth. Profiles of eel vertical distributions during transit through the estuary, the bay, and the tidal passages that connected the bay to the open sea differed and differed from analogous profiles for silver European eels A. anguilla transiting the Baltic Sea where tidal forcing is negligible. The underlying mechanism(s) associated with their direction of net displacement appears to be innate, although local circulation features may influence transit speeds and exit routes to the open sea. Our results in combination with published information for both the silver American and the European eel suggest that the horizontal migration speed for both species is relatively invariant, around 0.5 body lengths per second. The rate of ascent/descent of eels exhibiting a change in swimming depth of greater than 5 m could not be estimated with precision from the available records of eel swimming depths. The observed average rate of 0.11 m/s was, however, within the relatively narrow range of the average vertical swimming speeds (0.11–0.18 m/s) reported in the literature for silver eels executing vertical dives.