Swimming ability of eels (Anguilla rostrata, Conger oceanicus) at estuarine ingress: contrasting patterns of cross-shelf transport?

The transport of eel early life stages may be critical to their population dynamics. This transport from ocean spawning to freshwater, estuarine and coastal nursery areas is a combination of physical and biological processes (including swimming behavior). In New Jersey, USA, the American eel (Anguilla rostrata) enters estuaries as glass eels (48.7-68.1 mm TL) in contrast to the Conger eel (Conger oceanicus) that enters as larger (metamorphosing) leptocephali (68.3-117.8 mm TL). To begin to understand the mechanisms of cross-shelf transport for these species, we measured the potential swimming capability (critical swimming speed, U-crit) under ambient conditions throughout the ingress season. A. rostrata glass eels were collected over many months (January-June) at a range of temperatures (4-21 degrees C), with relative condition declining over the course of the ingress period as temperatures warmed. C. oceanicus occurred later in the season (April-June) and at warmer temperatures (14-24.5 degrees C). Mean U-crit values for A. rostrata (11.7-13.3 cm s(-1)) and C. oceanicus (14.7-18.6 cm s(-1)) were comparable, but variable, with portions of the variability explained by water temperature, relative condition, ontogenetic stage, and fish length. Travel times to Little Egg Inlet, New Jersey, estimated using 50% U-crit values, indicate it would take A. rostrata similar to 30 and similar to 60 days to swim from the shelf edge and Gulf Stream, respectively. Travel times for C. oceanicus were shorter, similar to 20 days from the shelf edge, and similar to 45 days from the Gulf Stream. Despite differences in life stage, our results indicate both species are competent swimmers, and suggest they are capable of swimming from the Gulf Stream and/or edge of the continental shelf to estuarine inlets.