Why Did the Eel Cross the Creek: Understanding American Eel Behavior in an Estuarine Setting

American eels (Anguilla rostrata) serve key roles in the ecological and economic health of the mid-Atlantic region.  Resource managers facing a combination of declining landings and changing environmental regimes are in need of information on the factors influencing American eel movement.  In the spring of 2009, a combined mark-recapture and biotelemetry study was initiated in the St. Jones River, Delaware.  Monthly mark-recapture events took place using fixed locations (n = 40) stratified by commercial fishing practices (i.e. intense, occasional, and rarely harvested).  Transmitters were randomly assigned to each strata and were surgically implanted in an American eel (>305 mm).  Telemetered American eels were allowed to recover before released at initial site of capture, and were monitored using a combination of active and passive telemetry.  Using data from telemetered eels, we conducted home range analysis and used general linear mixed models (GLMMs) to assess the importance of covariates in determining eel movement rates.  The vast majority (98/102) of telemetered individuals were detected at least once over the course of the study (May 2009 through January 2011).  During the summer months, site fidelity was very high for the majority of detected eels which remained at the initial tagging locations.  In the late fall, when water temperatures began to decline, detected American eels moved to higher saline waters of the Delaware Bay, possibly for thermal refugia.  Despite increased movement in the fall, average home range estimates remained low (5.98 ha) over all seasons.  In addition to season, modeling efforts indicated that eel movement was also dependent on water temperature, tide, turbidity, diel periods, and size of the animal; there was also evidence for considerable heterogeneity of movement rates between eels of similar size class.   Insights from this study have improved our understanding of movement ecology of American eels in the mid-Atlantic region, and should allow for better management and conservation decisions.