Selective tidal stream transport in the estuarine migration of glass eels of the American eel (Anguilla rostrata)

Abstract

During the spring of 1980 and 1981 systems of buoyed and anchored plankton nets were used on ten nights and at two stations in the Penobscot River (Maine, USA) to determine if and how selective tidal stream transport provided the mechanism for migration up an estuary by glass eels of the American eel (Anguilla rostrata). In stratified portions of the estuary glass eels remained on or near the bottom during the ebb tide and moved higher in the water column, but in or below the halocline, on the flood tide, thus effecting rapid landward transport. In tidal freshwater portions of the estuary glass eels behaved similarly on the ebb tide, and moved throughout the water column on the flood tide, again effecting rapid landward transport. The behaviour on the flood tide was adjusted to local hydrographic conditions rather than location within the estuary. Net upriver transports of glass eels of 0·8 to 4·2 km per tidal cycle were calculated, based on catch rates and velocity profiles, for eight of the nights. On one night following increased discharge there was a net down-river transport at the tidal freshwater station. Odour, turbulence, electric field detection, and circa-tidal clocks were considered as potential cues for timing the glass eels' sojourn in the water column and as cues for vertical positioning on the flood tide.