American eel

DepthBenthic and shelter-oriented by day; emerges from cover and may move shallower at night to feed.

Moving waterSmaller eels can use stronger current, while larger yellow eels more often occupy slower deep water, cover, soft bottom, and margins.

LightStrongly nocturnal; daylight usually favors snags, plants, burrows, tubes, depth, and other shelter.

Seasonal activityStrongly temperature-gated. ASMFC: activity decreases below 10°C, and at about 5°C feeding ceases and dormancy can persist for months. USFWS records yellow eels overwintering in mud bottoms in both freshwater and estuary habitats.

Temperature-driven behaviourASMFC gives movement onset around 14–20°C with activity peaking above 20°C, decreasing activity below 10°C, and feeding ceasing near 5°C. The catalog band of 58–68°F (about 14–20°C) sits inside the active range. USFWS gives a 10–12°C (50–54°F) minimum for elver upstream movement and a roughly 18°C threshold for yellow-eel upstream movement in Maine.

Depth movementYellow eels are benthic and hold on or in the bottom. In the marine phase, silver eels perform diel vertical migration — swimming within 100 m of the surface at night and descending toward 400 m in daylight (USFWS, from tagged European silver eels).

Change in flowUSFWS notes that discharge influences elver upstream movement rates, and describes the seaward silver-eel migration as occurring in spurts, with long periods of no movement punctuated by peaks of intensive movement.

CoverUSFWS: primarily benthic, using substrate — rock, sand, mud — plus large wood and submerged vegetation for cover. Cover use is size-specific: smaller eels associate with cobble and gravel, larger eels with larger cobbles, boulders and sand. ASMFC adds weedy bottoms, detritus and shell substrate, and notes riparian vegetation and complex substrate as important cover in impounded systems.

Open waterUSFWS calls American eel the fish occupying the broadest array of habitats in the world — open ocean, coastal shelf, estuary, river, small stream, lake and pond — and describes yellow eels in fresh water as habitat generalists with little consistent preference for habitat type. Growth is roughly twofold higher in brackish water than in fresh water.

Predator avoidanceASMFC: eels burrow into sediment and shelter within aquatic macrophytes to avoid predators. Reduced movement around the full moon is read the same way. USFWS notes elvers may burrow during the day between upstream movements.

Spawning (conservation context)Catadromous and believed semelparous. Spawning occurs in the Sargasso Sea in winter and spring, and adults are believed to spawn once and die (ASMFC, USFWS). Maturing silver eels leave continental waters and move seaward; that migration, and any structure that constricts it, is conservation context and is excluded from target guidance. Larval leptocephali drift back on ocean currents, transform to glass eels at the coast, and enter continental waters as elvers.

ASMFC's habitat chapter reports a freshwater study in which 43% of stomachs contained insects, 26% fish and crayfish, and 20% gastropods, with mayflies, stoneflies and caddisflies dominating the invertebrate fraction. In brackish water the diet shifts to blue crabs, bivalves and alewife. USFWS gives benthic invertebrates for smaller eels and mussels, fish and other eels for larger ones, plus plant material and carrion. This is dietary capacity, not evidence of a current forage event.

Feeding style

Opportunistic

Feeding zone

Benthic

Diet capacity is not proof that a hatch or prey event is occurring. Observed current forage still comes from a user observation or Hatch Match.