Eel

Eels are elongated fish, ranging in length from 5 cm (2 in) in the one-jawed eel (Monognathus ahlstromi) to 4 m (13 ft) in the slender giant moray. Adults range in weight from 30 g (1 oz) to well over 25 kg (55 lb). They possess no pelvic fins, and many species also lack pectoral fins. The dorsal and anal fins are fused with the caudal fin, forming a single ribbon running along much of the length of the animal. Eels swim by generating propulsive waves that travel the length of their bodies. They can swim backward by reversing the direction of the wave.

Eels begin life as flat and transparent larvae, called leptocephali. Eel larvae drift in the sea's surface waters, feeding on marine snow, small particles that float in the water. Eel larvae then metamorphose into glass eels and become elvers before finally seeking out their juvenile and adult habitats. Some individuals of anguillid elvers remain in brackish and marine areas close to coastlines, but most of them enter freshwater where they travel upstream and are forced to climb up obstructions, such as weirs, dam walls, and natural waterfalls.

Gertrude Elizabeth Blood found that the eel fisheries at Ballisodare were greatly improved by the hanging of loosely plaited grass ladders over barriers, enabling elvers to ascend more easily.