Torrentfish

Description

The torrentfish (Cheimarrichthys fosteri), or panoko (Māori), is an amphidromous freshwater fish that is endemic to New Zealand. Torrentfish are well adapted to life in shallow, fast-flowing riffles and rapids. They grow to a maximum of 20 cm (7.9 in) in total length, but more commonly reach 10–12 cm (3.9–4.7 in). Torrentfish are stocky, with a flattened underside, arched back and a broad, downward-tapering head with eyes set high. The lower jaw is very undercut and is surrounded by a fleshy upper lip an adaptation for picking invertebrates off the surface of stones. The fins are very robust. The pectoral fins are very large and triangular, angled so that water flowing over them presses the fish against the riverbed, helping them to stay in position in fast-flowing water. The pelvic fins are set underneath the head. The dorsal and anal fins have very long bases, the dorsal fin extending for nearly half of the body length. The tail fin is short and truncated or slightly forked, suited to sudden burst swimming rather than sustained swimming. All torrentfish have a similar pattern of five dark stripes on a lighter background. There are three stripes on the body, generally angled downwards towards the head, with a fourth stripe vertically at the base of the tail and a fifth passing through the eyes and angling downwards towards the base of the pectoral fin. Their colouration is either dark grey on a lighter grey background, or dark brown on a lighter brown background, changing according to the surroundings of the fish. This colour pattern camouflages the torrentfish and help it to blend in with its stony habitat. Torrentfish are primarily found in shallow, fast-flowing riffles and rapids. They spend little time actively swimming against the rapids, living instead amongst and beneath loose gravels and cobbles. They emerge from the rapids at night to feed. Torrentfish are solitary and benthic, but may be found in high densities where there is a large population. Because part of their life cycle is spent in the sea, they are found in higher numbers near the coast. Torrentfish are mainly found in gravelly rivers, particularly braided rivers with wide, open channels. They favour rivers with highly unstable substrates, as the regular movement of the gravels maintains open gaps around and underneath the stones where the torrentfish can take refuge from fast water.