Pygmy whitefish

Content sourced from Wikipedia, licensed under CC BY-SA 3.0.

The pygmy whitefish (Prosopium coulterii) is a small freshwater fish in the salmon family. It lives in mountain streams and lakes of western North America, with isolated populations in Lake Superior and in Ekityki Lake on the Chukchi Peninsula.

Description
The pygmy whitefish is small and herring-like, with a long, cylindrical body. It is silvery, with a brown back and whitish belly. Young fish often show dark side markings similar to parr marks seen on young trout; some adults in Alaska also have them. Size ranges from about 3 to 15.3 cm, usually around 12 cm, and it can reach up to 28 cm. It has a large eye, a small snout, and fins that are generally clear. Its tail is clear with a faint dark spot, while the anal and pelvic fins are plain white. It has relatively large scales (56–70 along the lateral line) and fewer gill rakers, which helps distinguish it from other Prosopium fishes. It is considered the most trout-like member of its genus.

Taxonomy
The pygmy whitefish was first described in 1892 as Coregonus coulteri and named after botanist John Merle Coulter. It later moved to the genus Prosopium. The genus name means “mask” in Greek, referring to large bones in front of the eyes. Prosopium fishes have a single flap of skin over the nostril, parr marks, a round body, and a small toothless mouth.

Habitat and range
The species is mainly found in the northern Rocky Mountains, with three separate populations elsewhere: Lake Superior, southwestern Alaska, and Ekityki Lake in the Chukchi Peninsula, Russia. Its historical range may have been continuous until the late Pleistocene. In the Rockies it lives in cold, fast streams and cool deep lakes (more than 6 meters deep). In Lake Superior it prefers cooler water at depths of 18 to 89 meters.

Spawning
In Lake Superior, spawning occurs in November and December at depths of 31 to 46 meters, with females laying about 362 orange eggs about 2.57 mm in diameter. In other areas, pygmy whitefish migrate upstream to spawn, usually in November or December in gravelly streams.

Diet and predators
Pygmy whitefish feed mainly on aquatic insect larvae and crustaceans. In Lake Superior, the diet is dominated by amphipods of the genus Pontoporeia and other crustaceans, especially ostracods, making up about 77% of their food. Predators include burbot, kingfishers, terns, and pikes.