The Sheefish is easily identifiable by its white or silvery coloration without spots or other markings. One notable feature is its large scales, which contribute to its distinct appearance. Another distinguishing characteristic is its jaw structure, with the lower jaw extending beyond the upper jaw. Within its mouth, one can observe an abundance of small densely-packed teeth.
In terms of size, Sheefish can grow to impressive proportions. In the Selawik-Kobuk area of Alaska, these magnificent fish can weigh up to 27 kilograms (~60 lbs) and reach lengths of over 1 meter (~42 inches). However, it's important to note that in different regions, such as the interior rivers of Alaska, Sheefish tend to be smaller in size.
Growth and Reproduction: The life cycle of Sheefish begins in early spring when young fish hatch before the winter ice breaks up. These juvenile Sheefish are carried downstream by rapidly flowing spring-melt water. They find shelter and ample food sources in backwater eddies along the river, off-channel lakes, and estuary regions at river mouths.
Sheefish exhibit rapid growth. For example, on the Kuskokwim River, Sheefish can reach a length of 406 millimeters (16 inches) by the age of 2 and 762 millimeters (30 inches) by the age of 8. During their early stages, juvenile Sheefish primarily feed on insects and other small prey. However, as they mature, their diet shifts to predominantly consist of other fish.
The age at which Sheefish reach sexual maturity varies depending on their location. In the Mackenzie River in Canada, they may mature as early as age 6, while males in Alaska streams typically mature between 6 and 9 years. Females, on the other hand, reach maturity between 7 and 12 years.
Feeding Ecology: Sheefish display a diverse feeding ecology throughout different stages of their lives. During the winter and spring months, Sheefish captured under the ice near Kotzebue have stomachs filled with herring. As the spring ice breaks up, they move to estuaries, particularly those of the Yukon and Kuskokwim Rivers, where they feed heavily on oil-rich smelt. Following this feeding period, many Sheefish migrate upriver to the mouths of major tributaries, indulging in the abundance of outmigrating juvenile fish species.
It's important to note that the feeding behavior of Sheefish can vary between populations and even within individual fish. Some Sheefish exhibit site fidelity, returning to the same spring/summer feeding areas year after year. In contrast, others explore different tributary mouths or visit multiple locations throughout the summer.
Reproduction and Spawning: Sheefish embark on extensive journeys to reach their spawning areas, which can take up to two months. Interestingly, they tend to return to the same area where they themselves hatched. During the spawning process, Sheefish behave as "broadcast spawners," releasing their eggs and milt into the water column instead of digging nests. The fertilized eggs drift downstream, eventually sinking and becoming lodged in the gravel.
The spawning season for Sheefish occurs from late September to early October. They prefer to spawn in shallow waters with temperatures ranging from approximately 0.5° C (33° F) to 5.5° C (42° F). Within one to two weeks after spawning, Sheefish typically leave the spawning area. It's worth noting that most Sheefish spawn only every two or more years, with females often skipping a spawning season due to the energy-intensive process of egg production.
Migration: Sheefish are known for their remarkable long-distance migrations. Some individuals travel over 1,600 kilometers (~1,000 miles) within a single summer. In major Alaskan river drainages, such as the Kuskokwim, Yukon, Selawik, and Kobuk rivers, Sheefish overwinter in brackish waters of the bays. With the arrival of spring, many Sheefish migrate upriver to feed, while others migrate later in the summer to spawning areas. Interestingly, there are also resident Sheefish that remain in freshwater throughout their entire lives, never venturing into the bays.
Range and Habitat: Sheefish inhabit the large northern rivers and associated lakes of northwestern North America, as well as the drainages of northern Asia up to the White Sea. There is also an isolated population found in the northern Caspian Sea and its inflowing rivers. These magnificent fish are often found in glacial, silt-laden rivers, and they can spawn in both clear tributaries and those with higher sediment content.
Here are some quick facts about the Sheefish: