Fish of the subfamily Coregoninae (family Salmonidae) are commonly known as whitefish. The center of whitefish diversity is in the North. While California has only a single species of native coregonid, the Kobuk River is home to at least six species (Morow 1980). The largest member of the family, the sheefish (Stenodus leucichithys), is found in three river systems in Alaska, the Yukon, the Kuskokwim and the Kobuk-Selawik. The range of the species is circumpolar, yet in Alaska it is not found north of the Kobuk River (Morrow 1980). Sheefish in the Yukon and Kuskokwim Rivers exist as both anadromous and resident populations (Alt 1988). In the Kobuk and Selawik Rivers, however, the entire stock is anadromous, migrating from estuarine winter-feeding grounds in Kotzebue Sound, Hotham Inlet and Selawik Lake, 450 km up the Kobuk and 20 km up the Selawik to spawn (Alt 1988).
Longtime Alaska Department of Fish and Game sheefish researcher Kenneth Alt speculates that lack of suitable spawning habitat limits sheefish abundance and distribution (Alt 1988). Only 12 spawning locations have been recorded in Alaska. Spawning occurs in clear swift streams, in water 1.2 to 2.3 meters deep over a substrate of differing sized gravels and sand (Alt 1988). Current speeds of 0.9 to 2.7 m/s, and mixed gravels of 10 to 100 mm diameter are common to all sheefish spawning sites (Alt 1988).
The mixed-size substrate can be seen in this video clip of sheefish agragating on their Kobuk River spawning grounds directly upstream from the confluence with the Pah River.
As river clarity in the Upper Kobuk is great (visibility exceeding 4 meters through out the spawning grounds) aggregations of sheefish can be visually located reliably. Between our first observations of sheefish 15 river miles below the Beaver River to downstream of the Mauneluk River sheefish were reliably found in large schools. Single fish were a rarity with almost all sightings of sheefish occurring in schools of 20 –300 fish. Riffles 1-3 meters deep at the head of pools held fish but sheefish were most often observed in the tail-outs of large pool/glide complexes, in water 0.5 to 2 m deep. Often the substrate in these areas was characterized by small sand dunes regularly interspersed with gravel bands. Such distinctive alternating bands of differing substrate look like “waves” on the river bottom to observers passing over in rafts.
Here is a video of sheefish over this distinctive substrate:
These dune formations are dominated by sand and the gravel present is relatively uniform and small. Such facts differ markedly from published spawning habitat criteria. As our trip took place before actual spawning begins, we did not observe spawning fish. Spawning takes place in mid September through October when water temperatures are between 1 and 4.6˚ C, often occurring just as the rivers begin to freeze (Morrow 1980, Alt 1988). It seems likely that prior to spawning fish may hold in river sections characterized by these dunes, drifting back into the higher gradient riffles with larger substrate at the time of spawning. On our trip sighting of these distinctive dune “waves” meant that we would soon be pushing schools of magnificent sheefish 2 to 3 feet long ahead of the rafts, the schools splitting to let us pass and then reforming as soon as we were down stream.
I leave you with an video of sand migration up the dune faces. This is what a sheefish sees all day long. I recommend watching it in slow motion.
Referenced Sources:
Alt, K.T. 1988. Biology and Management of Inconnu (Stenodus leucichthys) in Alaska. Finnish Fisheries Research 9. p 127-132.
Morrow, J.E. 1980. Freshwater Fishes of Alaska. Alaska Northwest Publishing Company.