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I received a query from an Alaska Fly Fisher that went something like this: I have something that I wonder about. Where are the rainbow trout during winter? Where do they go in the fall, winter, and spring in the KenaiRiver?
This is a good question and a very important question for a management biologist because this is what “critical habitat” is all about and it must be protected. Unfortunately, this is also a very difficult question to answer because only limited research has been done on overwintering fish to provide a good answer. For me, however, the good news is that a colleague completed a research project on KenaiRiver rainbow trout about ten years ago. The study was designed to learn about seasonal movements of rainbow trout in the KenaiRiver. Doug Palmer surgically implanted radio transmitters in rainbow trout and tracked their movements for two years. Much of this Fish Talk is derived from his report from that study.
Transmitters were implanted in forty fish between Kenai and SkilakLakes and thirty-nine fish between SkilakLake and Naptowne Rapids. Subsequently, fish locations were documented by ground based and aerial surveys and from fixed, constant recording monitoring stations. Rainbow trout included in the study were 14 to 26 inches long. (Bruce King, formerly with Alaska Department of Fish and Game, reported that most spawning size rainbow trout in the KenaiRiver are 16 inches or longer.)
Over 90% of the tagged rainbow trout overwintered in either Kenai or SkilakLake. Of these, most overwintered in SkilakLake or its outlet. Only 9% of the fish in the study overwintered in the mainstem KenaiRiver. Movement to overwintering areas began in late September and was complete by late November or mid December.
Rainbow trout that overwintered in lakes made more extensive movements during winter than those which overwintered in the river. Average movements of the fish in the lakes during winter was 30 miles while those in the river generally stayed in the same 0.6 mile reach of the river all winter. I suspect that these differences in winter movements are related to feeding. The rainbow trout in the river did not need to move far to find food because the current is a conveyer belt for invertebrate drift. Rainbow trout in the lakes would have to move about to find food.
What is it about lakes that make them good overwintering locations for rainbow trout? The lakes provide a refuge from more rigorous conditions that typically occur in the mainstem of the river. The lake water is not moving so much less energy is needed for swimming and there is no danger of being caught off balance and getting swept downstream, especially during flooding outbursts.
Movements from overwintering areas to spawning and feeding areas occurred during May and June and the RussianRiver is an important spawning area for the upper KenaiRiver rainbow trout population. One survey by Alaska Department of Fish and Game reported over 500 rainbow trout spawning in the RussianRiver in late May. Other rainbow trout spawned in the mainstem and braided side channels of the KenaiRiver. In addition, this study, along with some other studies, suggested that the stocks of rainbow trout from above and below Skilak Lake exist as two separate semi-isolated stocks with little intermingling. One genetic study indicated that rainbow trout from below SkilakLake are more closely related to rainbow trout from Bristol Bay than to those from upper KenaiRiver
What specific habitat do the rainbow trout that overwinter in the river use? And for that matter, where do rainbow trout overwinter in rivers that do not have lakes? Like Willow Creek. And what about those fish smaller than 14 inches that were not included in this study? The tagging and tracking study was not designed to answer these questions, however I speculate that it is probable that at least some of the smaller sized rainbow trout in the KenaiRiver join the larger sized individuals.
A smattering of other studies offers additional information. One report summarized: in general, overwintering salmonids prefer sheltered, low velocity habitats. In other words, slow, deep holes, side channel sloughs or beaver ponds. Side channel sloughs are especially important for overwintering rainbow trout, especially in glacially influenced rivers such as the Susitna River, because the sloughs often have upwelling spring like conditions. Sloughs are slow moving and, in winter, upwelling water is relatively warmer than mainstem water. Many rainbow trout from streams such as Willow Creek migrate downstream to overwinter near the confluence with the SusitnaRiver or in the side channel sloughs.
Additional complexity of these habitats only make things better. Large rocks, large woody materials, and irregular shoreline are valuable. Young rainbow trout hover close to the bottom and between coarse cobble, particularly during the daytime. Some studies have reported that juvenile salmonids may burrow into loose bottom materials.
Oh, and by the way, did you notice that rainbow trout occupy wintering areas for about 7 or 8 months of the year? Although fish require less energy in the cold water, this underscores the extreme importance of summer feeding and fat accumulation and, therefore, the extreme value of salmon eggs and carcasses as a food and winter energy source in late summer.
The rigor of the winter season is usually the primary cause of death of young salmonids particularly during their first year. One report found only 0 to 23% overwintering survival for juvenile rainbow trout. Summaries of overwintering survival rates for other species include: for brown trout, 15 to 84%; brook trout, 35 to 75%; Atlantic salmon, 43 to 75%; and, coho salmon, 16 to 84%.
Where are the rainbow trout during winter? Where do they go in the fall, winter, and spring in the KenaiRiver? The short answer is lakes. The slightly more specific answer is that most rainbow trout overwinter in lakes and others find a place of low velocity water with complex cover.