All about winter survival

_{By STEVE TRAFTON
Henry’s Fork Foundation Director}

Winter habitat in the caldera section of the Henry’s Fork is critically important to the survival of juvenile rainbow trout – and the health of the trout fishery as a whole, so can we increase the amount of winter habitat available to juvenile trout?

Adequate winter flows below Island Park Dam are the most important habitat “fix” available to us. The improved flows of the past five winters and the improved trout numbers below the dam are not coincidence. Water is a commodity in short supply in the West, and if there are ways to improve trout habitat in the caldera at a scale that can be measured in trout populationlevel terms (improved survival of entire “classes,” or year groups of wild trout), those methods bear investigation. Jim Gregory, a fisheries biologist with a long history of work in the caldera and the author of the Henry’s Fork Foundation’s (HFF) recent caldera assessment report (go to www.henrysfork.org for more details) discussed the possibility of habitat in waters off the main Henry’s Fork thus:

Besides the Buffalo River, five perennial tributaries enter the Henry’s Fork between Island Park Dam and Riverside Campground — Blue Springs Creek, Antelope Park Creek, Big Bend Creek, Thurmon Creek, and Fish Creek. While assessment of each of these streams has been minimal, they may have the highest probability of success for significantly increasing juvenile trout winter survival in the caldera section of the Henry’s Fork.

Blue Springs Creek originates east of Highway 20 and flows into the Henry’s Fork at Last Chance. It is small (11 cubic feet of water – or cfs – per second measured in July 1997) and spring-fed and remains without ice through much of the winter. No research has been conducted on this stream during the winter, and a summer habitat assessment indicated that much of the stream substrate consisted of silt or embedded cobbles. During the summer of 2006 the HFF conducted some additional assessment of the stream and found some juvenile rainbow trout and a few juvenile brook trout but minimal winter habitat. Therefore, concealment habitat is likely limited in this stream. The extent to which juvenile trout use this stream during winter is unknown and the thermal benefit it may provide has not been evaluated.

Antelope Park Creek and Big Bend Creek are small, highly degraded streams that flow through the Railroad Ranch and flow into the Henry’s Fork on the east side just upstream of Big Bend. While they are small enough that they may not provide substantial amounts of habitat, they are also small enough that they likely “snow bridge” (are completely covered by a layer of suspended snow), which may serve to keep the water temperature relatively high through the winter and may also make concealment habitat less critical for juvenile rainbow trout. Incidental observations indicate that a substantial number of age-0 (juveniles in their first year) whitefish currently spend at least part of the winter in these streams. It is unknown how many, if any, juvenile trout use them for winter habitat.

Thurmon Creek is also a spring-fed stream (34 cfs in July 1997) that enters the Henry’s Fork from the west, upstream of Osborne Bridge. This stream originates at springs at the heads of East, Middle, and West Thurmon creeks on the Targhee National Forest, all of which flow into Golden Lake. Thurmon Creek flows over the dam at Golden Lake, through the Railroad Ranch, through Silver Lake, and into the Henry’s Fork. The only portion of this stream accessible to fish from the Henry’s Fork is that section of stream between the Silver Lake Dam and the Henry’s Fork, a stream reach of approximately 980 ft (300 m) in length. An estimated 2,000 juvenile rainbow trout, along with a comparable number of juvenile whitefish, were discovered in this section of stream during the winter of 2000 – 2001, after a single juvenile trout implanted with a radio tag (part of study by the HFF) at Last Chance was located there.

This short section of stream contains little habitat that would typically be thought of as winter concealment habitat. However, most of the fish discovered in this section were located downstream of a bottom-release pipe that was leaking 39°F (4° C) water and upstream from where that channel met the spillway channel, which was passing 33° F (0.5° C) water. Fish within this section, where concealment habitat did not exist, were selecting a thermal advantage over concealment habitat that was available in other areas.

Fish Pond Creek is similar to Thurmon Creek in that its waters flow from a man-made pond, Fish Pond, into the Henry’s Fork. However, it has a lower discharge than Thurmon Creek, the section of stream between the dam and the Henrys Fork is longer, and the dam may be passable to fish. Trout were not observed in this stream by an electrofishing crew during the Winter (1998) but many trout of various sizes were observed in mid-July of that same year. Biologists speculated that these fish may move seasonally between the pond and the stream downstream from the dam.

Further evaluation of each of these streams is needed to assess their benefit, or potential benefit, to wintering fish. In all the spring-fed tributaries, it may be possible to increase over-winter habitat through standard habitat improvement techniques. Additionally, providing passage over Silver Lake Dam may provide a large block of winter habitat (Silver Lake) for juvenile trout. The benefits that may be realized by these projects are unknown but may be similar to those expected from the Buffalo River.

However, the mechanism may be through juvenile trout migrating upstream during early winter and returning sometime after late winter, whereas the Buffalo River project provides spawning and rearing habitat in addition to favorable winter habitat.

This is part of the online edition of Henry's Fork Country.

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