_{By JIM DE RITO, Henry’s Fork Foundation Conservation Director}

A couple weeks ago in this column, Steve Trafton introduced the Caldera Project, an effort led by Henry’s Fork Foundation to address concerns about the Island Park Caldera’s fishery.

This project was launched primarily because anglers have repeatedly said the number of rainbow trout has declined in Harriman State Park over the years, specifically in the Ranch section. One of the Caldera Project’s components is to work with the Idaho Department of Fish and Game (IDFG) to estimate how many rainbow trout are on the Ranch.

IDFG uses electrofishing equipment to estimate the number of trout in the Henry’s Fork. This equipment consists of a drift boat with a generator that produces electricity, a unit that controls the output voltage and frequency of the electricity, and two bow-mounted booms that suspend electrodes (metal rings with steel cable droppers) into the water. The electric current in the water temporarily causes the fish to swim toward the electrodes. Electric current only affects a limited area around the electrodes and only a small percentage of the fish in the river are affected. Trout are captured as the two electrofishing boats move downstream, typically one boat along each bank of the river. Fish are netted out of the water by a person in the bow of the boat and placed into a holding tank.

All trout captured over 6 inches are marked with a hole punch on the tail and then released back to the river. A “recapture run” is then made after the fish have redistributed in the river section, typically about a week after the fish marking. The same electrofishing methods are used as the marking run, but this time it is noted if any captured fish have a hole punch on their tails, signifying a recaptured fish. The number of marked fish relative to the number of unmarked fish (previously uncaptured) is used to estimate the total number of fish in the river.

This mark-and-recapture methodology is used on the Henry’s Fork in various sections typically at least three miles in length, to look at trends in fish numbers, among other characteristics. In the caldera, this method is annually repeated in the Box Canyon and the estimate of rainbow trout numbers is used as an indicator of the population in the caldera. The estimated number of rainbow trout in the Box Canyon since 1994 has fluctuated between 1,638 per mile (2003) to 5,583 per mile (2000), with an average of a little fewer than 3,000 fish per mile. The estimate of rainbow trout this year was 2,525 per mile in the Box Canyon.

Because the Henry’s Fork in the Box Canyon is immediately below Island Park Dam, and is also deeper and narrower than the river downstream through Pinehaven, an extrapolation of fish numbers per mile from the Box to the remainder of the river would not be accurate. In addition, the IDFG has generally not attempted to directly estimate fish numbers in the Henry’s Fork downstream of the Box, because the wider and shallower river make it difficult to obtain accurate estimates using electrofishing gear. Therefore, the fish number estimates in the Box have generally thought to be representative of trends throughout the remainder of the caldera without having specific estimates for other sections of the Henry’s Fork. However, reports from anglers that there were few fish in the Ranch and HFF’s development of the Caldera Project led IDFG to conduct a rainbow trout estimate this year on the Ranch

IDFG completed the mark-and-recapture of rainbow trout from the upstream Harriman State Park boundary to Osborne Bridge in mid May. The estimate was 383 rainbow trout per mile, much lower than the Box Canyon estimate or about 15% of the per mile number in the Box this year. Prior to this year, 441 rainbow trout per mile for the Ranch was estimated in 1994, or about 11% of the Box Canyon estimate for that same year.

These low estimates of rainbow trout on the Ranch confirm what anglers have been saying for many years, that there are relatively few fish there. In addition, this most recent estimate of rainbow trout numbers on the Ranch gives HFF, IDFG, and others a baseline of current conditions and a measure against which future research and restoration work can be compared. The recently completed “Response to Frequently Asked Questions about the Henry’s Fork Caldera Fishery” points to several additional research and restoration steps, beyond the current focus on winter flows and improved fish passage on the Buffalo River, that may be taken to improve fish habitat, and thus fish numbers on the Ranch.

Given the increased focus of all those involved in the Caldera Project, we hope to see the tangible results of these efforts manifested in greater numbers of rainbow trout on the Ranch in coming years.

For more information on rainbow trout numbers in the caldera, visit the Caldera Project link at the bottom of the HFF Web site home page - henrysfork. org and navigate to the “Literature” section for pdfs of the FAQs document noted above and its companion Caldera Project brochure. Also, please visit the link to the HFF blog site if you’d like to post a question on fish numbers in the caldera.