Utah Division of Wildlife Resources

A strategy for Scofield

SCOFIELD RESERVOIR is one of the most important flat-water fisheries in Utah. When the Division of Wildlife Resources (DWR) surveys anglers every five years, Scofield consistently ranks as the second- or third-most-utilized reservoir fishery in the state. Recently, however, the reservoir has experienced some challenges that have reduced the number of anglers who fish there.

This photo was taken from the north side of Scofield Reservoir in 2006.

In 2007, the Bureau of Reclamation (which owns the dam) was forced to lower the reservoir’s water level to perform maintenance on the spillway structure. The construction lasted for over a year, and the low water interfered with angler access to some degree. Unfortunately, rumors were widespread at this time that the reservoir was so low that it was unfishable from a boat. This was never true.

What appeared to happen, though, was that the combination of low water and a longer-than-usual ice cap in the winter of 2008–2009 caused oxygen depletion and a fish kill. No dead fish were observed at Scofield itself, but there was a fish kill in Lower Fish Creek (below the reservoir) the following spring. That fish kill was attributed to a release of low-oxygen water from the reservoir.

Fishing was also slower than normal in 2009, and the rainbow trout were much smaller than the typical Scofield Reservoir “footballs.” The good news is that stocking in 2009 and 2010 seems to have rectified this problem, and Scofield is once again producing the healthy fish it has always been known for.

Low water and fish kills are events that can be expected periodically in many reservoirs throughout the West. Fortunately, they can be remedied fairly quickly through an effective stocking program. The bigger challenge to the overall fishery at Scofield was the discovery of Utah chubs in the reservoir in 2005.

A tiger trout and three rainbow trout at a Scofield fish-cleaning station in May 2007.

Utah chubs (Gila atraria) are a member of the minnow family native to the Great Basin. They are not native to the Colorado River drainage where Scofield is located. It is likely that someone illegally introduced the chubs into the reservoir by using live chubs as bait.

After discovering the chubs in 2005, the DWR responded immediately. Our fisheries personnel stocked approximately 105,000 tiger trout fingerlings that same year. Biologists hoped the aggressive tiger trout would be able to control the chubs through predation (similar to the way Bear Lake cutthroat trout have helped control chubs in Strawberry Reservoir).

A follow-up survey in 2006 showed low numbers of chubs and good growth of the stocked tiger trout — it appeared the plan was going to work. Then, in 2007, Scofield’s chub population skyrocketed. It continued to increase in 2008 and 2009, despite the stocking of additional tiger trout.

Many anglers became concerned about the drastic increase in chubs. In 2009, fishery managers took an additional step to address the problem. They introduced Bear Lake cutthroat trout, hoping the cutts would also prey on the chubs. That same year, a slot limit on tiger trout and cutthroat trout was implemented, with the hope that it would help protect the larger, more predacious fish.

Has it worked? We don’t know yet. There are some positive signs: the previously abundant redside shiners in Scofield have nearly disappeared. This phenomenon also occurred in Strawberry Reservoir after the slot limit was implemented and the number of predatory fish increased.

However, reduction of chubs at Scofield has not occurred yet (the lower number for spring 2010 shown in the graph below was probably due to cold weather during the sampling period). But again, using Strawberry Reservoir as a model, it took a couple of years after the redside shiner disappearance before there was a notable decrease in the number of chubs.

The DWR is not just sitting back and waiting to see what happens at Scofield. It is too important a fishery for that approach.

We put together a plan of action this past winter, and it will begin in 2011 with a study by Utah State University (USU). The study will examine the interaction between the three trout species (rainbows, cutthroats and tigers) and the chubs. The study will take about two years to complete and should answer the question about how effective it is to biologically control chubs. It’s important to keep in mind that there weren’t very many chubs in Strawberry when the DWR began using other fish to control the population. There are a substantial number of chubs in Scofield.

The USU study may reveal that Utah chubs cannot be controlled by biological means. To prepare for that possibility, the DWR will begin planning for a potential chemical treatment of Scofield. This treatment may never take place, but the planning requirements and necessary environmental clearances take a long time to complete. We do not want to wait until 2013 to begin the planning phase. A treatment plan will be finalized by 2013 and then, if the USU study shows that it is necessary, we will assemble the funding and equipment and plan on treating the reservoir in 2014 or 2015.

A treatment of Scofield Reservoir would likely cost between $750,000 and $1 million. Sportsmen and women would fund the treatment effort (via license sales and federal excise taxes on the sales of hunting and fishing equipment). The funding would pay for the rotenone, additional fish stocking and follow-up studies required to renovate Scofield. It’s funding that could certainly be used elsewhere, but might be needed for Scofield because someone selfishly used live minnows as bait. Remember, it is illegal to use live fish as bait anywhere in the state. This rule is in place to protect Utah’s fisheries and to hopefully prevent what’s happened at Scofield from happening at other popular waters.