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When lake trout was stocked in the Deerfield Reservoir in 2015, South Dakota Game, Fish and Parks officials had an idea of what might happen to the ecosystem.

So far, everything seems to be going according to plan.

The fish were stocked to help combat a problem with a smaller predatory fish, the rock bass.

According to a report from South Dakota GF&P that tagged fish in 2016 and 17 as well as interacted with local anglers, the fish have been able to survive, and that's music to angler's ears.

The fish came to Deerfield as retired broodstock fish from the federal system, which meant they would have to adjust from a hatchery lifestyle to more rough life of living in a reservoir.

According to aquatic resources chief for GF&P John Lott, survival wasn't guaranteed going into this process. But the 44 fish caught last season from the 2015 and 2016 stocking indicates that the fish are doing well adjusting to reservoir life.

"At Deerfield one of initial concerns at the time of stocking was if the temperature and oxygen level in the summer period would be sufficient, and it was believed that lake trout would do okay there," Lott said. "With the lake trout, and with all fish, it’s a combination of the temperature and the oxygen level at that same temperature range, and in the summer time a lot of the lakes you get a zone at the bottom where there’s not a lot of oxygen but that's a temperature range where a lot of species of fish will need. Lake trout are a species like that."

Lott said the combination of Deerfield's size, depth, water temperature and oxygen level has made it a successful place so far for the species.

It's a comparison that can be seen in another body of water in the Black Hills; Pactola Reservoir. Although larger and deeper, the success of lake trout gave waterlife biologists hope that they would work in a lake that still had some of the characteristics of Pactola.

Any predator helps, because Deerfield has had an issue with the overabundance of rock bass. 

So far, Lott said the Deerfield management plan doesn't call for the introduction of any new predator species.

"Lake trout are something that we want to evaluate and see how they do," he said. "It's very hard to evaluate a number of species at any one time, and another thought is we want to have a predator in there that will have the least amount of negative impact on anything that’s a positive in the lake."

The diagnosis isn't 100 percent rosy however, as the author of GF&P's completion report, Gene Galiant said.

Galiant is a waterlife biologist with GF&P and said that while the fish are surviving, some are looking smaller.

"We have seen condition declining in our fish, they are getting skinny, which is to be expected," he said. "These fish spent about 10 years living in a hatchery feeding on pellets and now they have to feed on a more natural diet of fish and insects, so they are getting pretty skinny on us right now and we’re watching that."

The report said that the fish have dropped from an average relative weight of 86.3 and dropped to 77.0 in 2017.

"We’re always concerned about it but we put quite a few in so we knew from the start we were going to have a problem with that," Galiant said. "We’re not overly concerned because we have seen that some fish have taken to the reservoir life very well."

One of the conclusions to the study is that rainbow smelt might be a good addition to Deerfield, providing an easy and plentiful food source for the lake trout.

The reason the stocking is such a big hit with the anglers is that they are getting a much larger fish than they are accustomed to, even if that fish might be shrinking from hatchery life.

Galiant said it took Pactola 20-30 years to have fish the size that they are in Deerfield. Because they are coming from a hatchery already weighing as much as eight pounds, the natural growth that took place in Pactola doesn't need to happen in Deerfield.

Both Lott and Galiant said although it is too early to declare victory, it appears that lake trout could have a promising future in Deerfield Reservoir.

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Contact Geoff Preston at geoffrey.preston@rapidcityjournal.com

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Sports Reporter

Sports reporter for the Rapid City Journal.