Great Lakes Article:

Two inland lakes have unique strains of lake trout
Jim Lee
Gannett Wisconsin Newspapers
May 9, 2004

You can't see, feel, smell or even taste the difference, but a crop of lake trout being raised at the Department of Natural Resources' Woodruff hatchery are unique.

The fingerlings are the progeny of fish from Vilas County's Trout Lake, one of only two inland lakes in Wisconsin with native lake trout populations.

"It's a pure genetic strain," said Wes Jahns, DNR fisheries technician at Woodruff.

"It's the only one of its kind in Wisconsin, and probably the only one in the whole Mississippi River drainage area. Genetic tests show the Trout Lake strain of lake trout has been isolated since the last Ice Age."
DNR fisheries staff members became aware of the genetic differences in the Trout Lake population largely as a result of two unfortunate incidents.

The first was a discovery that lake trout in Trout Lake were not reproducing in sufficient quantities to sustain a natural population. That trend continues and is the subject of ongoing study. Rusty crayfish are viewed as one likely culprit.

A second problem developed as attempts, which began in the 1950s, were made to stock fingerling lake trout raised from eggs of Lake Superior and Trout Lake fish to make up for the deficient natural reproduction. Lake trout from each location were marked differently prior to stocking so that future identification was possible.

It was discovered that lake trout hatched from Trout Lake eggs survived considerably better than the Lake Superior fish, and the stocking of Lake Superior trout was halted.

"We felt those Trout Lake fish had a natural adaptation to the lake that enhanced their survival," Jahns said, adding that surviving Lake Superior fish did not appear to have mated with the Trout Lake stock.

A subsequent bout of infectious disease at the DNR's Bayfield Hatchery, where lake trout were raised, resulted in the Trout Lake stock being isolated from other lake trout at the hatchery. It also prompted genetic testing, which detected genetic variations in the Trout Lake fish, when compared with the Lake Superior trout.

Trout Lake, a deep, clear 3,816-acre lake located near Boulder Junction, and Black Oak Lake, a 584-acre mix of deep and shallow water located about six miles west of Land O'Lakes, are the only two inland lakes in the state with natural lake trout populations, according to Jahns.

Unlike Trout Lake, Black Oak continues to maintain an adequate lake trout population through natural reproduction though the number of trout is considerably lower than in Trout Lake.

"We're not planting any lake trout in Black Oak," Jahns said. "Studies have shown that stocking can be detrimental to natural reproduction in that lake."
Why have Black Oak lake trout succeeded in reproducing?
Biologists are still searching for the answer. It is known that rusty crayfish densities are lower in Black Oak.

There also is a difference in fall spawning site selection.

"We have one population of lake trout on Trout Lake that spawns on rocky areas in about 4 feet of water, while another population spawns in about 20 feet," Jahns said. "On Black Oak, the fish spawn in 40 to 50 feet of water."
The eggs are large and unprotected from the time they are laid in November to hatching in March, making them vulnerable to predation and fungus.

Lake trout stocking was halted in 1988, when hatchery disease issues first surfaced, and was not resumed until 1998. A total of 300,000 fingerlings from Trout Lake stock were released into the lake between 1998 and 2002. In 2003, 40,000 yearling fish, which are larger and have better survival rates, were stocked. That was followed up with 20,000 fingerlings this spring.

Lake trout are notoriously slow growing, and the fish in Trout Lake and Black Oak are no exception.

Anglers on the two lakes are allowed to harvest one lake trout daily during a limited, open-water season. The fish must be a minimum of 30 inches.

"A fish that size could be anywhere from 20 to 40 years old," Jahns said. "A 31-inch laker from Trout Lake was found to be 41 years old."
The DNR has plans to introduce lake trout in other northern waters from Trout and Black Oak lake stock.

"We have prioritized a list of suitable lakes," Jahns said.
Clear Lake in Oneida County recently was stocked with Trout Lake strain lake trout. Future recipients of Black Oak strain lake trout include Long Lake in Vilas County, Big Carr Lake in Oneida County, Lake Lucerne in Forest County and Lac du Lune in Vilas County.
"We hope to take spawn from Black Oak lake trout this fall and raise them for stocking as yearlings in 2006," Jahns said.

As a result, additional Wisconsin lakes eventually might support a naturally reproducing lake trout population, but that probably won't be known for 20 or 30 years.