She started out as a patrol boat for law enforcement. For a while, she was outfitted for sportfishing. But for the past 35 years, her main duty has been trawling on Lake Huron monitoring fish populations.

Meet the RV (as in "Research Vessel") Chinook, a 50-foot steel-hulled boat that has spent more than 60 years on the Great Lakes serving Michigan's sportsmen and sportswomen.

The Chinook was one of three 42-foot gasoline-powered vessels launched by the Department of Conservation in 1947 to police the commercial fishing industry on the Great Lakes. Designed much like a commercial trap net boat -- with cabin forward and an open aft -- Patrol Boat No. 3 (as the Chinook originally was called) also was outfitted with a gillnet lifter inside the cabin.

Her original cost was $8,000, but modifications came quickly. Within a few years, the gasoline engine was replaced with a GM 671 diesel.

"When the boat was built the Great Lakes were commercial fisheries," said Jim Johnson, DNR fisheries research biologist at Alpena who is intimately familiar with the Chinook's history. "There was very little sportfishing in the Great Lakes other than some trolling in protected bays. Commercial boats were fishing for whitefish and lake trout; it was still a native ecosystem.

"By the early 1950s, the lake trout in Lake Huron were declining dramatically because of sea lamprey predation and too much commercial fishing, and by the late 1950s they were virtually gone."

With the alpha predators virtually removed from Lakes Michigan and Huron, the alewife populations exploded. And that preponderance of baitfish gave state fisheries managers an idea: Why not stock the Great Lakes with salmon?

In 1968, the fisheries division acquired Patrol Boat No. 3 for its new Great Lakes Fisheries Research Station at Alpena. She was renamed the Chinook, in honor of the Pacific king salmon that had been introduced to the Great Lakes the year before.

Just one season into her new career, the boat was rebuilt with 40-foot outriggers to accommodate West Coast salmon trolling gear. The Chinook could troll up to 96 lures as fisheries biologists attempted to understand the new Great Lakes denizens.

"We wanted to help guide anglers through this new fishery, to develop methods we could share with the public on how to catch chinook salmon," Johnson explained.

The following winter, the Chinook was taken to Gaylord where she was further customized; the vessel was cut in half and another eight feet of hull was added to make her 50 feet. An additional four feet was added to the cabin, too. The gill net lifter was moved to the aft deck.

In 1973, with her trolling mission accomplished, the Chinook was outfitted again; the trolling gear was removed, a larger gillnet lifter was installed and the hull was equipped for trawling. And except for some new electronics -- radios, radar and GPS systems -- she's configured the same way today.

These days, the Chinook is largely involved in fisheries assessments from Port Huron to Sault Ste. Marie. The vessel conducts annual lake trout surveys that assess not only the status of the populations but also yield other important information; for instance, in 2007, the Chinook verified naturally produced young-of-the-year lake trout in Thunder Bay. If such natural reproduction continues to increase, stocking may no longer be necessary.

In addition, the Chinook routinely conducts fall assessments of the walleye and perch populations in Saginaw Bay, but the Chinook is pressed into duty as new situations present themselves.

"Now we're into gobies, zebra mussels, quagga mussels and the collapse of the off-shore fishery," Johnson said. "She spent the whole month of August doing nothing but examining the effects of the mussels on the off-shore and in-shore fisheries."

"But that doesn't mean the old research isn't of value. Fact is, some of the early data collected by the Chinook is becoming increasingly important to scientists.

Scales collected from salmon she netted back in the 1970s are being asked for and used by geneticists today to compare what's happened to them," Johnson said. "Now that 80 percent of the chinook in Lake Huron are wild fish, we assume there's been a great deal of fast-moving genetic drift. We should have the results of those studies back soon."

The Chinook is laid up each November, when her crew performs routine maintenance and minor repairs, which over the years have become more costly.

A few years ago, for instance, the crew became aware of significant rusting on the bottom of the boat and scheduled replacement of her bottom plates for June before the start of her summer season.

"While doing the bottom plates the through-hull fittings broke off, and we discovered the rudder shaft tube also had rusted through," Johnson said. "With the help of technicians from the Marquette State Fish Hatchery, the damage was corrected."

But as repair costs continue to mount, DNR fisheries officials will have to decide whether to sink a good deal of money into the Chinook or look for other alternatives.

One idea is to move her into a slip at Tri-Centennial State Park in Detroit, where she will be an educational exhibit.

It would be just one more assignment for a vessel that has served the state well for more than 60 years.