By Chris Niskanen
It was a simple question: Why do some Lake Minnetonka crappies get exceptionally large?
Curious if there was a biological or fish management issue at play, Minnesota Department of Natural Resources biologist Mike McInerny decided to investigate with a bit of high-tech sleuthing. He asked fish geneticist Loren Miller, in the Department of Fisheries, Wildlife and Conservation Biology at the University of Minnesota, to study the genetic makeup of the largest crappies entered in the Crappie Contest, held each April at Lake Minnetonka. DNR biologists hoped the results might aid them in managing for big crappies in other lakes.
Fishy DNAA fish geneticist demonstrates his science
Miller got fin samples from the 16 largest crappies entered in the 2003 fishing tournament and performed a test familiar to viewers of television crime shows: He isolated DNA to see how it was similar to or different from other specimens (in this case, black crappies).
Indeed, this crappie DNA was unique. The top three prize-winning crappies (two were true trophies, weighing more than 2 pounds) weren't true black crappies but hybrids of black-crappie and white-crappie parents. Miller says it is not unusual for first-generation fish crosses to grow exceptionally large, but since the Lake Minnetonka hybrids looked almost identical to their black-crappie parents, the difference wasn't noticeable to even a trained eye.
Miller further tested more crappies from 20 other lakes and found many large crappies were also first-generation hybrids between the black and the white species. "Presumably, this crossing happens with regularity," he says.
It turns out Mother Nature hides a lot of her secrets in fish chromosomes. Advances in the study of fish genetics are producing more and more "aha" moments for the DNR in its study and management of brook trout, walleye, muskies, and other species.
Of course, the discoveries in fish science don't happen as quickly as a one-hour crime show, but researchers like Miller use the same basic DNA tools to identify fish strains as forensic scientists use to match blood and hair samples to catch criminals.
DNA becomes an important tool to help decide which genetic strain of fish to stock where, says Don Pereira, DNR fisheries research manager. For example, Miller, who oversees the university's aquatic genetics laboratory, helped the DNR determine which genetic strain was best suited for the recovery of walleye on Lower and Upper Red lakes. He determined the genetics of the Pike River walleyes, which migrate from Lake Vermilion and spawn in the Pike River, most closely matched the genetics of native Red Lake fish.
Pereira says that for nearly a decade, the DNR has been increasingly using Miller's research to ferret out the pitfalls -- and successes -- of fish stocking in Minnesota. "In the early 1900s, we started moving fish around without knowing much about what we were doing,'' says Pereira. "Loren's work is showing us today that local, native fish have adapted best to their surroundings."
Fred Utter, a University of Washington professor, is considered the "father of fish genetics" for his 1970s work that identified the various strains of Pacific salmon that spawn in the Pacific Northwest. Few Midwestern states have fish genetics laboratories, Miller says, though the field is made up of a "small but growing group" of scientists.
Modern fisheries scientists use fish genetics to differentiate fish populations from each other.
"Mostly we can tell if a fish population is isolated," says Miller, "or whether individuals are mating with other populations of fish."
That kind of research cracked the case of the missing native brook trout in 2004 in southeastern Minnesota's blufflands. The Wisconsin glaciation of the last glacial age did not reach this so-called driftless area, a unique ecoregion. The native brook trout adapted specifically to the region's spring-fed limestone creeks. Early fisheries managers assumed that European settlers nearly wiped out most of these native fish by the 1870s, due to overfishing as well as deforestation and grazing that ruined the pristine, cold water.
By the 1880s, the Minnesota Fish Commission was using railroad cars and custom-designed wooden barrels to bring in stocks of East Coast brook trout (and a lot of other species, including Pacific salmon). By the late 20th century, native brook trout were assumed to have been extirpated from most southeastern waters and replaced by East Coast strains. The only possible exceptions were remnant native populations in small, remote headwater streams.
In 2001 the DNR decided to investigate the missing natives and the agency hired Miller to look into the case. He gathered genetic samples of brook trout from New Hampshire, Maryland, and other East Coast streams and hatcheries and compared their DNA sequencing with the DNA of southeastern Minnesota brook trout and a sample from a remnant population of native brookies in far northeastern Iowa. The samples were collected from adipose fins and stored in ethanol. By 2004, with the help of DNR fisheries and graduate students, Miller had enough samples to make comparisons, and he spent the better part of a year analyzing 20 different populations.
The results showed some Minnesota brook trout populations were more closely related to the Iowa natives than they were to the East Coast fish. It confirmed what current DNR fisheries managers thought: There might be remnant native brook trout populations persisting in southeastern Minnesota. Now DNR fisheries managers believe some of the brookies that anglers catch in driftless area streams today are likely descendants of Minnesota natives that never disappeared. As water quality improved in the 20th century, their ancestors persisted, eventually moving downstream out of the headwaters and prospering, while the nonnatives diminished.
"If we leave native fish alone over a period of time, those locally adapted genetics will reassert themselves," Pereira says. "That appears to be the situation with some populations of brook trout in southeastern Minnesota."
The DNR's walleye stocking program is guided by research done a decade ago and by continuing genetics work by Miller.
A walleye from the Red River in western Minnesota has slight genetic differences from a walleye living in the St. Louis River near Duluth. Ditto for a walleye from the St. Croix River, which is different from the other two. The differences are on a genetic level (there are no noticeable physical dissimilarities). DNR fisheries managers believe that if the wrong strain were placed in the wrong water body, the introduced strain could have poor performance or even negatively affect any existing population of the same species. A negative effect of mixing genetically distinct populations is known as outbreeding depression. For example, researchers at the Illinois Natural History Survey, a research group based at the University of Illinois in Champaign, found that hybrid crosses between Illinois and Wisconsin largemouth bass populations had increased susceptibility to the infectious disease known as largemouth bass virus.
The DNR hired the Illinois Natural History Survey research group to help guide the agency's fish-stocking decisions. Researchers tested walleyes throughout Minnesota. The study results, reported in 1997, showed six distinct regions where walleye stocks shouldn't be mixed.
Today, the DNR does not stock walleyes from southern Minnesota in northern Minnesota. Nor are walleyes from the St. Louis River stocked in the Red River watershed. This no-mixing policy raised a potential problem when it came to restocking Upper and Lower Red lakes after a walleye population collapse led to a fishing closure in 1999.
Henry Drewes, DNR northwest regional fisheries supervisor, says the native Red Lake walleyes were classified as Hudson Bay drainage fish. But the DNR didn't have any major egg-taking facilities where walleyes had the Hudson Bay genetic markers. So Miller made some DNA comparisons and found that the Pike River walleye from Lake Vermilion, where the DNR annually collects walleye eggs and milt to create fry, were the closest match to the Red Lake walleye. Lake Vermilion is in the Rainy River basin, another major basin within the Hudson Bay drainage.
The stocking in the Red lakes has been a success: The stocked walleyes are reproducing on their own, and the lake was reopened to recreational fishing in 2006, sooner than expected.
"We have documented outstanding natural reproduction of walleye in the Red lakes since 2004," says Drewes. "Selection of the wrong strain -- one that may not have performed as well -- would have delayed this critical milestone in the recovery effort."
Drewes is a firm believer in maintaining genetic purity of fish.
"We have denied all requests for stocking of any catfish in the Red River watershed," he says, because the trophy catfish of the Red River are genetically unique and managers do not want to add any risk of diluting the gene pool to this population. "They are adapted to living much longer than their southern cousins and grow and mature at a much slower rate."
Miller says fish evolve and adapt to survive in their specific environment, but fisheries scientists can't yet identify those adaptive traits. Those studies in Minnesota may be decades away; however, biologists now know that the wrong strain can fail to reproduce over the long term.
Drewes recalls a study conducted several years ago comparing the growth- performance of bass from Wisconsin, Illinois, Texas, and Florida that were experimentally stocked into a lake near Brainerd. The hypothesis was this: Would stocking bigger bass from southern states produce bigger bass in Minnesota waters? "The answer was no," Drewes says.
The Wisconsin fish survived and grew larger in northern climates. The southern strains of bass were not able to contend with our winter climate and eventually perished.
The modern understanding of fish genetics allows today's fish managers to look back and realize stocking mistakes made in the past.
University of Colorado researchers recently discovered that a 20-year effort to restore the greenback cutthroat trout in Colorado was failing because the wrong cutthroat strain was being stocked. Instead of the endangered greenback cutthroats, genetic testing revealed that five of nine populations of restocked fish were actually the more common Colorado River cutthroat trout. The two fish look similar, and until genetic testing was done, managers didn't realize they had been reintroducing the wrong fish. Researchers told the Associated Press in 2007 that the recovery effort still had a chance of success because four of the populations tested were indeed endangered greenback cutthroats.
After stocking muskies derived from Shoepack Lake into other Minnesota lakes from the 1950s until the 1980s, the Minnesota DNR conducted genetic testing and switched to Leech Lake strain muskies, which grow bigger in Minnesota. In the process, the DNR created one of the world's best muskie fisheries.
"If not for the knowledge of and application of genetics for enlightened muskellunge management, Minnesota would not be the destination for trophy muskies in North America,'' Drewes says.
Muskie angler Josh Stevenson, owner of Blue Ribbon Bait and Tackle in Oakdale, says he's thankful the DNR made the correct choice in picking the Leech Lake strain, which grows to weights up to 50 pounds and greater.
"We have the best muskie fishing in the country because we're stocking the correct strain of fish," says Stevenson.
From a nondescript office and laboratory in the university's Hodson Hall, Miller and his graduate students are working on the next important genetic discoveries about Minnesota's fish that will guide fisheries management for decades to come. And thanks to Miller's past research, next spring the DNR will stock the correct strains of walleye, muskie, and trout in the waters where they belong.
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