How far South did Brook Trout (Salvelinus fontinalis) Range in Georgia during Ice Ages?

Illustration of a Brook Trout from Cornell University.

No scientist has ever studied this topic, so my blog post is the first of a kind.  Scientists are limited due to a lack of data–no trout fossils have ever been found in the region.  There’s also a lack of interest, probably because I’m the only person in the world who daydreams about what it would be like to live in Georgia 37,000 years BP.  I want to know, if I traveled in a time machine to live then and there, whether or not brook trout would get caught in fish traps I would place in the Broad River.  I’ve concluded that it’s possible, maybe probable, that they did occur in the Broad River during stadials and even interstadials because summers were cooler then than they are today, an age within a full blown interglacial with hot summers.

Present day Georgia brook trout range map.  Map from the University of Georgia Museum of Natural History.

Today, brook trout are the only native species of trout in Georgia, and as the above map shows, their natural range is restricted to the extreme northeastern region of the state with a few outlying disjunct populations, suggesting formerly a  wider, more continous distribution.  Temperature is one obvious limiting factor in their present day range.  Brook trout suvive best in waters with temperatures between 55-65 degrees F.  They can withstand water temperatures between 32-72  F, however, they can only live in warm waters of up to 78 degrees F for a few hours.  Stocking trout in waters with temperatures greater than 70 degrees F generally fails in the long term.  The temperature of the Savannah River near Augusta, Georgia on June 13, 2011 was 73 degrees F.  (Modern reservoir temperatures are much warmer.)  Water temperatures this warm and warmer are common during summer months in southeastern rivers, making them unsuitable for trout.

Summer air temperatures in the southeast were much cooler during the last glacial maximum, and even during interstadials within the Ice Age, therefore water temperatures were as well.  Data from isotopic oxygen ratios of fossil foraminifera in the Atlantic Ocean suggest average annual summer temperatures were 9 degrees F cooler.  (See my May 15, 2011 blog entry for more detailed explanation of this.)  Taking the present day summer water temperature reading from the Savannah River near Augusta (73 degrees F) and subtracting it by 9 = 64 degrees F…within the brook trout’s comfort zone.  Potentially, trout could’ve lived as far south as what’s now Augusta.

Trout also prefer shallow fast moving water–fingerlings like water that’s just 16 inches deep, and adults prefer the water slightly deeper than that.  Trout stay in shallow water and avoid deeper water even if temperatures are cooler in the latter.  Much of the present day Savannah River, besides being too warm, is too deep, but during the LGM arid climate lowered the water table, creating more shallow fast moving streams for trout.  Suitable trout habitat likely existed in 2 notable streams that flow into the Savannah River: the confluence of the Broad River with the Savannah is only ~60 miles south of present day trout habitat, and the Little River’s confluence is only ~70 miles south.  The headwaters of the Broad River is just a scant few miles from present day trout habitat.  Both the Broad and the Little Rivers have the gravel and rocky bottoms brook trout prefer.  During cooler climate phases, there was no ecological reason brook trout couldn’t have expanded their range at least that far south.  The fossil record shows that many other species from northern climates extended their range farther south during the LGM including caribou, elk, woodchucks, bog lemmings, red backed voles, red squirrels, gray jays, pine siskins, wood turtles, and others.  So it’s not only possible but probable brook trout did too.  Unfortunately, finding Pleistocene fish bones in a region almost devoid of Pleistocene fossils is not likely, rendering my hypothesis difficult to prove.

A Review of Giants in the Storm by Mark Renz

There are so many fossil sites in Florida that academic experts don’t have the time and money to excavate the all.  Mark Renz, a professional fossil-hunting guide but not an academic expert, sought and gained permission to excavate 2 productive retention ponds in LaBelle, Florida.  Giants in the Storm is about his experiences spear-heading that effort.  His team found a truckload of fantastic fossils including lots of mammoth, mastodon, and horse material as well as specimens of llamas, flat-headed peccary, long-nosed peccary, white tail deer, Harlan’s ground sloth, saber-tooth, jaguar, bobcat, Armbruster’s wolf, alligator, and giant tortoise.  The site is unique because it dates to the middle Irvingtonian Land Mammal Age (~500,000 BP).  Though Florida has many fossil sites dating to the Rancholabrean Land Mammal Age (~300,000-~10,000 BP), and plenty from the late Pliocene/early Pleistocene (~2 million BP), the LaBelle fossil site is a rarity that helps fill in a gap in the fossil record, and it provides more specimens for comparison of the evolution of individual species.  The site is located in southwest Florida.  The types of mammals excavated here suggest the region was mostly grassland interspersed with riverine and lacustrine forests.

Like the other Mark Renz book I reviewed, Fossiling in Florida, the best thing about this book are the copious photographs of fossils that are useful for amateur  and serious fossil collectors.  Some of my favorites include parts of the anatomy that don’t automatically come to mind such as a mammoth’s calcaneum (heel bone) and an hyoid (tongue bone) from either mammoth or mastodon.  Of course, there were the usual proboscidean teeth, and spectacular fossils–a relatively complete mammoth skull and a 9 foot long, 300 pound tusk found in place but broken into 4 pieces.

Mr. Renz imagined a single dramatic event, a storm of great magnitude causing a flood, as an explanation for why these fossils concentrated here.  It gave him an excuse for a dramatic title for this book, but he concedes his title is a misnomer.  After consulting with Richard Hulbert, curator of the University of Florida Natural History Museum, he agrees with the conclusion that all the fossil specimens were accumulated here gradually over a long period of time.

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6 Responses to “How far South did Brook Trout (Salvelinus fontinalis) Range in Georgia during Ice Ages?”

  1. James Robert Smith Says:

    I doubt Brook trout ranged that far south. For the simple reason that the species needs very clear, highly oxygenated water to thrive. They don’t need just cool water, but the kind of water that you can only get in the kinds of streams and rivers you find in the mountains and along the Blue Ridge escarpment.

  2. markgelbart Says:

    The Broad River is not that far south–the headwaters are just a few miles south of the brook trout’s current range. During the Ice Age the water table was lower. Streams running through rocky shoals would have been shallow, highly oxygenated, and clear. Actually, all of Georgia’s rivers and streams were usually clear just a 150 years ago. There are rocky shoals as far south as Augusta.

    Brook trout live throughout the entire state of New York, not just in the mountains. And they occur in plenty of midwestern states far from any mountain ranges.

  3. James Robert Smith Says:

    Could be, then.

    What is fossil evidence like for freshwater fish species in Georgia?

  4. markgelbart Says:

    There is very little–the reason why my hypothesis will probably never be proved. Catfish bones that compared favorably to channel catfish were found at Kettle Creek in Wilkes County. Predatory birds left a whole bunch of fish bones at Kingston Saltpeter Cave in Bartow County including gar, pickerel, minnows, 6 kinds of suckerfish, 7 kinds of catfish, largemouth bass, and bream. KSC dates to about 13,000 years ago which was during the last relapse of the Ice Age known as the Younger Dryas but it followed a warm phase of climate. Garfish bones have been found in coastal fossil sites. Garfish bones are very robust and easy to identify. Most other fish bones are not. The Isle of Hope site near the coast yielded many marine fish in addition to the gar, and, of course, sharks teeth.

  5. JoeinATL Says:

    After realizing that nearly every trout I have ever caught is either stocked or descended from stockers, I have developed a special love for Brook Trout in Georgia and looking for them. My first trout this year was an 8″ brookie from the restored section of Stover Creek at about mile 4.0 of the Appalachian Trail.
    My brother works in the municipal water system industry and met a Dougherty County/Albany, GA water system official who told him that Radium Springs, a local feature there, has a population of resident Brook Trout that are remnant from the ice age. He called me to ask whether I thought this was plausible, and I don’t think it is. Radium Springs flows at about 68 – 72F year round, at the upper limit of Brook tolerance, and it gets warmer quickly so the habitat would be literally a few hundred meters. This point is quite far south, quite flat, and I simply don’t think it’s possible. Plus, he’s a municipal employee of a backwards place and I think he’s both a promoter and a little slow on the science. It would be easy to verify their presence or absence with a shock survey. Any thoughts?

    • markgelbart Says:

      If there are trout there, they are almost certainly stocked.

      I doubt brook trout lived that far south, even during the Ice Age, because South Georgia didn’t get that much colder then than it does today. The Gulf Stream kept the Atlantic Coastal plain fairly warm.

      In any case, I haven’t come across any scientific studies about relict brook trout in South Georgia. Believe me, I’ve looked.

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