A tale of two fishes

December 30, 2008

By Jennifer Yauck

Fish biologist Rick Goetz aboard the research vessel Lake Char with a large siscowet caught in Lake Superior  ~courtesy Rick Goetz Lake Superior may be the perfect fishing destination for Jack Sprat and his wife, the nursery rhyme duo who ate, respectively, no fat and no lean.* The lake is unique among the Great Lakes in that it is home to both a lean and a fat form of lake trout.

Though they are the same fish species, the so-called leans and fats (or siscowets) have different physical characteristics-and Great Lakes WATER Institute (GLWI) fish biologist Rick Goetz wants to find out why.

Fish biologist Rick Goetz aboard the research vessel Lake Char with a large siscowet caught in Lake Superior ~courtesy Rick Goetz

One difference between the two forms, as their names indicate, is body fat. Only about 15 percent of a lean’s body weight is fat, compared to about 55 percent in siscowets. “When you grill the fillets of siscowets, they are dripping with fat,” said Goetz. The name “siscowet,” in fact, is an Ojibwa word meaning “that which cooks itself.” 

The fish also differ in outward appearances. Lean lake trout have small eyes and fins, and long, straight snouts. Siscowets, on the other hand, have large eyes and fins, and short, angled snouts.

So why these differences? One possible explanation, said Goetz, is genetics. Earlier work by other researchers has shown lean and siscowet lake trout have some genetic dissimilarities. However, no one has yet determined whether genes might be responsible for the physical differences between the fish.

Another possibility is environment. Lean lake trout tend to inhabit Lake Superior’s shallower waters, preferring depths of about 300 feet or less. But siscowets favor deeper waters and are most abundant 400 feet or more below the surface. Siscowets have even been found at Lake Superior’s deepest point, at a depth of some 1,300 feet-more than twice the height of Milwaukee’s tallest building.

Environmental conditions such as pressure, temperature stability, and light penetration differ markedly between shallower and deeper waters. Lake Superior’s shallowest waters, for instance, receive some light, while its deepest waters receive virtually no light at all. So, the thinking goes, perhaps these external environmental forces are driving the physical differences observed between the leans and siscowets.

Genetics or Environment?

Housed in the GLWI fish lab are two green, round 1,000-gallon tanks that each hold 200 leans and 200 siscowets-and possibly the answer to the question of what makes these fish different.

Goetz raised the fish, now 2 years old, from fertilized eggs. He obtained the lean eggs through the Les Voigt Fish Hatchery in Bayfield, Wis., which collected them from fish at the nearby Gull Island Shoal in Lake Superior. He and his research team collected the siscowet eggs themselves from Lake Superior fish they netted offshore near Marquette, Mich. in about 400 feet of water.

Compared to lean lake trout (top), fat lake trout (bottom)—also known as siscowets—have bigger eyes and fins, shorter and more angled snouts, and a higher body fat content. ~courtesy Shawn Sitar For the past two years, Goetz has kept all the fish under identical environmental conditions-temperature, light, diet, and the like-and is watching to see if they still develop differently. He regularly measures the length and weight of a sample of fish, and evaluates their body shape by measuring the distance between specific points, such as two fins. He also analyzes their fat content.

Compared to lean lake trout (top), fat lake trout (bottom)—also known as siscowets—have bigger eyes and fins, shorter and more angled snouts, and a higher body fat content. ~courtesy Shawn Sitar

So far, with an average weight of 1.7 pounds and length of 16 inches, the siscowets are considerably heavier and slightly longer than the leans, mirroring growth trends seen in the wild. Goetz also has observed a quantifiable difference in their body shapes. “These fish are actually differentiating morphologically and you can see it when you pick them up,” he said. What’s more, the siscowets have a higher body fat content.

Goetz said the results so far suggest the physical characteristics of the leans and siscowets are inherited. “There’s a strong genetic component to them, otherwise they would not persist in fish that have been raised in identical environmental conditions,” he said.

Two-year-old leans and siscowets swim in an eight-foot diameter tank at the GLWI fish lab where they are being raised under identical environmental conditions. ~courtesy Rick Goetz Goetz plans to continue monitoring the fish for several more years. Eventually, he would like to tag them and release them in Lake Superior in order to see where they reproduce-especially the siscowets. “We know leans spawn on reefs in the fall, but nobody knows where siscowets spawn because they live deep,” he said.

Two-year-old leans and siscowets swim in an eight-foot diameter tank at the GLWI fish lab where they are being raised under identical environmental conditions. ~courtesy Rick Goetz

Goetz also wants to develop a way to raise siscowets for fish oil, which contains high concentrations of omega-3 fatty acids, for use in dietary supplements. Although siscowets are plentiful in Lake Superior-they account for most of its lake trout biomass-the fat of these wild fish often contains contaminants like PCBs, which means extra processing is necessary to purify their oil. This wouldn’t be the case with farmed fish, said Goetz, making siscowet farming a safer and more economic option. ]

Jennifer Yauck is a science writer at the Great Lakes WATER Institute. GLWI (glwi.uwm.edu) is the largest academic freshwater research facility on the Great Lakes.

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