Lake Superior is the most heavily fished of the Great Lakes, which makes the DNR’s surveys of lake trout populations all the more important. The research helps the DNR when it comes to establishing regulations for anglers large and small.
“It gives us a better estimate on how the lake trout are doing,” said Capt. Kevin Rathbun of the DNR fisheries research vessel Lake Char. “They do a lot of length and weight measurements, and they do maturity on the fish as well. They take that information back to the office and run it through models and then they come up with quotas for everybody.”
“We do this annually and it’s similar to what the census bureau does with the human population; we do this with lake trout in Lake Superior,” DNR fisheries research biologist Shawn Sitar said.
Part of the DNR’s research involves tagging some of the fish to study survival and immigration patterns.
“It’s like having a pulse of the population,” Sitar said. “We try to study and estimate the key mortality sources and one of those is the sea lamprey, which is a non-native parasite that has a strong effect on lake trout populations and we study that by look at the wounding rates. This is in partnership with the sea lamprey control program.”
The other major factor in lake trout mortality is fishing, which is where the safe harvest quotas come in to play. Another study that aids the DNR in determining survival and mortality is diet.
Sitar said, “To see what lake trout eat helps us understand how well or how poorly they grow in the population and sometimes when food resources are limited lake trout forage on anything they can find and we do see insects and birds in their diet often.”
While the fish are cataloged on route back to the harbor, the researchers extract an important structure from near the fish’s inner ear to work on back at the labs.
“We extract structures from the fish, to age them,” Sitar said. “We use otoliths; we’ll cross section them and look at them under the microscope. They have rings, similar to tree rings, and we count those to age the fish and mortality is derived from that by looking at the changes in the number at each age across the age classes. That gives us an estimate of the survivorship in the population.”
The Lake Char was delayed in getting started due to this year’s prolonged ice cover on Lake Superior , but now that it’s back in home port, the crew can begin assessing this year’s catch.