Through the looking glass

“I’ve learned more about fisheries science in the last four or five years than I have in the past 30.”
What a remarkable statement, especially when you consider who it was made by—Dr. John Casselman.
Casselman is among the most respected fisheries experts on earth. As the senior research scientist in the Ministry of Natural Resources’ Aquatic Research and Development Section, he specializes in, among many things, fish population dynamics.
But he probably is most famous for the pioneering work he and Dr. Ed Crossman undertook by analyzing the cleithrum bones from muskellunge so they could better understand the fish’s life history and its management implications.
By removing the cheekbones from thousands of muskellunge and northern pike—as well as the otoliths (“ear bones”) from lake trout and the scales from bass and other species—and then putting the calcified structures and tissues under a microscope, Casselman has been able to peer into the fascinating life of individual fish and fish communities.
It is like aging a tree by examining its growth rings. And what he has discovered is fascinating.
For example, when we catch a northern pike—and presumably most other species—in the middle of a Sunset Country summer and we unhook it and let it go, the fish stops feeding for anywhere from five-10 days.
“If you catch it during the time that it is actively feeding at its optimal temperature, it will form a check on the scales and the bones that looks identical to the winter months,” Casselman explained.
“Now you may ask, ‘How can that be?’ Because they stop growing in the winter from mid-November to mid-April. But in the summer, when they’re growing quickly and they stop feeding, their metabolic rate is high. They’re burning up body reserves and body energies.
“Then they go into a growth check that is synonymous on the calcified tissue to a stoppage in growth,” Casselman noted.
To illustrate why this growth check is so important, Casselman said we need to understand how a fish grows over the entire open water season. He uses muskellunge to explain the point.
The big toothy predators typically spawn in mid-May and early June. Then they feed for a month to regain their condition. This puts them into the first of July, he says, before the food they eat actually is converted into “growth”
In other words, additional length and weight.
“By the last week in August, however, the muskies are starting to develop eggs again,” said Casselman. “So their somatic growth is a limiting problem. The fish have to build eggs and sperm over the autumn because they don’t grow and barely feed during the winter.
“So they only have about eight weeks worth of time to optimize their growth and energy.”
It is a key reason muskellunge need to be handled so carefully after we hook them. And why so many of the leading muskie conservation groups and top anglers are vehemently opposed to muskie tournaments in lakes where natural stocks sustain the population.
With a growing season that literally measures only a precious few days in total length, holding a fish out of the water to weigh it—or excessively handling it for any reason—can have severe physiological consequences.
Casselman sees similar disturbing signs when he examines the cross-sections of calcified tissue taken from lake trout.
For the past several years, Casselman has participated in a study in the Arctic funded by the prestigious National Science Foundation, where he and two other scientists are examining the impact of climate change and global warming.
“I looked at lake trout samples from the Northwest Territories and all of a sudden I found a set of lakes that have very ancient lake trout in them,” he said. “The trout were all born in 1912.
“So I asked myself, ‘What is important about 1912?’ It was a year when the weather was such that it produced a fantastic year class of trout. I can go back in our records across several lakes and these very ancient fish are all from the 1912 year class.”
When Casselman studies the similar calcified bone structures taken from lake trout in central Ontario’s Haliburton Highlands (which presumably model the behaviour of our trout in Sunset Country), he says the samples teach us another valuable lesson.
Moderate- and large-size trout caught in the summer time need to be carefully released. Why? Because the vast majority are mature egg-laden females. But why would females be so much more vulnerable to angling in the summer than males?
“Female lake trout build their eggs from the summer solstice (June 21) until they spawn in the fall,” Casselman explained. “They become extremely active and feed heavily during this time.
“By late summer, 60-70 percent of the trout that you catch are maturing females.”
But there’s more, says Casselman. Just as what happens when we catch pike and muskies, the stress of being hooked and handled often is enough to cause the trout to cease feeding.
But since trout are early fall spawners, they’re thrown for a loop. They often reabsorb their eggs. As a result, instead of a bumper crop of lake trout eggs being laid in the fall in Clearwater Bay, Whitefish Bay, Silver Lake, or wherever (and another year class of trout hatching the following spring), the trout don’t even spawn.
As insightful as all this is, however, it pales in comparison to what Casselman now is seeing when he studies the otoliths, cleithrum bones, and scales taken from walleye, bass, pike, trout, and muskies.
What the aging structures are telling him about global warming and the impacts of climate change is shocking.
“Am I likely going to see those changes reflected in Northwestern Ontario lakes and fish populations?” I asked him.
“You already are,” he assured me.
But that is another story for another day.

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