HOW OLD IS THAT FISH? - Captain Mike Schoonveld

HOW OLD IS THAT FISH? - Captain Mike Schoonveld

Every fish starts out small and grows progressively larger as it gets older. But just as with salmon, the older a year-class of fish becomes, the wider the size range of the fish in that class will become. Some will be smaller or larger than most of their brothers, sisters or cousins.

With the current forage base, there’s no way to age wild-spawned kings with a quick glance. 


I called a fishing buddy a few days ago for a fishing report and he detailed the results of his latest trip to Lake Michigan. “We caught... (blah, blah, blah)” and ended by saying “and one of the kings was a big four-year-old.” If you’ve hung around places where king salmon fishermen are frequently found, you’ve heard guys telling about catching a mix of two-, three- and four-year-old salmon.

I was talking with a Lake Michigan fish biologist recently and he said something about “this fall’s run of three-year-olds.”

“What about this year’s run of four-year-olds?” I asked.



He looked as puzzled to me as I probably did to him. Then it dawned on both of us we were talking about the same thing. We just referred to it differently.

When my biologist friend was talking about this year’s run of three-year-old kings, he was referring to the “nearly four-year-olds.” When my fishing friend talked with me about catching a four-year-old, he also was referring to a fish approaching its final birthday.

Who was “more correct?” Now that I’ve given it some thought, I think the biologist was right—or at least more precise.

I’ll explain it this way. Let’s say I have a three-year-old son. If I tell you I have a three-year old, you know my future fishing buddy is somewhere between his third birthday and fourth birthday. Perhaps I’ll say he’s three and a half or almost four, but if I’m filling out a form, I’ll list his age as three until he actually has his fourth birthday. Regardless, if his birthday is in October and I tell you how old he is in July, I won’t say he’s four.

Neither do biologists when they are talking about the age of the fish with which they are working. What they will do is list the salmon (or other fish) by the number of birthdays they’ve had and then add a plus sign after the number.

So the king salmon my friend reported to be a four-year-old was really a three-plus-year-old (3+)—or was it? All he had to judge it by was its size.



One would think since all Great Lakes kings spawn at approximately the same time, spend most of their lives in the same lake and grow relatively fast, it would be easy to tell how old a king salmon is, just by its size. They should be roughly the same size on their first birthday. They should all be similarly sized on their second hatching anniversary and all of the 2+ fish should be sized different than the 3+ aged fish.

In that same conversation with my biologist friend, I found out the truth. He admitted he used to think he was good at aging Great Lake salmon strictly by size. He now admits, these days, it’s not that easy.



Before the abundance of alewives in the lakes faltered and most every salmon in the lakes had plenty to eat, regardless of where in the lake it was swimming, perhaps age-guessing by fish-size was an almost sure bet. With a steady and ready food supply, most of the fish grew at similar rates. That’s not at all true, these days.

Nowadays, the total abundance of alewives (the main prey of most Great Lakes king salmon) is just a fraction of their pre-mussel invasion abundance and the alewives which are in the lakes are not distributed evenly. There are few alewives left in Lake Huron at all. In Lake Michigan some areas seem to be blessed with alewives, while other areas are as desolate as Lake Huron. Lake Ontario is somewhere in between and the king salmon in Lake Superior have never had an abundance of alewives.

Remember also, for the past decade or longer the actual number of king salmon in each year-class has been highly variable. Stocking efforts have trended down as fishery managers try to bring balance to the predator/prey relationships in each lake and then Mother Nature upsets these management efforts by either under or over performing in places where natural reproduction occurs.



Couple the plight of the alewives with drastic swings in king salmon abundance and the result is years with mostly hungry salmon in the lakes along with years with plenty of food and fast-growing fish, at least in some areas. If every king salmon had exactly the same amount to eat, they’d all weigh within a few ounces of each other. Instead, weights of same-age salmon now vary widely based almost entirely on how much they have been able to eat.

The coded wire tag program has proved this decisively—at least for hatchery-reared king salmon. For several years all the king salmon stocked in the Great
Lakes were injected with a tiny, micro-tag when they were only a few months old. The tag positively identified each salmon’s age, the hatchery where it was reared, the location it was subsequently stocked and several other details more interesting to biologists than fishermen.

As these tagged salmon lived, grew and migrated through the lakes, fisheries technicians made a special effort to collect as many of these coded wire tags as they could gather. The tags were removed from the fish, examined and pages of
information was recorded on spread sheets, compiled into charts and explained
with graphs showing information about the tagged fish’s travels, growth rate, longevity and other details.


The coded wire tag program implanted one of these tiny wires in every hatchery-reared king salmon. 


The coded wire tag program proved judging angler-caught king salmon strictly by size to be a totally inaccurate rule of thumb. Whether the tags the researchers recovered came from net captures, from sport angler caught fish or from fish actually making their spawning run, the data shows quite an overlap on the size vs. age chart. There are fully mature, 3+ aged fish on their spawning run which don’t weigh 10 pounds and others which weigh 30 pounds. Those unexpectedly small fries or big guys aren’t just outliers, either. A rough average in recent years shows only about half of the 3+ aged kings in late sum-mer or fall are “normal” sized for Great Lakes kings while about a third are under-sized and the remainder are outsized.

There are also significant numbers of age 2+ kings which weigh in the teens, as well. The next time you catch a king salmon in a Great Lake you can guess its age if you wish. Chances are you will be wrong as much as you are right. On the other hand, now that most Great Lakes kings are not fitted with coded wire tags, your guess will be as good as the next persons.

Since 2017, due to budget restraints and other issues, only a small portion of the hatchery kings are tagged with coded wires. (All hatchery-spawned king salmon do still have adipose clips however, so hatchery vs. wild-spawned fish can be differentiated at a glance.)


For several years, all Great Lakes hatchery-reared kings were marked with an adipose fin clip and a coded wire tag. 


Since none of the Great Lakes wild fish and only a portion of the hatchery fish can be aged definitively by examining coded tags and aging them by size is more guess than golly, can fish biologists determine the age of a particular, untagged fish definitively? Certainly, but they’ll have to do it the old fashioned way—just as fish biologists have to do with most of the other species of fish in the Great Lakes.



Every fish starts out small and grows progressively larger as it gets older. But just as with salmon, the older a year-class of fish becomes, the wider the size range of the fish in that class will become. Some will be smaller or larger than most of their brothers, sisters or cousins.

By the second or third year for most predator fish, some of the two-year-olds will be larger than some of the three-year-olds and the overlap of size and age will only become greater as the fish continue to age. On the really old specimens—“old” depending on the species of fish being dis-cussed—it will become even more difficult since growth rates slow as a fish matures.

A walleye (in Lake Erie) may go from a fraction of an inch long when it hatches to 10 inches long in its first year. It could easily be 14 inches on its second birthday and maybe 17 inches the following year. At that rate, it would be a trophy-sized 30-incher by age six or seven.


Both of these pre-spawn kings were caught the same day. They aren’t the same size, are they the same age?


Right? Not at all. First, it will depend on whether the walleye is a male or female. Most males won’t ever grow to be 30-inchers. They’ll die from old age first. Second, and more important, is how fish growth slows with age. In Lake Erie, studies have shown 10-year-old female walleyes “average” 27 inches long. In Saginaw Bay, the average age 10+ walleye is 25 inches. In Lake Nipigon, far to the north with a totally different climate, growing season and food base, the “average” 10-year-old walleye is only 20 inches long.



Without fin clips, coded wire tags or other “man-made” markers to identify individual fish or groups of fish, how can a biologist definitively determine a particular fish’s age? The easiest way is to use a microscope to examine the scales of a fish to reveal its age. It’s the only way to do it if the fish is to be released. A scale can be popped off and the fish will swim away no worse off than you would be if some one pulled out a tuft of your hair.

Fish growth is seasonal here in Great Lakes region. Fish, whether they are salmon, walleyes, perch or minnows, grow relatively fast in the summer months and go into a slow-growth period in the winter.



So do trees and that’s what makes the “tree rings” you can see on a tree stump. Count the rings on a cut-off stump and quickly learn the age of the tree.

Fish scales show “rings” similar to age rings on a tree. Biologists count the rings apparent on a fish’s scale and learn how old a particular fish was when the scale sample was taken.

Examining scales is quick, easy and in-expensive. But on slow growing and especially on slow-growing older fish, the read-the-scales method isn’t perfectly adequate. Better accuracy (though it requires killing the fish) is to use a high power microscope look at a cross section of bone from a fish—the cross section cut from a vertebra or more commonly from the fish’s otoliths—a bone-like nugget which grows in a fish’s head. The otoliths or fish bones will also show fast and slow “growth rings” just on a scale or on a tree.


Fish scales show growth rings but not all are as easy to read as this one. 


Whether a fish is a salmon, a walleye, bass, perch or alewife, its size and growth rate is the best predictor of the health of a lake’s fish population. Skinny, stunted or at least fish growing at substandard rates is a sign something is out of whack in the lake. High growth rates would seem to be good and can be a positive sign. It could also mean the predator/prey relationship is skewed heavily to the prey side.

That was the reason salmon were first put in the Great Lakes. The lakes were full of alewives and little else. The salmon grew extremely fast.





What fisheries biologists and managers strive for is average or normal growth rates. That’s why size and bag limits are higher on Saginaw Bay than at Bay de Noc for walleyes. That’s why king salmon stocking numbers have fluctuated widely and wildly in past decades. The biologists are striving for that sweet spot where predator fish are growing at normal rates and the prey populations are stable.

To find that sweet spot, however, they first have to be able to answer the question posed in this story’s title. “How old is that fish?”

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