Barbs and Backlashes

How Old? by whitetips
April 8, 2010, 12:29 pm
Filed under: Fishing | Tags: ,

Age and Growth

Some of the most important data that a fisheries biologist can have for any population of fish would be age and growth rates.  Fish have “indeterminate” growth meaning that they can continue to grow throughout their lives.  Obviously there are some limitations as a bluegill is not going to reach 20 pounds no matter how old it is, but generally the older the fish, the bigger the fish.  As fish grow their scales and bones grow, enlarge, and those scales or bones can be used to age fish.

Most times when fisheries managers sample fish populations they collect scales from a variety of sizes of fish.  Those scales can be used to determine the age and growth rates of the fish.  It is a lot like looking at the growth rings in a tree with one important difference:  when you look at the growth rings in a tree stump you are seeing one ring for each year of growth.  When you look at growth rings in a fish scale you see rings that represent growth throughout the year, throughout the growing season.  To determine the age of a fish by looking at a magnified image of a scale we look for patterns in the growth rings as the fish grows relatively fast during the warm-water growing season and then growth slows as the water cools.  In most species of fish there is no growth during the winter and this creates a mark on the scales that signifies an “annulus” or one year of growth.  We look at fish scales and interpret the growth rings and count the number of annuli or the number of years that fish has lived.

When we get a scale sample back to the “lab” or office, we do not look directly at the scales themselves.  The scales can be dirty and that can make it difficult to see the growth rings.  By making an impression of the scales onto an acetate, plastic, slide it becomes a lot easier to see the growth rings.  An interesting side note:  the scales have a “rough” side and a “smooth” side.  When running the scales through a press in order to make impressions on a plastic slide, only one side of the scale will have the “ridges” and make an impression.

An impression of 3 muskie scales made onto an acetate/plastic slide.

I have an old microfiche reader sitting on the back of my desk and that is there mostly because I might like to take a look at some fish scales now and then.

Magnified image of a muskie scale on a microfiche reader.

Here is a closer look at a portion of a magnified scale image.

I know this will not show up well; look close and you can see the growth rings and get an idea of what some annuli look like.

If you are thinking ahead of me, you are thinking about species like catfish which have no scales.  How are those fish aged?  Earlier I mentioned that fish bones can also be used to determine the age of fish as those bones also continue to grow, enlarge, through a fish’s life just like the scales do.  There are a variety of bony structures that can be taken from fish and also used to determine the age of those fish.  On catfish a pectoral spine is usually used for aging.  Scales and fin spines can be taken from fish without sacrificing the fish.

Other bony structures like otoliths (i.e. ear bones) or cleithrum can be used to determine age.  In fact it may be easier to determine ages from bones especially on large, old fish.  The problem with using bony structures like otoliths or cleithra is the fish have to be dead or sacrificed in order to collect those bones.

Let me continue telling my story by going back and telling you what a “cleithrum” is.  The cleithrum is an “L”-shaped bone located just behind the gills; it is found just under the skin, right below the gill cover.  Aging pike and muskies using scales is notoriously difficult and that is especially true for old, large pike and muskies.  Cleithra are often used to accurately age large muskies and pike.


That brings me to Nebraska muskies.  We have some age and growth data for some Nebraska muskies, but that information is limited and represents mostly small, young muskies that were aged using scales.  In the past year I had a couple of opportunities to collect cleithra and age some older, larger Nebraska muskies.

I am betting many Nebraska anglers remember this fish caught from Merritt Reservoir last year.

Vicky took her fish to Lewon’s Taxidermy in Randolph , and I was able to work with Vicky and Lewon’s to obtain a cleithrum from that beautiful Merritt muskie.  I also had an angler extract and send me a cleithrum from a 42-inch muskie that was found dead last fall at L.E. Ray Lake in Grand Island.

Now keep in mind that I am talking about 2 fish, and any fisheries biologist would tell you that sample size is too small to be meaningful.  But remember that we have very little age and growth data for Nebraska muskies and almost none from large Nebraska muskies.  So, even though it represents only a couple of fish, let me tell you what I found.

Cleithra from the L.E. Ray muskie (top) and Vicky's Merritt muskie (bottom) along with 3 scales that were also collected from Vicky's fish. If you look close you can see some of the annuli in the cleithra.

Gotta build the suspense a little bit . . . everyone do a Clark Griswold drum roll . . . brbrbrbrbrbrbrbrbrbrbrbrrrrrrrr . . . . The L.E. Ray fish was 6 years old, would have been stocked in the spring of 2004, and Vicky’s fish was 12 years old, stocked in 1998.  Interestingly the scales from Vicky’s fish were not that difficult to read and the age determination from both the scales and the cleithrum were similar.

Now let me tell you a couple of interesting things about the ages of those two muskies.  First of all, those are relatively fast-growing fish.  I pulled a reference off of my bookshelf that includes growth rates determined from cleithra for 500+ muskies taken from the entire North American range of muskies.  On average a 6-year old muskie would be about 38 inches long and on average a 12-year old muskie would be about 44 inches long.  I would suspect that Nebraska muskies being a little bit farther south than classic muskie waters would grow a little bit faster especially where they have an abundance of prey.  Those ages confirm my suspicions.

Secondly, the only muskies we now stock in Nebraska waters are pure-bred muskies, and those fish are raised in hatcheries until they are 1-year old.  Those yearling muskies are raised on minnows and other small fish and stocked in the spring when they are approximately 12-14 inches long.  We shifted to that stocking strategy about 13 years ago and have found that we have excellent survival of those age-1 muskies we now stock.  I believe Vicky’s fish was one of the first of those age-1 muskies stocked into Merritt.  That is significant because the Merritt muskie fishery is probably better now than it has ever been and that is the result of that stocking strategy!  Consider that a muskie that is only 12-years old has the potential to grow a lot bigger especially in a water body like Merritt where there is an abundance of prey, primarily alewives, to feed all the predator fish and Merritt has enough water to offer some cool-water habitat that big muskies require.  Muskies can grow a lot larger than Vicky’s fish, and I believe we may have the numbers of muskies in Merritt right now that one of those fish could grow large enough to beat our state record of 41 pounds 8 ounces.  A 52- or 53-inch muskie likely would be a state record candidate and some of the first muskies stocked at age-1 could reach that size in the next few years.

If I catch a new state record muskie from Merritt or any other Nebraska water, I am warning you, that is all I will blog about for at least a year!

Now, please do not go harvesting every legal muskie just to collect cleithra.  On the contrary, I would strongly encourage you to release all muskies you catch including the legal-size fish (we have a 40-inch minimum length limit on muskies, statewide).  Muskies are not a fish that are stocked so some angler can put some fillets on the table; they are intended to be trophy fish and they are worth a lot more in the water.  Check this website out for some tips on handling and releasing large fish, especially muskies,|MINC%20Images/MINC%20forms%20and%20pamphlets%20and%20KTD/2006-TipsRele.pdf , and if you want a trophy for the wall consider snapping a few photos, releasing the fish and getting a graphite replica mount, .  If you do find a dead muskie the cleithra is relatively easy to remove, and I would love to have some more cleithra from large Nebraska muskies to age.  If you catch a big muskie, take that fish to a taxidermist, and would like to have them take a cleithra for aging, let me know and I can work with the taxidermist to accomplish that.

Thank you to those folks who helped obtain cleithra from these two muskies!


5 Comments so far
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Excellent read, (as usual) Mr. B!

Comment by Harold F.

So, how old ws that Muskie?

Comment by Steve Trybus

Oooops, I didn’t see the rest of the aticle.

Comment by Steve Trybus

Learn something new every day.That’s very interesting.

Comment by Mel W.

[…] fish.  In that post I specifically highlighted the age and growth of a couple of Nebraska muskies, […]

Pingback by Another muskie « Barbs and Backlashes

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