How many?

Many of you will recall that a couple of weeks ago I helped with a northern pike tagging project at the new Wanahoo Reservoir in eastern Nebraska.  First of all, just a reminder, Wanahoo is NOT OPEN YET.  The opening date has been set for April 28; you will have to wait a few more days to fish there.

Anyway, I already posted this, but here is a video that shows the pike tagging that was done:

I want to return to this subject because now I can tell you part of what was accomplished with that project.  We were tagging pike at the new Lake Wanahoo in order to estimate the actual number of pike in the reservoir.  By tagging the pike we could accomplish a mark/recapture population estimate.  WARNING:  Science content ahead; let me explain how a mark/recapture population estimate is done.  Imagine you have a jar full of jelly beans, let’s say they are all Jelly Belly cherry jelly beans, my favorite.  Now, you do not know how many jelly beans are in that jar, but let us say you take 20 of those jelly beans out of the jar and color them black with a magic marker.  After marking those 20 jelly beans, you put them back in the jar and then stir/shake the jar to make sure the black jelly beans are thoroughly mixed into the rest of the jelly beans in the jar.  Then let us say you randomly sample 50 jelly beans from the jar and let’s say that out of those 50 jelly beans, 3 of them were marked black.  With some simple algebra you can now calculate an estimate of the total number of jelly beans in the jar–you know you marked 20 of them black and when you randomly sampled 50 jelly beans, 3 of them were jelly beans that you had marked.  A simple proportion can be used to calculate the estimate of the total number of jelly beans.   The proportion of marked jelly beans in your random sample of 50 is the same as the proportion of the total number of marked jelly beans to the total population: 3/50 = 20/x  where x is the estimate of the total population.  Solving that equation tells you that the estimate of the total number of jelly beans is 333.

Tagging a Wanahoo pike (that fish was typical of the male pike we collected, the females were larger).

Now, to make a population estimate of the actual number of fish in a body of water there are some assumptions that must be met (e.g. all the fish retain their marks or tags, none of the fish leave the population, marked and unmarked fish are equally likely to be captured, etc., etc.).  There are some advanced statistical methods that can be applied to come up with a more accurate estimate, but the jelly bean example is basically how pointy-headed fisheries biologists conduct a mark/recapture population estimate.

If I have not lost you in the numbers, yet, let me throw some more at you.  In the northern pike tagging efforts at Wanahoo a couple of weeks ago, 718 adult northern pike were tagged.  The estimate of the actual number of adult northern pike in Wanahoo at the time of tagging was calculated at 2,118 pike or about 3 adult pike per acre of water.  Let me have some more fun with numbers and assume that all of the adult pike in Wanahoo now would be fish that came from the 2010 stocking of 26,650 northern pike fingerlings.  If we have 2,118 adult pike in Wanahoo now, from the original stocking of 26,650 fingerlings, that would mean that about 8% of those fingerling pike that were stocked in 2010 survived to adulthood.

The rest of the story will be told after Wanahoo opens to fishing on April 28.  We will also be able to use the tagged pike to estimate angler exploitation of that pike population.  All of the tags have a number and information for reporting the catch of that tagged fish.  That information will be invaluable and I hope every angler that catches a tagged pike from Wanahoo will report it.  If the pike is caught & released you can still report the tag number and size of the fish, but please leave the tags in released fish.

Stay tuned, over time we are going to learn a lot more about northern pike in a new, eastern Nebraska reservoir.

Waiting in a livewell to be tagged, and now waiting to be caught.