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Largemouth Bass Study
Hello this is Carson ***, District Fisheries Biologist with the Kansas Department of Wildlife
and Parks. I’m frequently asked “what does a fisheries biologist do in the winter”
and my answer is I analyze the data I collect during the spring, summer and fall. This presentation
is an assessment of largemouth bass length limit change from 15 inch minimum to a 13-18
inch slot on three selected Kansas lakes. The three study lakes were Madison City Lake
a 114 acre lake located in Greenwood County; Moline New City Lake, a 185 acre lake located
in Elk County and Sedan New City Lake a 70 acre lake located in Chautauqua County. Remember
that Moline City Lake is a little bit larger than the others and that will become an important
factor which we will discuss later in the presentation. The objectives of the study
were to reduce largemouth bass density by increasing angler harvest of bass less than
13 inches. Secondly to compare largemouth bass growth rates when changing a 15 inch
minimum length limit to a 13-18 inch slot length limit. I used an electro-fishing boat
to sample bass in the month of May. That’s my boat pictured on the left. A generator
is located under the seat on which I’m sitting. It is capable of producing 1,000 volts. I
can control the voltage, amps and pulse rate from the control panel located on the center
console. A netter stands on the front of the boat and dips the stunned bass from the area
between the two arrays and puts them in the recovery tank in the middle of the boat. This
is the desk in my office. On the left side are two boxes filled with about 2,000 scale
envelopes for this study. On the right side is a projection microscope. It magnifies the
scales onto a piece of paper so I can measure length at age. This is what a typical scale
looks like. I can determine how long a fish was in any given year by measuring the ratio
of the annulus to the total length of the fish at capture. This is the Madison City
Lake catch/hour from electrofishing. Notice that the objective catch rate is 100 bass
per hour of electrofishing. From 2001 to 2007, all of those catch rates were above the objective
rate. When this happens there were too many mouths to feed for the available food supply.
However, in 2008 we changed the length limit from a 15 inch miminum to a 13-18 inch slot.
Notice that in 2008 and 2009, we saw a reduction in the catch/hour electrofishing catch rate.
This is the Madison City Lake largemouth bass age and growth comparison. The red line shows
the age of the fish in 2006 when there was a 15 inch minimum while the black line shows
the age of the fish in 2009 when had changed it to a 13-18 inch slot. There was a mean
increase in growth of 1.5 inches when we changed the length limit. Moving on to Moline New
City Lake, this is the largemouth bass catch/hour electrofishing. Once again we have the objective
catch rate of 100 bass/hour. This is the level at which we can sustain growth in the bass.
Above this as you can see from 2002 until 2006, we have way more fish than we were able
to support with the available food supply. It was above the objective catch rate, however
in 2006, we changed that length limit from 15 inch minimum to 13-18 inch slot and the
response in 2007, 2008 and 2009 was a reduction in the catch/hour electrofishing; however,
it is still slightly above the objective catch rate. This is the Moline New City Lake largemouth
bass age and growth comparison. The red line shows the growth rate when we had the 15 inch
minimum while the black line shows the growth rate when we had the 13-18 inch slot. Overall
there was a .7 inch increase in growth however, most of that growth occurred in bass less
than 13 inches. Once those bass got into that protected slot between 13-18 inches the growth
rate really didn’t increase. This is likely due to the size of the reservoir being 185
acres. Probably there was just too much area for the anglers to effectively reduce all
of those bass. However, we did see more big bass. This is the Moline New City Lake largemouth
bass size comparison. The red squares show the catch rate by year of the bass between
8-12 inches. These are really your smaller fish that we were targeting with that 13-18
inch slot. The blue triangles show the bass that were greater that were 15 inches. These
are the bass that we wanted to increase. So you’ll see that the trend from the red or
small bass decreased from 2003 to 2009 while the trend on the big bass increased between
2003 and 2009. That is really what the objective of this changing length limit was all about.
Getting more big bass into the population and it seemed to work. Finally this is the
Sedan New City Lake largemouth bass catch/hour electrofishing. Once again notice the objective
range of 100 bass per hour of electrofishing during 2002 to 2006 we were above the objective
range. There were too many bass in the population for the amount of food available, however
in 2007 we changed the length limit to a 13-18 inch slot. Anglers were now able to harvest
those bass less than 13 inches and by 2008/2009 our catch rate had declined below the objective
catch range effectively increasing the amount of food available and increasing the growth
rates. This is Sedan New City Lake largemouth bass age and growth comparison. The red line
shows the growth of the bass when we had the 15 inch length minimum, while the black line
shows the growth of the bass after we changed it to a 13-18 inch slot. There was a mean
growth increase of 1.1 inches by changing to that slot length limit. In conclusion,
changing the length limit from 15 inch minimum to a 13-18 inch slot increased growth rate
at all three lakes. Secondly, changing the length limit decreased density of small bass.
This only works if anglers are willing to harvest bass under 13 inches which was difficult
at the larger surface area at Moline. Thank you and I hope you enjoy the larger bass at
these three lakes.