Hi, I'm Paul Michaeletz. I'm a resource scientist with the Missouri Department of Conservation.
We conducted a seven-year study on blue catfish in Mark Twain Lake to
assess their population and consider if new harvest regulations could improve the fishery.
Mark Twain Lake is a large reservoir in northeastern Missouri.
At normal pool, it has a surface area of 18,600 acres.
Mark Twain Lake supports a popular sport fishery for blue catfish and flathead catfish.
At the beginning of the study, we knew little about the blue catfish and flathead catfish
populations or about their exploitation or harvest by anglers.
Currently, the daily creel limit is five blue catfish and five flathead catfish with no
size limit for either species.
We wanted to determine if there were opportunities to improve the sport fishery for these species.
We, Paul Michaletz and Ross Dames were the principal investigators on this study.
The objectives of this study were to 1) learn more about the blue catfish and flathead catfish
populations, such as their growth, mortality, sizes, and ages, 2) determine what proportion
of these populations are harvested by anglers which is called exploitation, and 3) if new
harvest regulation may improve the quality of these populations.
In other words, could we have more and larger fish, while still allowing satisfactory amounts
of fish to be harvested.
Unfortunately, we could not capture enough flathead catfish during our pilot sampling
in 2012 and 2013 to assess their population.
We tried several different methods to capture flathead catfish but none of them produced
many flathead catfish.
Consequently, we excluded flathead catfish from further evaluation.
We captured blue catfish with jug lines baited with gizzard shad.
Most anglers targeting blue catfish in Mark Twain Lake use this method.
We fished the jug lines on flats and points close to the old river channel.
The jug lines were anchored and fished overnight in the springs of 2012 to 2018.
When we captured a blue catfish, we measured it for total length and some of the blue catfish
were weighed.
Length and weight data were used to develop a relationship between length and weight which
was necessary for our assessment of length regulations.
We also removed a pectoral spine from some fish for ageing.
Removing a spine does not kill the fish and can be sectioned to determine the age of the
fish.
During 2014 to 2016, we tagged 1,396 blue catfish that were at least 15 inches long
with Carlin dangler tags.
Printed on the tag was a reward value of either $25 or $100.
Seventy-five percent of the tags were the standard tag with a value of $25 and 25% of
the tags were the high-reward tags of $100.
Anglers who returned the tags to us received the reward value printed on the tag.
The tags returned by anglers were used to estimate exploitation which is the portion
of blue catfish harvested annually by anglers.
The catch per jug or CPUE, which stands for catch-per-unit effort, was low averaging about
one blue catfish for every 10 jug sets.
Over the seven-year study, we caught 2,409 blue catfish which included 38 fish that were
caught more than once.
The blue catfish ranged in length from 10.3 inches to 45.6 inches.
We estimated the age of 698 blue catfish.
Growth of blue catfish in Mark Twain Lake is slow taking about 6-7 years to reach a
length of 20 inches.
Growth among individual blue catfish of the same age was also quite variable.
The graph shows the length at age for individual blue catfish identified by the various symbols
which represent different years of capture.
As you can see, at any given age, the length of blue catfish can vary quite a bit.
The curved line represents the average length of blue catfish at the various ages.
We captured blue catfish up to 20 years old.
We estimated the percentage of blue catfish that die every year with a catch curve shown
on the right.
We transformed the number of fish at each age and plotted these estimates across the
ages.
The slope of this line can be used to estimate total annual mortality or the percentage of
fish that die annually.
We only used age 5 and older fish because younger fish were not fully vulnerable to
our sampling method.
Age 5 fish roughly corresponds to our minimum tagging length of 15 inches.
Using the catch curve, we estimated total annual mortality to be 24.8%.
So, about 25% of the blue catfish population that were at least five years old died every
year.
This estimate includes both natural mortality and mortality due to harvest by anglers.
We received 297 tags that were returned to us by anglers.
We knew the location of where we tagged and released the blue catfish and where the anglers
caught the tagged fish.
We found that blue catfish in Mark Twain Lake moved throughout the lake.
The map of the lake shows the four areas where we tagged and released the fish and where
the anglers captured the tagged fish.
The different colored symbols indicate where the fish was tagged and released.
For example, the dark red symbols indicate that fish were tagged in the Indian Creek
Arm.
Note that the dark red symbols are scattered throughout the lake even up the North Fork
arm above the lake itself.
Using these tag returns, we estimated that annual exploitation by anglers ranged from
8% to 12%.
We used these estimates and our total annual mortality estimate to calculate the corresponding
natural mortality of 17% and 13%.
Most anglers who returned a tag captured the blue catfish with either jug or trot lines.
Far fewer anglers used either rod and reel or limb lines.
Hi, my name is Ross Dames, fisheries management biologist for Mark Twain Lake.
Paul just shared with you the important population information we need to
fully assess the status of the blue catfish fishery in the lake.
And as you have seen, it takes a long time and a lot of work to get that information.
So, what’s next?
How can we use all this new information to make catfish fishing better at Mark Twain?
The structure of a fish population, meaning the number and sizes of fish, are determined
by 1) how many fish are added to population each year, 2) how fast the fish grow, 3) how
fast they die naturally,4) how fast they die due to fishing , which we often call exploitation
or fishing mortality and 5) how long the fish can live.
6) So, in order to change the population structure to make fishing better, we have to alter one
or more of these five variables.
7) The most effective and direct way to manipulate fish populations is to alter fishing mortality.
We do this by using harvest regulations, like length limits.
Length limits alter fishing mortality by changing the age or length where fishing mortality
or harvest starts.
Harvest regulations, like length limits, however, only work when fish harvest is high enough
to noticeably impact the population.
Is 8 to 12 % exploitation of blue catfish in Mark Twain high enough for a length limit
to noticeably alter population structure to make fishing better?
Exploitation of blue catfish in Mark Twain is much lower than in some other reservoirs.
Now that we have good data on blue catfish growth, fishing mortality or exploitation,
natural mortality, and longevity, we used all of it in combination to predict how the
population might respond to new harvest regulations, specifically minimum length limits.
We used a computer population model to look at the potential impact of four different
minimum length limits, 20, 24, 26 and 30 inches compared to a 15-inch baseline minimum length
limit.
For clarity, there is not currently a 15-inch length limit on catfish in Mark Twain, but
our data tells us that very few blue catfish less than 15 inches long are harvested, so
we are using this measure as the current baseline for comparison.
Also, this model is just a simple, mathematical representation of something that is really
very complex.
The model does not consider any fluctuations in environmental conditions, which we know
vary widely from year to year.
It also assumes that fish growth is consistent, even after the length limits are imposed.
The output variables we were most interested in looking at were: YIELD (or total weight
of fish harvested), yield is a very important measure in fisheries that are mostly harvest
oriented, fisheries where anglers like to keep fish to eat.
Past surveys tell us that most catfish anglers like to keep fish.
The second output variable we looked at was the number of catfish harvested.
Anytime additional harvest restrictions are added to a fishery, the number of fish harvested
will decrease, but it is important to know how much harvest numbers decline.
The last output variable we’ve included here is the number of catfish in the population
at least 35 inches long.
This gives us an indication of how the length limit changes the number of large fish in
the population because increasing the opportunity to catch big fish may be important to some
anglers.
The results in this table are expressed in percent change from the baseline, which recall
is a 15-inch basically self-imposed minimum length limit.
A negative percent means a decrease.
First, let’s look at the impact of these minimum length limits on YIELD.
With a 20-inch minimum length limit we can expect yield to go up a bit, that means the
overall weight of catfish harvested should go up, not a lot but up.
With a 24-inch minimum length limit yield would not likely change much but could go
up slightly.
Most importantly it is not expected to go down.
The likelihood of yield decreasing is higher with both the 26 and 30-inch minimum length
limits, especially the 30-inch minimum length limit.
As I mentioned before, implementing more restrictive harvest regulations will always cause the
number of fish harvested to decline.
A lot fewer catfish would be harvested under a 30-inch minimum length limit than with a
20 or 24-inch minimum length limit.
A 30-inch minimum length limit would likely reduce the number harvested by about 70%
All of these minimum length limits increase the number of catfish at least 35 inches long
in the population and that increase grows with each longer minimum length limit.
I need to point out again that we are expressing these changes as % compared to the 15-inch
minimum length limit, and even though we might see a 100% increase from one minimum length
limit compared to another, the number of fish could still be very low, for example an increase
from 1 fish to 2 fish over 35 inches is a 100% increase, which sounds really good, yet
there still are very few in the population.
None of these minimum length limit resulted in a lot of big catfish.
To summarize, the 20, 24, and 26-inch minimum length limit improved the number of large
catfish without substantially reducing YIELD.
The 30-inch minimum length limit is far more likely to reduce yield.
Also, the number of fish harvested would decline greatly with the largest minimum length limit.
I think the 20, 24, or 26-inch minimum length limit would all be ok, but I like the 24-inch
minimum length limit because it doubles the portion of large catfish relative to 20-inch
minimum length limit and has a lower risk of reducing yield than the 26-inch minimum
length limit.
It is a good middle ground choice.
Note, that even though we couldn’t collect enough information to make these assessments
for flathead catfish, whatever minimum length limit we ultimately recommend for blue catfish
we will also be recommending for flathead catfish.
I’ll also add that we have no plans or desire to change the daily bag limit for either blue
catfish or flathead catfish.
Bag limits really don’t impact overall harvest much unless they are very low, like one or
two fish per day, and we are confident that most anglers would not support a daily limit
that low.
Changing the daily limit would also be counter to our objective to provide a fishery that
balances opportunities to keep fish to eat and to catch an occasional large fish.
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