Hello and welcome to Conservation Skills
in 10 Minutes or Less.
This series of short, skill based videos
is brought to you by the US Fish and Wildlife Services National
Conservation Training center in Shepherdstown, West Virginia.
If you have a couple of minutes, pull up a chair
and pick up a new conservation skill,
or maybe refresh an old one, on topics ranging from fish
culture the bird identification to stream restoration.
Enjoy.
My name is Matthew Patterson.
And I'm a course leader at the National Conservation Training
Center.
And I'm going to lead you through this skill exercise
today.
So what's the topic for today?
The topic is calculating the density index.
You're probably immediately thinking to yourself,
what is density index?
Well, I'm going to tell you.
Density index is a measure of how
crowded fish are in a raceway.
A lot of fish hatcheries raise fish in raceways. .
And this is getting at was the carrying
capacity of that raceway?
How tightly packed are the fish?
Now federal, state, and tribal fish hatcheries
have been raising fish and raceways
for endangered species recovery and recreational fishing
for many, many years.
But if fish are too crowded in that raceway,
growth, survival, and the general health of the fish
can suffer.
So it's really important to keep an eye on things
like the density index.
So how do I calculate density index?
Well typically, a hatchery will calculate it
with an Excel spreadsheet.
But we want to teach you how to do it by hand,
so that you know what the formulas are running
behind the excel spreadsheet, such
that if it spits out something that doesn't make sense,
you'll recognize that and make the corrections.
So the formula for density index, to calculate it by hand,
is w divided by l times v. And what does that mean?
w is the fish weight in pounds.
That's all the fish that are in the raceway.
l Is the fish length in inches.
And this is an average fish length
for all the fish in the raceway.
And v is the volume of the rearing unit in cubic feet.
So for many of these skills, we're
going to do a demonstration first.
I will show you how to do it.
And then we'll do a skill practice, where
you'll get a chance to do it.
So here's our example.
Shepherdstown Fish Hatchery is holding 120 pounds
of 2 inch brook trout in a raceway with the following
dimensions.
The length is 20 feet.
Width this 3 feet.
And the average depth is 2 feet.
Obviously, since volume is in the denominator
of our equation, the first thing that we need to do
is calculate the volume of the rearing unit--
in this case, the raceway.
The bulk of the formula for that is the volume
is the length times the width times the average depth.
You can see from this diagram that the bottom of the raceway
is sloped.
This allows water to drain to the back in the raceway.
Because of that, we have to take an average depth, which
is typically that center point of the raceway.
So in our example, we had a 20 foot long raceway,
with a width of 3 feet, and an average depth of 2 feet.
Plug those into your calculator.
You should get a volume of 120 cubic feet for that raceway.
So now we know what the volume is.
We can plug this into our density index equation.
If you'll remember, we had 120 pounds of fish.
Those fish were 2 inches in length.
And again, our volume was 120 cubic feet.
Plug those into your calculator, and you should
get a density index of 0.5.
OK, now it's your turn.
After I read through the problem,
I would recommend that you pause the video here,
practice, run through the calculations.
And then when you come back, we will provide the answers
for you.
So that Utopia Hatchery, you are holding 94.9 pounds
of 1.61 inch fish in a raceway.
The raceway is 15 feet long, 3 feet wide,
and an average depth of 2 feet.
What is the density index?
So go ahead and pause it here.
And when you come back, we'll walk through the answers.
OK, welcome back.
Hopefully, you had some time to run through that calculation.
And we will walk through it here with you.
Again, just like in the demonstration,
you should have calculated the volume of the rearing unit
first.
Our raceway was 15 feet by 3 feet
by 2 feet in average depth.
That would have given you a volume of 90 cubic feet.
Is that what everybody got?
Sweet.
So then you would have plugged that into your density index
equation.
Remember, we had 94.9 pounds of fish
in raceway that were 1.6 inches in length.
And the volume that you calculated was 90 cubic feet.
That gives you a density index of 0.65.
Hopefully, that's what everyone got.
But now you should be thinking to yourself, OK.
Now I know how to calculate density index.
So what?
What does that mean?
What's a good density index?
Well, the general rule of thumb for trout
is to hold them at densities in pounds
per cubic foot no greater than half their length in inches.
I know that's a lot of words.
What does it mean in terms of density index?
In other words, we want our density index to be less than
or equal to 0.5.
Now if you remember back to the skill practice,
you calculated that Utopia Hatchery
had a density index of 0.65, which
is obviously higher than 0.5.
So what do we do?
Well, we want to drop that density index back
below that threshold of 0.5.
And the way to do that is to temporarily corral the fish up,
net half of the fish into the next raceway,
and split them into two raceways,
giving the fish a lot more space to live.
So would you like to learn more about density index and skills
like it?
You can sign up for the National Conservation Training Center's
Coldwater Fish Culture course.
And if you hang on at the end of this video,
I'll quickly walk you through how
to find that course on the NCTC website.
So thanks for joining us and listening to the density index
calculation.
If you have any questions about this skill
or any course at the National Conservation Training Center,
you can contact me at either my phone number below
or drop me an email.
OK, as promised, I was going to show you
how to find cold water fish culture on our website.
Best thing to do is just go to Google and type in National
Conservation Training Center.
Hit Enter.
Typically, the first search that comes up, open our website.
And then go to the Search bar here.
Type in coldwater fish culture.
Hit Search.
The first PDF that comes up we'll take you to this PDF
with more information about the course,
including course description, course objectives, target
audience.
And then down at the bottom, we see
what course is coming up next.
Thanks again for joining us for Conservation
Skills in 10 Minutes or Less.
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Have a great day.
And always remember, the beautiful thing about learning
is that no one can take it away from you.