[Coral-List] Responses to "Measuring damselfish on SCUBA?"

Malin Pinsky malin.pinsky at gmail.com
Thu Jan 12 10:54:25 EST 2017


Hi all,

I want to say thank you to all who responded to my question about measuring
damselfish accurately underwater. There were some wonderfully creative
answers, from quick and dirty to accurate to technologically advanced. I've
compiled the answers below in case anyone else has similar questions.

Regards,
Malin Pinsky


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In order from approximately fastest and dirtiest to most complicated but
reliable:

The quick and dirty way is not to catch them at all but to estimate their
length versus landmarks on the substrate.  Then measure those landmarks.

Or you could use a ruler on a stick (or markings on the end of a stick),
get the stick close to the fish and then estimate.

If you transfer the fish to a small Ziploc, they are usually easier to
handle.  Crowd it into a corner and the confined space will encourage the
fish to flatten out.  You could also shoot a little ethanol into there to
calm them further.

If you have the fish in a net (is this a hand net or a holding net?), have
a flat piece of plastic (slate) with a ruler glued on.  Hold the fish flat
against that.  If you have a stop plate for the nose, that improves
accuracy and also the speed.

Now that I think about it, maybe you need a bigger net?  If you have a
small dipnet style net, you might not have enough netting to secure the
fish and keep it flat?  Whenever I measured fish underwater I used a large
mono net or I had transferred them into a ca. 5 gallon sized floating
holding net.

Even faster (in the field at least) would be to have the flat piece of
plastic with a ruler and photo the fish—have a go pro running on your head
so the same person can do the handling and photographing.  Then just
flatten the fish against the plastic with the ruler in sight.  Later you
can go through the video and record the size.  This is helpful if you want
to measure additional components of size, like depth.

You could also hook up two lasers in parallel.  And mount them on a Go Pro
or video camera.  Then you don’t even need to capture the fish (assuming
you only need length).  You video the fish and as long as you get a good
lateral shot and the two lasers are both on the fish at the time, you can
measure length back in the lab.  This is sort of the quick and non-dirty
way.  (FYI I’ve never done this one, but it should work in theory.  It will
be trickier with damsels than say with a larger fish, but I still think it
would work with practice.)

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I normally use clove oil to catch them. So they fall "sleep" for few
seconds. Enough time to do a precise measurement.

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I know it can be tricky measuring even sessile organisms on SCUBA while
dealing with surge, etc.  This is just a thought but if there was a clear
device which you could gently sandwich the fish between and read
measurements from that would fix the problem of keeping them straight.   It
could even be slightly flexible so that it could bend along the fish like
those clear flexible rulers.  Maybe a crazy idea but hopefully helpful.

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I'm no fish person and have never tried this but, How about putting a
graduated board inside net and taking photos of fish against it. wonder if
the option of lifting net slightly out of water so that water empties
briefly and damsel fish are lying against board so you can take a photo of
it then.

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It might be worth practicing on some model fish to see if you can get your
eye in to estimate.  You will get to 0.5 to 1 cm accuracy fairly quickly.
It's a recognised technique and means you don't have to handle the fish.

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For measuring and tagging small fish in situ (on snorkel or SCUBA), your
best bet is to use an anesthetic. By far the simplest and cheapest method
is to use half an Alka-Seltzer tablet in a one litre zip-loc bag full of
seawater. That’s about 25% of the lethal dose - just enough to knock out a
fish 10 cm or so. Put the captured fish in the bag until it stops moving,
then measure and release.  Be careful not to swirl the bag around too much
or you will transfer water and dilute the anesthetic. Just crack it open
enough to transfer the fish in or out then re-seal it immediately.

The only problem with Alka Seltzer is that it induces acidosis and hypoxia,
which are thought to be painful to fish. The better (but more expensive and
harder to source) options are use clove oil (Eugenol), or 2-phenoxyethanol
(Aqua-Sed). Aqua-Sed is probably the best but as far as I know it needs to
be ordered from the UK. You can order it here:
https://www.fish-treatment.co.uk/aqua-sed/. Follow the dosage per litre on
the bottle and use a zip-loc bag as described above.

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Hi Malin - I have been working on damselfishes for most of my career. We
use a few techniques: 1) Transfer the fish to a free diver who then
measures the fish on a floating platform or transfers the fish to someone
who does (we sometimes have another person in the boat who measures and
tags the fish); 2) Use a thin plastic ruler that can bend with the shape of
the fish, or a piece of thread that you can later measure; 3) Measure the
fish in situ by hovering above it and noting a feature on the substrate
that is even with the snout and with the end of the tail, and then measure
the distance between those two points. This can be surprisingly accurate.

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Perhaps a quick photo of the fish with a ruler or scale bar in the frame -
if the damsel behaves and stays still long enough for a photo! Image
analysis software (ImageJ, etc.) could later be used to measure the FL.

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Small fish measuring boards work well. Just lay the fish down while in the
net. Commercially made ones tend to be on the larger side, so I ended up
making my own using pieces of a Plexiglas plate with a thin ruler glued on
top. Just cut a long/wide piece for the ruler and an end piece, then meld
together with. Here is a simple guide (http://www.wikihow.com/Glue-Plexiglas).
I believe that acetone also works.

Another strategy is to use a pvc pipe cut in half. There are commercial
versions as well but easy to make your own. Glue a cap to one end of the
pipe then (carefully!) cut in half lengthwise. Glue the rulers and you are
ready to go. Bonus is you have two boards. The diameter of the pipe will
depend on the largest size of fish you are measuring. I have used these too..

If not measuring a lot of fish and you need precise measurements, an
alternative is to build a small long flat chamber where you can squeeze the
fish in for measurement. The width of the chamber again will depend on the
largest fish you will measure. In this case you may need to build multiple
versions to accommodate a large range of sizes. Easily built out Plexiglas
too. I built them just to take pictures (already had boards) but by adding
a ruler on one side you can use for both purposes

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Not doubt you have received lots of replies already, but I just thought I'd
thrown my two cents in. Having measured 1000's of damselfish for my PhD and
helping out with lab field trips (Geoff Jones lab, JCU) I have found that
smaller fish (<15cm) are best measured inside a ziplock bag. I choose a bag
that’s big enough for the fish to fit into, but small enough that they
can’t wriggle around, and then hold them flat against my slate and measure
with callipers (this technique is also great for tagging them at the same
time, through the bag). When we catch anemone fish we do something similar
but inside of the catch nets. If extreme accuracy is important then I would
probably use a ziplock bag, take a picture with a ruler for reference and
then use ImageJ or something similar for an exact measurement.

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The best option for measuring damsel with less intrusion is to place them
against a white board with known measurement of grid lines and take
photographs. You can later analyze it using ImageJ or other imaging tools.

I do it for measuring corals and very delicate marine ornamental fish such
as seahorses and seadragons.

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Is squirting a little clove oil in the fishes face not an options? It
sounds like they need to be briefly anesthetized, we used MS-222 when
measuring Eviota out of the water with a pretty high success rate.

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