Tracking the Alaskan Red King Crab - Post 4

June 27, 2019

Alaska fisheries researchers are attaching acoustic tags to Alaska Red king crab and following their movement, with a saildrone. They will use this information to better understand Bristol Bay's red king crab distribution and habitat during the year.

Crab survey captain and crew with the first crab pot of the day!

The Alaska Red king crab survey captain and crew with the first crab pot of the day.

A crab pot is brought on board.

A crab pot is brought on board

Tagging Crab

Well, we set our crab pots in the right spot! Lots of red king crabs couldn’t resist the bait. We had plenty of choices on which ones to tag. I was worried that we might not get enough crab – it’s always nice when a research project goes to plan on the first try!

For this project we’re only tagging mature male crab, which means they have a carapace (top shell) length of at least 120 mm (4.7 inches). When we brought up a pot, all the female and small male crabs were thrown back quickly. We kept the large males in a tank of nice, cold water on our boat until they were tagged. We wanted to keep them as healthy and happy as possible.

Tagging a crab is a bit tricky because every year or two crabs shed their shell and grow a new one in a process called molting. So, we can’t just glue an acoustic tag to a crab’s shell if we want it to stay with the crab for a long period of time. Instead we give each crab a piercing – kind of like for an earring – but, through a muscle at the back of the crab. A piece of plastic tubing goes through the piercing (pink in the photo) and the acoustic tag hangs from the tubing.  Now, when the crab molts, the tag stays attached.  I tested this technique in our lab, and it worked great!

A crab gets an acoustic tag “earring”, hanging from the muscle at the back of its shell.

A crab gets an acoustic tag “earring”, hanging from the muscle at the back of its shell.

To get all 150 crabs tagged we played a game of “pass the crab”. As a crab was passed down the line we measured it, assigned a tag number, and attached the acoustic tag. Luckily, it was a nice, calm, sunny day.  If the boat was really rocking around, I probably would have stabbed my fingers more with the needle that I use to tag the crab!

The crab tagging assembly line: 1) Madison Shipley measures a crab, 2) Charlie Heller holds a crab while Leah Zacher pierces the muscle, and 3) Julia Dissen attaches the acoustic tag.

The crab tagging assembly line: 1) Madison Shipley measures a crab, 2) Charlie Heller holds a crab while Leah Zacher pierces the muscle, and 3) Julia Dissen attaches the acoustic tag.

Once we got all the crabs tagged, we released them in the same exact spot where they were caught.  Since we’re looking at how the crabs move, we didn’t want to confuse their little crabby brains by releasing them in a different spot from where they were captured!  Now our crabs are back on the seafloor with just a little extra decoration attached to their backs!  I can’t wait to re-find them in October with the Saildrone – it will be so interesting to see where they’ve gone.

Releasing tagged crabs.

Releasing tagged crabs.

Red king crabs tagged with a small (left) and large (right) acoustic tag.

Red king crabs tagged with a small (left) and large (right) acoustic tag.

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