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My First Crab Survey with AWT

Crab survey at Braye - Credit: Thanh Doan

Ever wondered what lies beneath the rocks on Alderney's shores? My first crab survey with the Alderney Wildlife Trust (AWT) was a fun adventure!
Lou measures the carapace of a furrowed crab - Credit: Thanh Doan

Lou measures the carapace of a furrowed crab - Credit: Thanh Doan

‘That’s a really big one!’ I exclaimed in delight at the size of an Edible crab under the rock that Lou has just turned.

Last Wednesday, I joined Mel, the AWT Living Seas Coordinator, and Lou, a seasoned citizen scientist, for my first ever crab survey at Braye's rocky shore (east end) during low tide.

To start with, a transect line was set from the shore to the water mark. Lou then walked along the transect line, stopping every 2 meters to selectively turn a big rock near the transect line. The team identified and recorded crabs and other species found under each rock. We found plenty of crabs, mainly broad-clawed porcelain, furrowed, and a few Chancre crabs. The furrowed and Chancre crabs were then put into separate buckets, measured one by one and then carefully put back to where they were.

Record data from the crab survey - Credit: Thanh Doan

Record data from the crab survey - Credit: Thanh Doan

The data collected for each crab included species name, male/female, presence of eggs, measurements of the carapace, crusher, and the distance between two hind legs, as well as an assessment of their health (a healthy crab receives index 3, while an unhealthy one receives index 4). This detailed data helps track population changes, understand the health of the crab population, and gain valuable insights into the overall health of Alderney's rocky shores.

Turn a big rock near the transect line - Credit: Thanh Doan

Turn a big rock near the transect line - Credit: Thanh Doan

Put crabs into separate buckets before measuring them

Put crabs into separate buckets before measuring them - Credit: Thanh Doan

Measure the height of the crusher - Credit: Thanh Doan

Measure the height of the crusher - Credit: Thanh Doan

Crab survey at Braye - Credit: Thanh Doan

Crab survey at Braye - Credit: Thanh Doan

I was taught how to hold crabs and how to measure them! It was super fun! It takes skill to measure pinchy crabs and patience and dedication to record data regularly and precisely. The crab survey data has been collected for over 2 years, with the survey conducted roughly every 3 months.

A furrowed crab pinned and held onto the ruler, so Lou had to use another one

A furrowed crab pinned and held onto the ruler, so Lou had to use another one. Credit: Thanh Doan

Apart from crabs, we spotted many marine species like shore clingfishes, super-fast gobies, well-camouflaged shannies, oyster thieves, scaleworms, barnacles, sponges, painted top shells, flat top shells, dog whelk eggs, and, of course, various seaweeds.

Dog whelk eggs - Credit: Thanh Doan

Dog whelk eggs - Credit: Thanh Doan

Oyster thief - Thanh Doan

Oyster thief - Credit: Thanh Doan

Scaleworm - Credit: Thanh Doan

Scaleworm - Thanh Doan

Shore Clingfish

Shore Clingfish - Thanh Doan

Taking part in the crab survey made my day!

Joining a citizen science project with the AWT is a fantastic way to contribute to vital research and learn more about Alderney's incredible marine life. Whether you're interested in crabs, Pacific oysters (an invasive species), eelgrass, or diving with Seasearch, contact Mel at marine@alderneywildlife.org and get involved!

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