Trouble for Our Sandy Beaches?

LiMPETS Program Reveals Mole Crab Population in Decline

By Amy Dean

Student Scientists monitoring for Mole Crabs
Credit: FMSA

This school year, the teachers and student scientists that participate in the Sanctuary’s LiMPETS program (Long-term Monitoring and Experiential Training for Students) have been sifting sand along our beaches in search of the Pacific mole crab. Mole crabs—also known as sand crabs—are often the most abundant and important herbivore on our beaches. They are vital to the sandy beach food web.

Why monitor mole crabs? For one, the data is important to the Sanctuary in the event of an oil spill - like the large spill that occurred in the San Francisco Bay on November 7th. By monitoring, students provide important baseline data that the Sanctuary can use to determine the extent of environmental impact or damage that spilled oil has on our sandy beach habitats.

The program also has great value because it is a hands-on, scientific experience that connects students with the ocean. Students take their work seriously and collect data carefully because they know they are involved in a real scientific endeavor.

“I think it’s cool that our data is actually going to be used in a real study”, says student Eliana Pera. 

Beyond the data collection, students gain a new sense of wonder and awe of the natural world.

“I didn’t realize just how much fun the entire trip would be. I never thought I would get so excited when I caught a sand crab”, says a student from Lick-Wilmerding High School, Sarah Roach.

Over one thousand students from Bodega Bay to Los Angeles have been monitoring mole crabs on beaches since the inception of the program in 2001. The data collected by these hard-working student scientists have revealed a trend. Pacific mole crabs are decreasing in abundance along many Bay Area beaches.

Teacher Dai Own and his oceanography class at Marin Academy have monitored mole crabs on Stinson Beach since 2001. Their data has shown a continued decline in abundance since 2002. These findings are by no means minor, and represent a 90% decrease in abundance between 2002 and 2007.

High School interns at the California Academy of Sciences have been monitoring Ocean Beach in San Francisco since 2002. They too have found record lows in the numbers of sand crabs on the beach in the past two years. In previous years, abundance typically ranged from 100 to 300 crabs / m2.  In 2007, annual abundance dropped to 6 crabs / m2.

“Though we do not know the cause of these declines, we do know that fewer sand crabs means less food for the birds and fish that prey on the crabs,” explains Sara Heintzelman, Education Specialist for the Farallones Marine Sanctuary Association.  “Continued monitoring is very important to help us understand these trends and to determine whether this data reflects a natural cycle in the sand crab population or is a red flag signaling trouble for our sandy beaches.”