Dungeness crabs are a Bay Area holiday tradition. But long before these tasty crustaceans grace our dinner plates, they’ve made an amazing journey. They start in the Pacific Ocean, migrate at least 20 miles to San Pablo Bay, and later walk back along the bottom of the San Francisco Bay all the way out the Golden Gate.
From now through January, Dungeness crabs will be hatching from eggs offshore in the Pacific. Once hatched, tiny crab larvae float on ocean currents, eating plankton. At night, they rise toward the surface, where they find more food. During the day, they stay 75 feet below, where they are less likely to be eaten. The larvae develop through seven stages. In the later stages, they shed their shells and grow new ones to accommodate their larger bodies. They look like small crabs by May.
Some of these crabs stay in the ocean, but many more ride the tides or hitchhike on the tentacles of jellyfish into San Francisco Bay. Jellyfish are especially good transportation because the crabs get protection from predators like salmon and octopus.
After Dungeness crab youngsters swarm in the Golden Gate, they migrate all the way to San Pablo Bay. They spend the summer crawling along its soft, shallow bottom, or in nearby tidal marshes, where they usually find plenty of littler fish and shellfish to eat. If there’s not enough food, they will also eat smaller Dungeness crabs. The little crabs shed their shells and form new shells about 12 times until they grow to the equivalent of teenagers.
In the fall, these "teenage" crabs crawl along the Bay’s bottom to the deeper channels of the central Bay and out the Golden Gate. By then, they are about four inches wide. If you or I were to travel on foot the same distance in proportion to our size, we would walk from San Francisco to San Diego.
Once they’ve returned to the cold waters of the ocean, Dungeness crabs grow to their full adult size. Crabs that mature in the rich waters of the Bay grow faster than anywhere else along the Pacific Coast. A Dungeness crab that grows up in the Bay is usually large enough to be legally caught and eaten by the time it is three years old. Crabs that grow up in the ocean need at least four years.
Dungeness crabs have been making their journey for generations. Now, however, a new threat looms—excessive sand mining. Normally, sand washes down from the Sierra Nevada Mountains, into the Bay, and out the Golden Gate. Sand miners dredge the Bay’s floor to extract sand for use in making concrete. But in recent years, more sand has been taken out than washes down from the mountains.
Even so, the state wants to allow a massive increase in the amount of sand that can be mined. The problem for crabs is that most of the sand is mined from the central part of the Bay. Miners may be scooping up sand right in the middle of the pathway of teenage crabs trying to crawl out of the Bay, on the way to fattening up in the ocean. That’s one reason Baykeeper is suing the State of California to prevent excessive sand mining in San Francisco Bay.
Another deadly threat to Dungeness crabs is pesticide pollution. You can help keep the Bay’s crabs safer by not using pesticides in your yard, because rain washes these chemicals into the Bay or its tributaries.
Here’s to a healthy San Francisco Bay—with lots of happy young Dungeness crabs—in 2014!
Photo by Dan Hershman (Flickr/CC)