Temperature and Salinity Constrain Mitten Crab Abundance
Professor of Biology
California State University, Fresno
T. (559) 278-4244
September 28, 2010
April 8, 2008
Contact: Christina S. Johnson, 858-822-5344, firstname.lastname@example.org
According to new results from a Sea Grant study, Chinese mitten crab zoea cannot tolerate waters colder than 11.7 degree Celsius. The zoea (larvae) also appear to thrive only when salinities in San Pablo Bay in northern San Francisco Bay are between 10 and 22 parts per thousand.
“There is a window in which these zoea can survive,” said Cal State Fresno biology professor Brian Tsukimura. “When the window is small, you are going to have small adult mitten crab populations four years later. (It takes about four years for zoea to develop into sexually mature adults.) When the window is big, you can expect a large adult population later.”
The window is defined on one end, at the beginning of the zoea life cycle, by temperature, Tsukimura said. Zoea cannot survive winter temperatures in San Pablo Bay, which is reasonable since the crabs are native to warmer waters off China. If mitten crab eggs hatch before the bay has warmed to 11.7 degrees, which usually occurs in March, the zoea die.
It takes about two months for zoea to develop into the next life-history stage, megalopa (post-larvae). As zoea near metamorphosis, their survival window is defined by the salinity in San Pablo Bay, he said. Zoea perish if the salinity drops below 10 ppt. However, if salinity tops 22 ppt, plankton-eating anchovies can enter the bay and prey on them. Water diversions in the Sacramento-San Joaquin Delta raise salinity levels in San Pablo Bay and likely help reduce adult mitten crab numbers, he said.
Tsukimura’s research, which is now being funded by the U.S. Fish and Wildlife Service, is based, in large part, on analyzing the contents of plankton trawls in San Pablo Bay. (The surveys are conducted by the California Department of Fish and Game). To identify mitten crab zoea from zoea of other crab species, such as the European green crab, another invasive, he and students developed a zoea key for 14 common crab species. A key of megalopa is under development. The keys will allow biologists to better monitor and predict abundances of crabs in the region.