ESNERR has developed a new monitoring program focusing on some of the most characteristic organisms of the Slough: invertebrates living in the mudflats. These creatures have long intrigued local scientists, beginning with George MacGinitie in the 1920s. There have been many classic studies on the invertebrates in Elkhorn Slough, yet there has been no consistent monitoring of their populations over time. Our invertebrates deserve more attention because they are inherently interesting, as well as being some of the best indicators of the health of our estuary. Unlike transient shorebirds, fish, or marine mammals, the invertebrates found in the Slough are typically resident species that are found exclusively in the few remaining sheltered estuarine mudflats along our coast.

The modest goals of this monitoring program are to detect:

1) dramatic changes in abundance of key species (for example – a 50% increase or decrease);

2) changes in species diversity (for example – the disappearance of rare native species or the appearence of a new invader and;

3) changes in distribution (for example – species formerly found only near the mouth may extend their ranges further up the Slough as tidal erosion flushes fine sediments from upper Slough areas).

Invertebrate Monitoring

We are focusing our monitoring efforts on two kinds of large, relatively easy to identify invertebrates. The first are crabs, which can be easily caught in baited traps. Only a half dozen crab species are common, and they are easy to recognize. The second group of interest is large burrowing invertebrates, such as gaper clams, fat innkeeper worms, and ghost shrimp. Each of these species form distinctively shaped openings to their burrows which, with a little practice, can be distinguished while walking along the surface of the mud.The monitoring protocols for this program have been developed by ESNERR staff Susanne Fork and Kerstin Wasson, interns Rani Gaddam, Elvie Hall, and Jenn Everly, and have been used by CSUMB and Hopkins Marine Laboratory classes as well as ESNERR volunteers.


Overall, crab abundances are highly variable over time. However, during the first years of observation, abundance of green crabs (a nonnative introduced into San Francisco Bay in 1989) increased and was relatively high, while native crab abundances dropped over this period. Since 2006, green crab abundance in Elkhorn Slough has been very low and native crabs seem to be on the rise again.

Gaper clams and Fat innkeeper worms
Data from the first years of this monitoring program show considerable inter-annual variation between sites as well as between seasons and no obvious trends. (Monitoring began at the MLWA site in 2006).

How to get involved
If you would like to join us on one of our organized crab surveys or mudflat burrow monitoring excursions, please contact Susie Fork. No experience is required, just careful attention to detail and enthusiasm about wetland natural history. We will contact you to let you know the date of the next volunteer survey when it is about a month away. If you are a high school or college instructor interested in participating in this monitoring program, we’d be delighted to provide you with all the necessary field equipment, data sheets, and identification guides. We also will join you in the field for your first excursion to provide training; thereafter, you can continue with future classes on your own. We ask that you carry out the protocol consistently and send us your data; in return, we’d be glad to send you the database of all previous years to carry out analyses with your class.