Sanctuary Integrated Monitoring Network
Monitoring Project

Invertebrate Monitoring in Elkhorn Slough

Principal Investigator(s)

  • Kerstin Wasson
    Elkhorn Slough National Estuarine Research Reserve
Start Date: January 01, 2001

The goals of the ESNERR invertebrate monitoring program are to detect:
1) dramatic changes in abundance of key species (e.g., changes of 50 percent or more);
2) changes in species diversity (e.g., the disappearance of rare native species or the appearance of a new species; and
3) changes in distribution (e.g., species formerly found only near the mouth of the Slough may extend further up the Slough as tidal erosion flushes fine sediments from upper Slough areas).

The invertebrates found within the Elkhorn Slough deserve more attention because they are inherently interesting, as well as being some of the best indicators of the health of the 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.

We are focusing our monitoring efforts on two groups 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 differentiate. The second group of interest is large burrowing invertebrates, such as gaper clams, fat innkeeper worms, and ghost shrimp. Each of these species forms distinctively shaped openings to their burrows that can be identified, with a little practice, while walking along the surface of the mudflats.

Summary to Date

Green crabs have been noted in Elkhorn Slough since 1994 (Grosholz and Ruiz 1995). It is believed that the green crab population in Elkhorn Slough is not as numerous as that found in Bodega Harbor. The presence of sea otters (Enhydra lutris), which forage to some unknown extent on green crabs, in Elkhorn Slough has been offered as a speculative explanation as to why the green crab population has not yet reached that of Bodega Harbor (A. DeVogelaere, Grosholz, S. Benson pers. comm.). Reproductive green crabs have been found in Elkhorn Slough and it is presumed that the population is self-sustaining.

For more information on green crabs go to

Monitoring Trends

  • Native grapsid crabs dominate in Elkhorn Slough, however non-native European green crabs became increasingly abundant after the initial introduction. No other non-native crabs have been detected.
  • By 2008, European green crab numbers have stabilized or slightly decreased.


How to get involved:
If you would like to join ESNERR staff 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. ESNERR staff 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 highschool or college instructor interested in participating in this monitoring program, ESNERR would be delighted to provide you with all the necessary field equipment, data sheets, and identification guides. ESNERR staff will also join you in the field for your first excursion to provide training; thereafter, you can continue with future classes on your own. Please carry out the protocol consistently and send ESNERR your data; in return, ESNERR can send you the entire database to carry out analyses as a class exercise.

Study Parameters

  • Non-indigenous species
  • Abundance
  • Diversity
  • Distribution
  • Size structure
  • Sex ratio

Study Methods

baited minnow traps

Figures and Images

Figure 1. This is the invasive European green crab Carcinus maenas.

Figure 2. Collecting data on the mudflats.