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Research Technical Report

Trapping Results of Green Crabs (Carcinus maenas) in Elkhorn Slough

Estelle, V.B. (May 2000)

Report to the Monterey Bay National Marine Sanctary. Contract Number 40ABNC901298.


The European Green Crab (Carcinus maenas) is one of three exotic crab species introduced in the United States. It arrived in the US in San Francisco Bay around 1989, presumably in ballast water carried by commercial ships (Cohen et al. 1995), and was first observed in Elkhorn Slough by 1994 (Grosholz and Ruiz 1995 and Carlton, pers. comm.). The crab is a generalist predator that forages on numerous invertebrate taxa including, but not limited to, native bivalves, crustaceans, and polychaetes (Cohen et al. 1995, Jamieson et al. 1998). In the US, the species is found in estuaries and embayments, and its distribution on the West Coast of the US is now (as of 1999) from Barkley Sound, BC, Canada to Morro Bay, CA, USA (Grosholz, pers. comm.). Up until 1998, the southern-most extent of its distribution was Elkhorn Slough National Estuarine Research Reserve (ESNERR) in central CA. This introduced species is of particular concern throughout its range in the US because of its demonstrated ability to alter the biological diversity and abundance of native invertebrates (Grosholz et al. In Press), and thereby, its potential to affect multiple trophic levels in marine food webs.

The most extensive research on the green crab's effect on invertebrate communities in the US has been conducted in Bodega Harbor, CA. Coordinated surveys and monitoring efforts have also been done in the West Coast NOAA NERR sites (Padilla Bay, WA, South Slough, OR, Elkhorn Slough, CA, and Tijuana Estuary, CA) (Grosholz, pers. comm.) during 1998-1999. To date, it is known that the crab exists in two NERR sites, Elkhorn Slough, CA and South Slough, OR. The establishment of a reproductively viable green crab population in Elkhorn Slough NERR has necessitated the study of this species and its potential effects on the invertebrate and vertebrate communities of this reserve. To date, Elkhorn Slough has been sampled only cursorily for green crabs, and knowledge about its population status and effects in this reserve are limited.

In collaboration with, and supplementary to, the work of Grosholz in Elkhorn Slough, I was contracted by NOAA / MBNMS to trap for green crabs and assess their potential impacts on shorebirds in Elkhorn Slough Reserve during the fall of 1999. Herein, I report the trap results for green crabs, and then discuss improvements and future directions for the current research. This report is a summary of the work conducted under contract number 40ABNC901298.


Green Crab Trapping

During November and December of 1999, I conducted 4 days of trapping (n= 66 traps baited and deployed) for green crabs in the main and side channels of Elkhorn Slough, and within the Long Valley Loop of Elkhorn Slough NERR. I baited and deployed 12 collapsible traps on the mudflats of the Long Valley Loop of ESNERR during Nov 2-3, 24 traps at Kirby Park during Dec 2-3 (12 traps set at ~ -0.1 ft above mllw and 12 traps set at +0.7 ft above mllw), and 30 traps in the main and side channels of Elkhorn Slough during December 4-6. Trapping locations within the main channel of Elkhorn Slough included the Vierra Property near the mouth of the slough (n = 6 traps), and both sides of the main channel near the Moonglow Dairy property (n = 12 traps on each side of the channel). The traps I used are approximately 91 cm x 62 cm x 28 cm; they are made of soft, black nylon mesh and are collapsible, making them easy to transport and deploy. These traps are used now by other green crab researchers on the West Coast, and do not appear to differ from the previous trap type used by Grosholz et al. (1995) with respect to capture rate of individuals or sexes (Grosholz and VBE, pers. obs.). When deploying traps, I tied each to a 1.6-cm diameter dowel rod with a 2.5-m length of string and anchored the dowel in the mud flat; I placed traps along a line parallel to the tide and approximately 20 m apart on all mudflats. I secured traps at tidal heights ranging from +1.3 ft above mllw (Long Valley Loop mudflat) to +0.7 ft above mllw (all other sites).

Bird Exclosure Experiment

During November 19-24 and December 2-5, 1999 and January 22-23, 2000, Ialso conducted a pilot experiment to evaluate the forage of shorebirdson a mud flat near Kirby Park in Elkhorn Slough. At a tidal height of+0.7 ft above MLLW, I established a bird exclosure experiment. Becauseinvertebrate abundance and distribution was unlikely to be homogeneous at this tidal height, I stratified the placement of exclosure/controlpairs of 'cages' along a 78-m line parallel to the tide. In total therewere 6 strata in the experiment, and each stratum was composed of an exclosure 'cage' and a control 'cage'. These 'cages' were randomly ordered (by the flip of a coin) and placed 1 m apart within a stratum. The 'cages' enclosed a 0.5m2 square area of mudflat, and they prevented birds from foraging in this area while allowing other potential predators (invertebrates and fish) access. I constructed each bird exclosure by placing 8, 46-cm long PVC pipes (1.6 cm in diameter) along the perimeter of a 0.5 m2 area on the mud flat. I wound string among the tops of the pipes, which extended 10 cm above the mud surface, to keep birds from feeding in this area. Each control cage was composed of 4 pieces of 46-cm long PVC pipes, placed at the corners of a 0.5m2 area. I did not attach string to these pipes so that these control areas were open to bird foraging.

I collected 2 core samples (10 cm x 15 cm) immediately outside, and on either side, of each exclosure and control cage at the beginning of the experiment (before foraging) and one core sample from the middle of each exclosure and control cage six weeks later at the end of the experiment (after foraging). I divided each core sample into 0-5-cm and 5-15-cm sections, as measured vertically downward from the surface of the mud, and processed their contents in a 1-mm mesh sieve. I used 10% buffered formalin to fix the sample remains, and then I transferred them to 70% ethanol for storage and future analysis.


Crab Trapping

During the trapping period of November - December 1999, I caught 6 green crabs. I caught 4 males (67, 69, 65, and 70 mm maximum carapace width) at the Kirby Park boat ramp between the culvert and the boat ramp on December 4. One of the males had two small, re-generating chelipeds. I caught two female green crabs, one gravid, on December 6 near the Moonglow Dairy. One of these females (42 mm, not gravid) was caught on the mudflat just below the Moonglow Dairy, and the other (49 mm, gravid) was caught on a side-cut channel also near the dairy. No green crabs were caught at any of the other locations.

Bird Exclosure Experiment

I have not yet analyzed the sediment samples from this pilot experiment. I have 72 samples from this experiment, which I plan to analyze for abundance of invertebrates during the summer of 2000.


Green crabs have been noted in Elkhorn Slough since 1994 (Grosholz and Ruiz 1995), but they have not been intensively studied at this location. The current estimation of how numerous the species is and where it is primarily located in this estuary are largely speculative. 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.). The fact that green crabs are reproductively established in Elkhorn Slough is agreed upon, however, and this is evidenced by the presence of gravid females, commonly-found molted carapaces, and the observation of individuals ambulating on a mudflat in shallow water near the Long Valley Loop boardwalk (Carlton, pers. comm.).

Research specifically on the green crab in Elkhorn Slough has been limited to annual sampling events for monitoring purposes. For example, Grosholz has captured green crabs most recently in the main channel of Elkhorn Slough during August 1998 (n = 30 crabs, primarily at Kirby Park) and August 1999 (n = 15 crabs, primarily at Kirby Park). Originally when I began trapping in the early winter in Elkhorn, I wanted to verify that green crabs were present at this time, and that they could be caught in the same sites and at tidal heights where Grosholz had trapped them during the summer. Because I did not capture any crabs at Long Valley Loop (a place where green crabs have been seen and captured in the past), I became concerned that there might be a seasonal or behavioral component effecting the capture of green crabs. An alternative as to why I did not catch green crabs or other species could also be that I set the traps too high in the intertidal. I placed traps at the same tidal heights, however, where Grosholz and I sampled in August 1999, and at which we captured green crabs. In order to check for seasonal or behavioral difference between late, summer and early winter, I set traps intensively at Kirby Park, a location where green crabs have been most abundant in the past. Additionally, I set the traps at two tidal heights to verify that the crabs would enter traps during this time. My results indicate that there is likely an important seasonal, behavioral, or physiological component in the capture of this species, which could confound efforts to delineate its spatial distribution and abundance if trapping events are held during the winter. Given that colder water temperatures in the winter may cause green crabs to slow down metabolically, and thus potentially be less 'trappable', it is probably advisable to trap for this species during the late summer and early fall when water temperatures are warmer. If there is a real seasonality to their distribution on the mudflat, then this may also mean that shorebirds and green crabs are less likely to be in competitive contact with one another during the winter months. If green crabs do not utilize the same intertidal mudflat areas that shorebirds do during the winter, this still may not diminish the potential of competition between them because green crabs forage during the spring, summer, and fall in the same intertidal areas used by birds in the fall and winter (as noted in Bodega Harbor).

In order to determine where green crabs occur most frequently in the Slough, extensive and simultaneous trapping events would have to be undertaken at a time when there is potential for a high capture rate. In Bodega Harbor, Grosholz and Ruiz (1995) determined the tidal height at which crabs were most abundant (~+0.7 ft above mllw) by conducting a harbor-wide trapping effort. They did not detect any seasonal differences in capture rate in Bodega Harbor. They deployed pitfall traps (5-gallon buckets buried flush with the mudflat surface) in a grid on a variety of mudflats throughout the year. The exceptionally soft substrate of Elkhorn Slough mudflats is likely not amenable to pitfall trapping for this species, but one could set many collapsible traps simultaneously throughout Elkhorn Slough and at various tidal heights in order to determine green crab distribution.

I was unable to catch enough green crabs in order to conduct the crab enclosure experiment. Not having holding facilities for crabs meant I had to catch all the crabs I needed over a one or two day period, and this did not happen. I did construct crab enclosure cages (12 of these, 0.5m2) in preparation for the crab portion of the experiment, and I eventually transported these to Bodega Harbor where I ran a foraging experiment for birds and crabs at the same time as the foraging experiment for birds in Elkhorn Slough. Although this was unintended, I will be able to compare shorebird foraging during the same time period between Bodega Harbor and Elkhorn Slough, and I will be able to verify crab predation impacts during the winter in Bodega Harbor.

A comparative study on the effects of green crabs between Elkhorn and Bodega Harbor may be a worthwhile approach for additional research on the green crab in Elkhorn Slough because of its lower abundance in Elkhorn Slough. The extent to which results between Bodega and Elkhorn are comparable and result in generalizations will likely depend on the measurement and incorporation of physical variables in this research. By comparing Elkhorn and Bodega, which differ physically and with respect to green crab abundance, and incorporating physical variables, it may be possible to develop and test generalizations about green crab effects at both sites.


This project was made possible by funds from the Monterey Bay National Marine Sanctuary (MBNMS) and the American Museum of Natural History's Lerner Gray Fund for Marine Research. The work was enhanced by the staff and facilities of ESNERR and MBNMS, including the interest of Mark Silberstein and Andrew DeVogelaere, volunteer effort by Karen Grimmer of MBNMS, and logistic support by Kenton Parker and Martha Nitzberg of ESNERR. Ted Grosholz, UC Davis, provided field equipment and advice; Rico Tinsman, Bodega Marine Lab, and Julian Ellis assisted in the set up of the bird exclosure experiment and collected the final cores for that experiment. Joe Turk provided housing for VBE.


Cohen, A. N., J.T. Carlton, and M.C. Fountain. 1995. Introduction, dispersal, and potential impacts of the green crab Carcinus maenas in San Francisco Bay, California. Marine Biology 122:225-237.

Grosholz, E.D. and G. M. Ruiz. 1995. Spread and potential impact of the recently introduced European green crab, Carcinus maenas, in central California. Marine Biology 122:239-247.

Grosholz, E. D., G. M. Ruiz, C. A. Dean, K.A.Shirley, J. L. Maron, and P. G.Connors. 2000. In Press. The impacts of a nonindigenous marine predator in a California bay. Ecology.

Jamieson, G.S., E.D. Grosholz, D.A. Armstrong, and R.W. Elner. 1998. Potential ecological implications from the introduction of the European green crab, Carcinus maenas (Linneaus), to British Columbia, Canada, and Washington, USA. Journal of Natural History 32:1587-1598.

Reviewed: April 11, 2024
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