Technical Report No. 28

MONITORING FOR INVASIVE CRABS IN SEAGRASS HABITAT IN THE PADILLA BAY NATIONAL ESTUARINE RESEARCH RESERVE, SKAGIT COUNTY, WASHINGTON

Sharon R. Riggs

September 2003

Bibliographic Citation
Riggs, Saron R. 2003. Monitoring for invasive crabs in seagrass habitat in the Padilla Bay National Estuarine Research Reserve, Skagit County, Washington. Washington State Department of Ecology (Publication No. ), Padilla Bay National Estuarine Research Reserve Technical Report No. 28. 27 pp.

Abstract
Two methods of monitoring for invasive crab were compared: minnow trap collection and shallow trays lined with 3-mm mesh. The traps (3) and trays (7) were set along one transect on the eastern shore of Padilla Bay for one year. The traps were deployed 24-hours prior to tray collection and tray collection occurred once a quarter.

The minnow trap method took two hours per quarter whereas the tray method took 60-90 hours per quarter. The total number of crabs caught in minnow traps was low (63) compared to trays (9,326). In this study, minnow traps caught crabs ranging in carapace width from 15.5 to 85mm while the trays caught pagurids ranging from 0.9 to 8.8mm shield length and other crabs (Cancer magister, Hemigrapsus oregonensis, Telmessus cheiragonus and Pinnixa tubicola) ranging from 2.3 to 71.1 mm carapace width. No invasive crabs were caught with either method which would indicate there are no invasive crabs at this site in Padilla Bay.

Baseline data for native crabs was obtained from the tray method. The greatest number of new recruits (< 6.0 mm SL) for Pagurus hirsutiusculus were present in April, the highest number of females without eggs were present in October and the highest number of females with eggs were present in April. For Pagurus granosimanus, the greatest number of new recruits (< 6.0 mm SL) were present in October and the highest numbers of females with eggs were present in January. The greatest number of Hemigrapsus oregonensis were present in January and gravid females were only present in July. The highest number of Cancer magister were present in January. No females were present during any sampling and no C. magister were present in the trays in July.

Pagurids were found exclusively in non-native snail shells (Batillaria, Nassarius). Parasites were present on Pagurus hirsutiusculus (bopyrid isopods and rhizocephalans).