Crab Tagging Study: Adult male Dungeness crab (Metacarcinus magister) movements near Reedsport, Oregon from a fisheries collaborative mark-recapture study.

Abstract

Oregon’s fishing community is concerned about the potential effects that wave energy conversion devices could have on Dungeness crab movement, behavior and harvests. Though crabbing has been a mainstay of West Coast economies for decades, little information has been collected or analyzed on adult Dungeness crab movement, in general, and for the Oregon Coast, specifically. To help begin addressing this critical information gap, The Oregon Wave Energy Trust (OWET), the Oregon Dungeness Crab Commission (ODCC) and Oregon Sea Grant added value to a planned baseline study conducted by H.T. Harvey and Associates. Crabs collected for the baseline study were tagged after they were measured and before they were returned to the ocean. Commercial crabbers later returned the tags, and were rewarded for collaborating in the project. Methods: Legal sized male crabs (6 ¼” carapace width) were tagged at three locations near Reedsport, Oregon, in October and November of 2009. A total of 2788 tags was deployed and crabs were recaptured in the commercial crab fishery. Tags were returned with recapture locations to Oregon Sea Grant and a $20/tag reward was provided. Results: Out of 2788 tagged crabs released, 952 crabs were recaptured and the tags returned to Oregon Sea Grant. A total of 626 tags was returned with location data of sufficient detail for analysis. The overall return rate of 34.15% is notably high for a mark-recapture study; however, this is not unreasonable because a high percentage of male crabs is harvested during each season. Additionally, a historic Oregon crab tagging study found a similar return rate. Crabs travelled distances ranging from 0.27 km to 90.68 km (Table 1). 65% of crabs traveled less than 20 km, 77.7% of crabs traveled less than 30km, and 95.5% of crabs traveled less than 50km (Figure 1). Crabs moved primarily in the alongshore direction, with minimal across shelf movement (Figure 4). See enclosed maps of recapture locations. Caveats: Because commercial crabbers target their fishing effort based on past experience, it is likely that some locations had a higher chance of being sampled than others. This sampling bias might inflate the tag recovery rate in those areas and has the potential to skew our results. That said, the tag recapture locations should not be interpreted as reflecting the distribution of fishing effort in the Reedsport study area. The distances reported here represent a minimum distance that the crab traveled. Additional tags were returned with general locations indicating that crabs may be moving as far north as Tillamook, Oregon (approximately 140 miles). It remains unknown to what degree Dungeness crab movements vary from year to year. The findings of this study should be treated as preliminary rather than definitive. Additional studies are needed to fully characterize adult Dungeness crab movements in Oregon’s coastal waters.