Dungeness Crab Harvest Trends
Over the past dozen years, Dungeness crab (Cancer magister) harvest in Puget Sound has more than doubled. Crab populations in the Whidbey Basin and Northern Puget Sound seem to be doing fine, but in Hood Canal catch has fallen from 698,000 pounds in 2005, to just 168,000 pounds in 2008. Catch has recovered somewhat in north Hood Canal, but remains very depressed in southern Hood Canal.
Crab Management Assumptions
For any other species, intense fishing followed by a precipitous drop in the population would indicate a collapse due to over-fishing. But crab management is supposed to prevent such a collapse. The time tested "3 S" management approach, which limits the fishery to size (6.25 inch), sex (males only) and season (closed during molting), should allow harvest of all legal sized males with no impact to the sustainability of the fishery. In theory, there should be enough sexually mature, sub-legal sized (not fished) males to mate with all the females. All the males that are legal fishing size are considered a harvestable surplus. Fifty years of Pacific Coastal crab catch data show wide swings in abundance, but the magnitude of the Hood Canal decline is unprecedented, leaving fisheries managers and crab harvesters alike very concerned.
Possible Causes of the Decline
In looking for the cause of this decline we first started looking into the basic assumptions of crab management. Are all large males superfluous? Are there enough sexually mature males left after the fishery to mate with all receptive females? Are the smaller males that are left, big enough to mate with the large females? These questions are being addressed in an ongoing mating success study. Details of this study and preliminary results were presented in a poster at the Puget Sound Georgia Basin Ecosystem Conference in February. The poster has been reformatted for viewing on this site.
The decline in the crab population may also be related to the low dissolved oxygen levels in Hood Canal that caused fish die-offs. Such environmental effects are another potential cause which needs to be looked at.
Project Goals
In this project we are examining origins of the larval crab supply and the natural fluctuations in that supply. The Dungeness crab life cycle includes a very long, 4 to 6 month, pelagic larval phase. On the Pacific Coast, they drift over a hundred kilometers out to sea and back on the currents. Juvenile crab which originated as Pacific coastal larvae, have been documented in several Puget Sound areas (Dinell et. al. 1993). Megalopae from the Pacific Coast have been documented in the San Juans (Orenzans and Galuchi ). But thus far, we don't know if or how often they drift into Hood Canal.
Implications for Fisheries Management
If most of the crab larvae drift in from outside Hood Canal, the fishery may have little impact of the overall health of the crab population in Hood Canal. However, if most larvae originate in Hood Canal, the Hood Canal crab fishery could be having a much greater impact on the health of the population. Several years of observations will be needed to map larval settlement patterns. But with the help of dozens of volunteers watching for megalopae, we hope to increase our understanding of the larval dispersion patterns to improve management of the crab fisheries.
Additional Information
The sources of those papers cited above are listed in the attached, Draft Quality Assurance Project Plan which covers this project in great detail.
Step by step direction can be found on the Megalops Collectors Instruction page.