Evidence for multiple recruitment-cohorts of Puget Sound Dungeness crab, Cancer magister

Abstract

While sampling intertidally in Puget Sound, Washington, USA, for juvenile Dungeness crab (Cancer magister) in 1984, we found evidence of two distinct “cohorts” of the same year-class based on sizes of first-instar juveniles (J1) and the spatial/temporal patterns of settlement. In 1988, three distinct cohorts were observed to settle in Puget Sound and its approaches. Settlement of one cohort occurred during May in the Strait of Juan de Fuca and in those areas of Puget Sound closest to the Strait. J1 individuals of this cohort were large (x=7.4 mm carapace width, CW) and comparable in both size and timing of settlement to populations along the Washington coast (e.g. Grays Harbor and Willapa Bay). Initial settlement density of the May cohort was as high as 215 crabs/m2 in intertidal eelgrass beds along the Strait of Juan de Fuca and decreased to <2 crabs/m2 within Puget Sound and the Strait of Georgia. A second cohort apparently originated in Hood Canal (a deep inland fjord), its size upon settlement in June was significantly smaller (J1 x= 5.3 mm CW) than the May cohort, and it was limited to Hood Canal and areas of Puget Sound close to the mouth of Hood Canal. A third cohort, which settled in late July and August, was the smallest of the three cohorts (J1 x= 4.8 mm CW), and was widely distributed around Puget Sound from Seattle in the south to the USA/Canadian border in the north. We hypothesize that most juvenile recruitment in Hood Canal and Puget Sound originates from parental stocks endemic to their respective basins (“Hood Canal” and “Puget Sound” cohorts), but that, on occasion, oceanographic conditions allow substantial influx of Pacific Ocean Dungeness crab larvae (“oceanic” cohort) through the Strait of Juan de Fuca into Puget Sound. Tracking of spatial/temporal settlement patterns and comparison of J1 sizes proved useful for estimating the probable sources and dispersion of Dungeness crab larvae. Differences in size and time of settlement between various larval cohorts of C. magister may prove useful as “biomarkers” for tracing circulation patterns within and between inland waters of Washington and the Pacific Ocean. Causes of smaller size and later settlement of the “Puget Sound” cohort relative to oceanic conspecifics of the same year-class are discussed.