|Snow Crab Life History and Diet in the Chukchi Sea|
The snow crab is a widely distributed and abundant epibenthic species on the Bering and Chukchi Sea shelves (Bluhm et al. 2009). The recent northward contraction of the distribution range of C. opilio in the Bering Sea (Orensanz et al. 2004), the assumed biomass increase in the Chukchi Sea (Bluhm et al. 2009), and the increased interest in the Chukchi Sea for oil and gas-related exploration activities warrant comprehensive estimates of snow crab reproductive potential and energetics, role in the food web, population size and stock characteristics.
|Reproduction and Energetics|
(1) Determine fecundity and sperm reserves in spermathecae in female snow crab.
(2) Determine energetic expenditure of reproduction across a temperature gradient.
To determine energetic expenditure of reproduction, caloric content will be measured on homogenized, pelletized tissue (somatic tissue, male reproductive tissue, and ovaries in immature, pre-pubescent, and egg-carrying females, and eggs) with an oxygen bomb calorimeter (Hondolero et al. 2011). Objective status: Laboratory dissections underway.
(3) Synthesize results on reproductive potential with retrospective data on snow crab in the Chukchi Sea.
The capability of sperm storage enhances the reproductive potential (Gravel and Pengilly 2007). Reproduction and somatic growth are processes that typically compete during resource allocation and both are influenced by temperature. In areas with water temperatures below 1ºC, female C. opilio switch from an annual to a biennial reproductive cycle (Moriyasu and Lanteigne 1998). The Chukchi Sea shelf experiences a substantial gradient of bottom water temperatures (Woodgate et al. 2005) including areas with <1ºC that contribute to structuring female reproductive cycles. To determine sperm storage capacity, spermathecae (Figure 3) were removed when present, fullness estimated, spermathecal load recorded and sperm cell counts conducted (Slater 2009). In our preliminary analysis (>167 individuals), female spermathecal load (measured as weight of the sperm stored in the spermathecae in female crabs) does not strongly relate to female body weight, shell condition or water depth.
Male snow crab size at maturity and fecundity is currently under way. Male crabs were consistently categorized as immature, sublegal males based on carapace width and chela height measurements (Figure 4). Male chela to carapace size ratios generally concur with immature status. Though, a few offshore stations near Point Hope and Cape Lisburne had 20-30% mature, sublegal male catch. Overall, immature males were generally 50-75% of the snow crab catch per station across the Chukchi Sea.
|Diet and Trophics|
- Divine, L. M., Iken, K., Bluhm, B. A. 2015. Regional benthic food web structure on the Alaska Beaufort Sea shelf. Marine Ecology Progress Series 531: 15-32.
- Divine, L. M., Bluhm, B. A., Mueter, F. J., Iken, K. in press. Diet analysis of Alaska Arctic snow crabs (Chionoecetes opilio) using stomach contents and δ13C and δ15N stable isotopes. Deep Sea Research Part II: Arctic Eis Special Issue.
(4) Identify diet constituents of snow crab in different geographic areas and size classes.
(5) Identify trophic position of snow crab using stable isotope analysis in different geographic areas and size classes.
1) increasing proportions of polychaete prey (P. hyperborea) toward the north and east,
2) increasing proportions of thinner-shelled bivalve prey toward the north and west, and
3) higher proportions of crustacean prey (amphipods and decapods) in the Chukchi.
This is reflected in the isotopic data (Figure 7) where there is a clear separation of snow crab in the Central Beaufort as compared to those to the west. This is related to prey field overlap, which could be attributed to more closely linked habitats through oceanographic currents in the Chukchi and Western Beaufort Seas. Snow crab trophic level determinations appear to be in line with Iken et al. 2010 and match other relatively large predators such as Arctic/Saffron cod (age1+), while appearing to be more benthically oriented (Figure 8).
|Population Assessment: with Bob Lauth and Bob Foy|
(6) Assess crab population dynamics in light of oil and gas extraction and possible harvest using a sustainable yield model.