Snow Crab Ecology

Snow Crab Life History and Diet in the Chukchi Sea

Bodil Bluhm, Katrin Iken, and Lauren Divine

Reproduction and Energetics
Diet and Trophics
Population Assessment

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
Objectives
(1) Determine fecundity and sperm reserves in spermathecae in female snow crab.

Figure 1. Percent of mature female Snow crab across all size classes. At 46 mm Carapace Width, 50% of females in that size class have reached maturity.


Figure 2. Female Snow crab total egg count has a relatively strong correlation to body size (carapace width). Thus, larger females are more fecund and are valuable to overall reproductive success. There were no significant relationships between egg number and water depth, bottom water temperature, or latitude.


Figure 3. Female snow crab spermatheca. Photo: Lauren Divine


Figure 4. Male Snow Crab size at maturity from the 83-112 net. Note: all Chukchi Sea males are below the established legal harvest size limit used in the Bering Sea Fishery.

(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.

Snow crab stock structure is strongly shaped by life cycle and reproductive characteristics which are poorly known in the Chukchi Sea. C. opilio are long-lived (~15 years for large males) with a life expectancy after terminal molt of about 5-7 years in males and 3-4 years in females (Comeau et al. 1998, Shirley and Bluhm 2005). Smaller size at maturity in colder waters at higher latitudes than in warmer temperatures at lower latitudes (Jewett 1981, Orensanz et al. 2007) has implications for fecundity estimates. From Arctic Eis and other Chukchi Sea samples, we estimated that 50% of female snow crabs in the Chukchi Sea reach maturity by 46 mm carapace width (Figure 1). Based on Arctic Eis samples only, R. Foy estimated that size at maturity is slightly smaller in the Chukchi versus the Bering Sea. For fecundity estimates, the dry weight of 250 sub-sampled eggs per female was determined (Paul et al. 1997, Slater 2009). The remaining eggs per clutch were removed from the pleopods and dried to constant weight at 60°C. The Pearson correlation coefficient for the number of eggs per female versus carapace width was r=0.69, confirming that female fecundity scales to body size as it does in other regions (Figure 2).

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

Manuscripts:

  • 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.
Objectives
(4) Identify diet constituents of snow crab in different geographic areas and size classes.

Figure 5. Snow crab stomach (white triangle) is located in the gastric region aft of the eyes. Photo: Lauren Divine


Figure 6. Snow crab diet consituents Polychaetes and Molloscs.





(5) Identify trophic position of snow crab using stable isotope analysis in different geographic areas and size classes.


Figure 7. Male (M), immature female (IF), and mature female (MF) muscle tissue stable isotope values across 4 regions. Carbon values are associated with prey source and Nitrogen values with trophic level.

Crabs are omnivorous predators and scavengers (Lovrich and Sainte-Marie 1997, Iken et al. 2010) with regional and age-specific variability in dominant prey items (Feder and Jewett 1980). In the southern Chukchi Sea C. opilio occupied a high trophic level (3.6, Iken et al. 2010). For dietary analyses, frequency of occurrence of food items in stomachs (Figure 5) will be determined for several crab size classes in 4 regions: Southern Chukchi, Northern Chukchi, Western Beaufort, and Central Beaufort. Preliminary analyses suggest Polychaetes and Molluscs make up the majority of snow crab diets (Figure 6), with:
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).

Figure 8. Trophic level position of snow crab (Black symbols = North Chukchi, White symbols = South Chukchi).

Population Assessment: with Bob Lauth and Bob Foy
Objectives

(6) Assess crab population dynamics in light of oil and gas extraction and possible harvest using a sustainable yield model.

Bluhm et al. 2013 - AMSS Poster

Crabs will be collected by the bottom trawl component using both gear types to ensure that we sample the entire size spectrum of crabs present. The data will be related to gradients in bottom temperature and food density across the study area. As indices of food density we will use water column chlorophyll and epifaunal abundance data to be generated by other study components. Macrofaunal prey biomass available across the study area will be inferred from recently updated biomass maps (e.g., Grebmeier 2012).

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