Growth, distribution, and abundance of juvenile Dungeness crabs (Cancer magister) in the Juneau, Alaska area

Mistee VinzantUniversity of Alaska Southeast

August 2005

ABSTRACTThe Dungeness crab (Cancer magister), harvested throughout its range from

California to Unalaska Island, express a complex life cycle with multiple planktonic larval stages. The transition from a planktonic larval stage to a benthic life style may be the most vulnerable point in its life history; however, little is known about early life stages in Alaska. I studied the distribution of discarded molts to determine juvenile abundance in areas that are productive for adults. Juvenile molt density was sampled during spring tides in soft-bottom intertidal areas from May to August 2005 at 8 locations around Juneau, Alaska. Spatial and habitat distributions were observed for 1+ (CW 10-20 mm), 2+ (CW 20-30 mm), and sub-adult (CW 35-100 mm) juvenile molts; 0+ (CW 6-9 mm) molts were not observed since settlement of YOY juveniles generally occurs in late August for Southeastern Alaska. All soft-bottomed intertidal areas at each site were surveyed; however, juvenile molts were quantitatively measured where molts were most present. At least two year classes were found. Sizes of juvenile molts in the field resembled concurrent observations in lab-reared juveniles. Size data from lab-reared juveniles showed that the average CW for J1 instars were 6.59 mm (length: width ratio = 0.98), J2 instars were 9.17 mm (length: width ratio = 0.79), J3 instars were 11.65 mm (length: width ratio = 0.79), and J4 instars were 13.74 mm (length: width ratio = 0.79). However, J2 instar molts from the field may have been early settlers, which occurred 5 months after the last J2 individual molted in the lab. On the other hand, early juvenile molts were concentrated within sandy habitats and lowest in open mud and complex substrates like cobble/algae, which does not fit the habitat preference pattern that occurs in California and Washington. Older juvenile molts were concentrated in areas that provided shelter (i.e. gravel and eelgrass). I also identified two locations (Sunshine Cove and Echo Cove) that supported large densities of molts and may serve as functional nurseries for juvenile Dungeness crabs. The implications of juvenile molt density, distribution, and growth of Dungeness crabs are discussed and compared to similar data from California and Washington.

INTRODUCTIONThe Dungeness crab (Cancer magister) is a soft bottom Cancrid that is sport and

commercially harvested throughout its range, from Southern California to Unalaska Island (Jensen and Armstrong, 1987; Iribarne et al., 1995). Sport and commercial harvest of Dungeness crab is productive within Southeastern Alaska, and the overall 2001-2002 commercial harvest of Dungeness crab was 4.1 million pounds worth nearly $7.4 million (Rumble and Bishop, 2002). Juvenile/adult growth rates are not well known and are variable in Alaska; however, the study of larval and juvenile transport is vital to predict Dungeness recruitment patterns for the fisheries in Southeastern Alaska.

The Dungeness crab is a meroplanktonic, benthic crustacean that expresses a broadcast reproductive strategy. Larvae are released around near-shore areas and hatch between January and June, depending upon latitude (Shirley et al., 1987; Reilly, 1983; Hobbs and Botsford, 1992). Dungeness crab larvae are independent and become planktotrophic with teleplanic dispersal (i.e., the larvae are able to swim as plankton in the water column for more than 2 months). The larvae molt 5 times (5 zoeal stages and 1 megalopal stage) in the plankton before they metamorphose to a non-swimming juvenile stage and settle to the benthos in near-shore areas.

Juveniles recruit in estuaries and intertidal habitats throughout the species range. (Gunderson et al., 1990; McMillan et al., 1995; Holsman et al., 2003). Young Dungeness crabs are more abundant in intertidal or shallow subtidal areas (<50 m in depth), while older sub-adults move in and out of nursery areas and between coastal and estuarine habitats (Gutermuth and Armstrong, 1989). Shallow areas provide for accelerated growth and higher survival presumably from warmer water temperatures (McMillan et al., 1995), greater food availability (Stevens et al., 1984), structures for refuge from predation (Fernandez et al., 1994), and high marine productivities (Stevenson, 1988).

Little is known about juvenile recruitment and population dynamics of Dungeness crab in Alaska. I studied the distribution of discarded molts to determine juvenile abundance in areas that are productive for adults. Juvenile molt density was sampled during spring tides in soft-bottom intertidal areas around Juneau, Alaska. Based on the distribution and abundance of juveniles in Washington and California, it is predicted that the largest concentrations of juveniles will be found in sand-mixed substrates and in areas that fit the description of a “nursery” that was defined by Armstrong and Gunderson (1985). Since Southeastern Alaska generally experiences older water temperatures, it is also predicted that the growth and size of field and lab-reared juveniles will be slower and smaller than warmer regions throughout the geographic range of the Dungeness crab.

METHODSJuvenile molts were collected in the intertidal within 8 shallow near-shore areas

around Juneau, Alaska (Figure 1A). Locations within cove areas were selected based on the presence of tidal flats, soft bottoms, adult crabs, estuarine conditions, and eelgrass (Zostera spp.). The sites (from south to north) included: Salisbury Beach (Taku Inlet), Spuhn Island (Auke Bay), Yankee Cove, Sunshine Cove, South Bridgett Cove, North Bridgett Cove, Echo Cove, and Sawmill Creek (Berner’s Bay) (Figures 1B-1D). Although the universal habitat had a soft bottom, the intertidal locations had a wide variety of substrates (e.g., cobble, gravel, sand, algae, etc.).

Juvenile molts were collected from late May 2005 to early August during low tides. Molt densities were quantitatively measured with 3 or 4 50m x 2m transects at MLLW. All soft-bottomed intertidal areas at each site were surveyed; however, juvenile molts were quantitatively measured where molts were most present. The type of substrate and percent cover where molts were found were identified. Spatial distributions of molts were displayed through ArcGIS software for all sites. Since juvenile growth rates can be variable (e.g. large J1, small J2; Poole 1966), juvenile molts were staged using data from

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laboratory reared individuals (Figure 2). Measurements of carapace widths included the 10th anterolateral spines and carapace lengths were measured from the longest point between the eye orbits to the posterior end of the carapace. Molts < 20mm were measured with an ocular micrometer from a dissecting scope to the nearest 0.01mm and molts > 20mm were measured with a hand-held dial caliper to the nearest 0.1mm.

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Figure 1: Study sites in Juneau, Alaska included Sawmill Figure 2: Dungeness crab. Carapace width vs. length: widthCreek, Echo Cove, N. Bridgett Cove, S. Bridgett Cove, ratios for the first 4 instars from laboratory reared individuals.Sunshine Cove, Yankee Cove, Spuhn Island, Salisbury Beach*.

* Site is not displayed in the map.

RESULTSSpatial and habitat distributions were observed for 1+ (CW 10-20 mm), 2+ (CW

20-30 mm), and sub-adult (CW 35-100 mm) juvenile molts. Concentrations of multiple juvenile molts were found within two shallow, near-shore areas: Sunshine Cove and Echo Cove. Sunshine Cove was the only location that had young juveniles (J1-J3 instars), and Echo Cove had a mixture of older juveniles (J4-J5+ instars, and sub-adults). However, the other 6 sites were less abundant with one juvenile molt stage. The percent substrate cover at Sunshine Cove (sand = 58%; algae = 23%; and other = 6%) and Echo Cove (gravel/algae = 52%; sand = 36 %; and other = 12%) were estimated only where juvenile molts were found.

Juvenile molts were also distributed according to the type of substrate, with the largest density of juvenile molts in small sand, the next highest density in simple sand mixtures (i.e. gravel, algae), and lowest in areas with open mud and complex substrates like cobble/algae. Unlike young juvenile molts (J1-J3 instars), older molts (J4-J5+ instars, and sub-adults) were more common throughout a variety of substrates. Early juvenile molts were concentrated within sandy habitats and lowest in open mud and complex substrates like cobble/algae. Older juvenile molts were concentrated in areas that provided shelter (i.e. gravel and eelgrass). Although juvenile molts were found

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within various substrates, molt densities were patchy, yet consistent throughout the 8 locations.

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Figure 3: Dungeness crab. Mean SE density (number/m2) of juvenile molts for all sites from South to North.

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Figure 4: Dungeness crab. Molt stage in each substrate type. Figure 5: Dungeness crab. Time frequency of field and laboratory molts for the first 4 instars.

At least two year classes were found to occur for this species within the Juneau area. Sizes of juvenile molts in the field resembled concurrent observations on lab-reared juveniles. Size data from lab-reared juveniles showed that the average CW for J1 instars were 6.59 mm (length: width ratio = 0.98), J2 instars were 9.17 mm (length: width ratio = 0.79), J3 instars were 11.65 mm (length: width ratio = 0.79), and J4 instars were 13.74 mm (length: width ratio = 0.79) (Table 1). The size of juvenile molts in the field (J3-J4 instars) closely resembled the size of lab-reared individuals. However, J2 instar molts from the field may have been early settlers, which occurred 5 months after the last J2 individual molted in the lab (Figure 5). By the end of the 2005 study, laboratory individuals did not surpass the J4 instar growth stage. Therefore, growth comparisons between laboratory and field individuals were only made for J2-J4 instars (J1 instar molts were not observed in the field because larval settlement typically occurs in late August for Southeastern Alaska).

Table 1: Growth and molt data for the first 3 juvenile instars from laboratory-reared Dungeness crabs.  J1 Instar (N=96) J2 Instar (N=68) J3 Instar (N=9)

Carapace Width (mm)      

Mean ± SD 6.59 ± 0.36 9.17 ± 0.38 13.23 ± 0.11

Maximum 8.2 9.9 13.4

Minimum 5.9 8.2 13.1

Carapace Length (mm)      

Mean ± SD 6.43 ± 0.31 8.02 ± 0.64 12.95 ± 0.08

Maximum 7.1 9.5 13.1

Minimum 5.8 7.3 12.84

Molt Interval      

Range (Days) 18 Oct.-04 Nov. (17 Days) 26 Dec.-03 Feb. (38 Days) 19 Jan.-02 July (163 Days)

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DISCUSSIONTemperature strongly affects Dungeness crab juvenile growth rates (Gutermuth

and Armstrong, 1989; Gunderson et al., 1990). Kondzela and Shirley (1993) observed a difference between Dungeness crabs from Californian and Southeastern Alaska waters with respect to megalopal/J1 instar timing of settlement and carapace size. There are differences in the timing of larval and juvenile stages for California, Washington, and Alaska (Table 2). Larval and juvenile life stage durations increase and growth rates decrease in higher latitudes. The data presented in this paper confirms that there is a difference in life stage durations and carapace size throughout the distribution of the Dungeness crab.

Table 2: Dungeness crab. Differences in growth and peak durations for various life stages (Puget Sound typically experiences colder inland water temperatures than Grays Harbor and was included to illustrate the effect temperature has on life stage durations).

Life Stage San Francisco Bay, CA Grays Harbor, WA Puget Sound, WA Southeastern Alaska

Egg Hatch January (1) January-February (3) January- February (3) May-June (7)

Zoeal ( I - IV) January-April (1) February-April (3) February-April (3) May-September (7)

Megalopae April (2) April (4) July (6) September-October (8)

Settlement May (2) May (4) August (6) September-October (8)

0+ Juvenile (CW) 7-58.6 mm (2) 11-54 mm (5) 5-30 mm (6) 6-9 mm (9)

1+ Juvenile (CW) 72-111 mm (2) 55-100 mm (5) 35-104 mm (6) 10-20 mm (9)

(1) Reilly, 1983; (2) Collier, 1983; (3) Hobbs and Botsford, 1992; (4) Stevens and Armstrong, 1985; (5) Armstrong Gunderson, 1985; (6) Dinnel et al., 1993; (7) Shirley et al., 1987; (8) G. Eckert, unpublished data; (9) M. Vinzant, unpublished data.

Differences in hatch time can alter development and intermolt durations for Dungeness crabs. Dungeness megalopae in Southeastern Alaska settle during a 3 month period, from August to late October (Eckert, unpublished data). Several studies from the Puget Sound area have suggested multiple molting pulses, or cohorts, of juveniles can occur from differences in hatch and settlement time (Dinnel et al., 1993; McMillan et al., 1995). Early settlers have an advantage because they experience more time for growth and development, move to the subtidal before the winter season, and contribute to the local fishery quicker than late settlers. (Dinnel et al., 1993). The J2 instar molts observed in field between late May and early June, 2005 could have been early settlers that may not require an extended duration in the intertidal in order to develop and prepare for the next year.

Armstrong and Gunderson (1985) were the first to define a “nursery” for juvenile Dungeness crabs as having the ability to support a large density of multiple age classes and a wide variety of suitable habitat compared to other nearby shallow areas. Sunshine Cove and Echo Cove both met these “nursery” requirements. On the other hand, the relationship between juvenile density and substrate distributions were highest in sand mixtures and lowest within open mud and complex substrates like cobble/algae. This habitat preference for juveniles does not fit the pattern that occurs throughout the Pacific Northwest and may be a result from patchy sources of suitable habitat in the Juneau area. Previous studies have shown that juvenile densities are relatively low in intertidal areas that have sparse protective covers like open sand, intermediate in eelgrass, and highest in gravel/algae (Dinnel et al., 1986; Dumbauld and Armstrong 1987; McMillan 1991; McMillan et al., 1995).

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Since the project was entirely concerned with the distribution of 1+ juveniles that over wintered in the intertidal around the Juneau area, the data presented in this paper were collected during 3 summer months (May-August). Seasonal and yearly densities would be interesting for further study. McMillan et al. (1995) described seasonal and yearly densities of YOY juveniles in Grays Harbor, WA, where the highest densities of juveniles (~300 crabs/m2) occurred during the summer months, and lowest during the winter seasons (~2 crabs/m2). Not only was this study conducted over a small range in time, my sampling methods were not diverse. I only observed molts that were discarded along sub-tidal areas. It would have been nice to diversify my sampling methods and capture live juveniles (live traps, settlement/mesh bags, sand sieving) to get a better idea of what the live densities are like. Lastly, the Stikine area supports the largest commercial harvest of adult Dungeness crabs, and it would have been nice to see if there is a functional juvenile “nursery” within that area that could show evidence of juveniles supplying adult stocks that recruit into the fisheries. All in all, these findings show the ability to understand juvenile population dynamics in order to predict juvenile/adult recruitment into the local fishery.

ACKNOWLEDGEMENTSThis project was supported and funded by an institutional undergraduate

fellowship award (Alaska Experimental Program to Stimulate Competitive Research, EPSCoR) from the University of Alaska. Computers and equipment was provided by the University of Alaska Southeast, Juneau Campus. The support and assistance from G. Eckert, D. Morgan, R. Vinzant, and several other University of Alaska facility and students made this project possible. I thank them for their assistance.

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Grays Harbor, Washington. In: Proceedings of the Symposium on Dungeness Crab Biology and Management. Alaska Sea Grant Report No. 85-3. 145-170.

Collier P. 1983. Movement and growth of post-larval Dungeness crabs, Cancer magister, in the San Francisco area. In: In: Life history, environment, and mariculture studies of the Dungeness crab, Cancer magister, with emphasis on the central California fishery resource. Fish Bull 172: 125-134.

Dinnel P., Armstrong D., and McMillan R. O. 1986. Dungeness crab, Cancer magister, distribution, recruitment, growth, and habitat use in Lummi Bay, Washington. Final Report to Lummi Indian Tribe, by the Fisheries Research Institute, University of Washington. Seattle, Washington. (FRI-UW-8612).

Dinnel P., Armstrong D., and McMillan R. 1993. Evidence for multiple recruitment-cohorts of Puget Sound Dungeness crab, Cancer magister. Mar. Bio. 115: 53-63.

Dumbauld B. R., and Armstrong D. A. 1987. Potential mitigation of juvenile Dungeness crab loss during dredging through enhancement of intertidal shell in Grays Harbor, Washington. Final Report for U.S. Army Corps of Engineers, Seattle District, by the Fisheries Research Institute. University of Washington, Seattle, Washington (FRI-UW-8714).

Fernandez M., Iribarne O., and Armstrong D. 1994. Ecdysial rhythms in megalopae and first instars of the Dungeness crab Cancer magister. Mar. Bio. 118: 611-615.

Gunderson D., Armstrong D., Shi Y., and NcConnaughey R. 1990. Patterns of estuarine use by juvenile English sole (Parphrys vetulus) and Dungeness crab (Cancer magister). Estuaries. 13(1): 59-71.

Gutermuth F., and Armstrong D. 1989. Temperature-dependent metabolic response of juvenile Dungeness crab Cancer magister Dana: ecological implications for estuarine and coastal populations. J. Exp. Mar. Biol. Ecol. 126: 135-144.

Hobbs R. C., and Botsford L. W. 1992. Diel vertical migration and timing of metamorphosis of larvae of the Dungeness crab Cancer magister. Marine Biology. 112. 417-428.

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Holsman K. K., Armstrong D. A., Beauchamp D. A., and Ruesink J. L. 2003. The necessity for intertidal foraging by estuarine populations of sub-adult Dungeness crab, Cancer magister: evidence from a bioenergetics model. Estuaries. 26 (4B): 1155-1173.

Iribarne O., Armstrong D., and Fernandez M. 1995. Environmental impact of intertidal juvenile Dungeness crab habitat enhancement: effects on bivalves and crab foraging rate. J. Exp. Mar. Bio. Eco. 192: 173-194.

Jensen G., and Armstrong D. 1987. Range extensions of some northeastern Pacific decapoda. Crustaceana. 52: 215-217.

Kondzela C., and Shirley T. 1993. Survival, feeding, and growth of juvenile Dungeness crabs from Southeastern Alaska reared at different temperatures. Journal of Crustacean Biology. 13(1): 25-35.

McMillan R. O. 1991. Abundance, settlement, growth and habitat use of juvenile Dungeness crab, Cancer magister, in inland waters of northern Puget Sound, Washington. Masters of Science Thesis. University of Washington, School of Fisheries.

McMillan R. O., Armstrong D. A., and Dinnel P. A. 1995. Comparison of intertidal habitat use and growth rates of two northern Puget Sound cohorts of 0+ age Dungeness crab, Cancer magister. Estuaries. 18 (2): 390-398.

Poole R. L. 1966. A description of laboratory-reared zoeae of Cancer magister Dana, and megalopae taken under natural conditions (Decapoda Brachyura). Crustaceana. 11:83-97.

Reilly P. 1983. Dynamics of Dungeness crab, Cancer magister, larvae off central and northern California. In: Life history, environment, and mariculture studies of the Dungeness crab, Cancer magister, with emphasis on the central California fishery resource. Fish Bull 172: 57-84.

Rumble J., and Bishop G. 2002. Report to the Board of Fisheries, Southeast Alaska Dungeness crab fishery. Alaska Department of Fish and Game. Juneau, Alaska. Regional Information Report No. 1J02-45.

Stevens B., and Armstrong D. 1984. Distribution, abundance, and growth of juvenile Dungeness crabs, Cancer magister, in Grays Harbor estuary, Washington. Fishery Bull. U.S. Dep. Comm. 82: 469-483.

Stevens B., and Armstrong D. 1985. Ecology, growth, and population dynamics of juvenile Dungeness crab, Cancer magister Dana, in Grays Harbor, Washington, 1980-1981. In: Proceedings of the Symposium on Dungeness Crab Biology and Management. Alaska Sea Grant Report No. 85-3. 119-134.

Stevenson J. C. 1988. Comparative ecology of submersed grass beds in freshwater, estuarine, and marine environments. Limno. Oceanogr. 33 (4, part 2): 867-893.

Shirley S. M., Shirley T. C., and Rice S. D. 1987. Latitudinal variation in the Dungeness crab, Cancer magister: zoeal morphology explained by incubation temperature. Mar. Bio. 95: 371-376.

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