Dungeness Crab Population Research Articles

Evaluating environmental controls on the exoskeleton density of larval Dungeness crab via micro computed tomography

Dungeness crab (Metacarcinus magister) have significant socioeconomic value, but are threatened by ocean acidification (OA) and other environmental stressors that are driven by climate change. Despite evidence that adult harvests are sensitive to the abundance of larval populations, relatively little is known about how Dungeness megalopae will respond to these stressors. Here we evaluate the ability to use micro-computed tomography (μCT) to detect variations in megalope exoskeleton density and how these measurements reflect environmental variables and calcification mechanisms. We use a combination of field data, culture experiments, and model simulations to suggest resolvable differences in density are best explained by minimum pH at the time zoeae molt into megalopae. We suggest that this occurs because more energy must be expended on active ion pumping to reach a given degree of calcite supersaturation at lower pH. Energy availability may also be reduced due to its diversion to other coping mechanisms. Alternate models based on minimum temperature at the time of the zoea-megalope molt are nearly as strong and complicate the ability to conclusively disentangle pH and temperature influences. Despite this, our results suggest that carryover effects between life stages and short-lived extreme events may be particularly important controls on exoskeleton integrity. μCT-based estimates of exoskeleton density are a promising tool for evaluating the health of Dungeness crab populations that will likely provide more nuanced information than presence-absence observations, but future in situ field sampling and culture experiments are needed to refine and validate our results.

The Effect of a Spatial Harvest Closure Area on the Abundance, Movement, and Demographics of an Intensively Fished Population of Dungeness Crabs (Cancer magister) Along Roberts Bank, British Columbia, Canada

In addition to regulating sex, size, and season of harvest, resource managers can implement spatial fishery closures as a management tool to protect Dungeness crab populations. Yet, we have little understanding of the role that spatial closures play in conserving crab populations, especially in fisheries management zones that are subject to intensive harvest. The primary purpose of this study was to evaluate the biological effectiveness of a crab closure area adjacent to an intensive fishery, by examining relative abundance, population demographics, and seasonal movement patterns within and outside of a closure area. Relative abundance and population demographics were assessed weekly from August to October, using standardized trap lines distributed at four locations, stratified by treatment (inside versus outside of closure area) and water depth. To obtain individual male crab movement data within and outside of the closure area, mobile acoustic tracking surveys were conducted one to three times per week in the autumn and winter. Male crabs were larger and in greater abundance inside the closure area than outside. The overall trend in male crab movement was southwest towards deeper water during autumn and winter. Our research supports the hypothesis that fisheries decrease the abundance and size of an exploited species, but closure areas may reduce these effects. Based on the observed seasonal movement patterns of crabs, it appeared that the closure area provided a temporary refuge from harvest. Resource managers should consider seasonal movement patterns, habitat use, and quality of available habitat when selecting closure area boundaries.

Ghost Fishing in the Southeast Alaska Commercial Dungeness Crab Fishery

AbstractEntrapment of crabs by derelict crab pots (also known as ghost fishing) can be a significant consequence of commercial fishing. The prevalence of lost commercial pots and ghost‐fishing entrapments was estimated for the commercial Dungeness crab Cancer magister fishery in southeastern Alaska during the 2009 and 2010 summer closures of the commercial season (16 August through 30 September). Teams of divers retrieved a random subsample of the derelict crab pots located using side‐scan sonar. Altogether, we retrieved 123 derelict crab pots containing 215 entrapped Dungeness crabs. The densities of derelict crab pots varied from 1.5 to 10.1/km2, while the densities of entrapped Dungeness crabs ranged from 0 to 54.5/km2, depending on the area surveyed. Derelict crab pots were discovered to effectively ghost‐fish for at least 7 years, indicating that there are long‐term cumulative impacts on Dungeness crab populations. The number of derelict crab pots and entrapped Dungeness crabs at each of the surveyed areas was highly correlated with the number of fishermen, the number of pot lifts, and annual harvest in numbers, allowing for extrapolation to a regionwide estimate of crab entrapment and derelict crab pot abundance. Overall, our findings show instantaneous entrapment of less than 1% of the commercial crab harvest with a cumulative annual loss of less than 3% of the regional commercial crab harvest. We challenge the efficacy of the biodegradable escape mechanism currently employed in commercial Dungeness crab pots in southeastern Alaska and present alternatives which may require further in situ or laboratory verification of their effectiveness.Received February 23, 2012; accepted December 27, 2012

RECRUITMENT LIMITATION IN DUNGENESS CRAB POPULATIONS IS DRIVEN BY VARIATION IN ATMOSPHERIC FORCING

Ecologists have long debated the relative importance of biotic interactions vs. abiotic forces on the population dynamics of both marine and terrestrial organisms. Investigation of stock size in Dungeness crab (Cancer magister) is a classic example of this debate. We first tested the hypothesis that adult population size was set by larval success. We found that during a five-year sampling period, adult crab population size from Oregon through central California, USA, as measured by the commercial catch, varied directly with the number of terminal-stage larvae (megalopae) returning to Coos Bay, Oregon, four years earlier; adult population size was largely determined (> 90% of the variation) by success during the larval stage. We then tested whether biotic interactions or abiotic forces caused the variation in larval success. Most of the variation (> 90%) in the number of returning megalopae is explained by the timing of the spring transition, a seasonal shift in atmospheric forcing that drives ocean currents along the west coast of the United States. Early spring transitions lead to larger numbers of returning Dungeness megalopae, while in four other crab taxa, species with very different life history characteristics, early-spring transitions lead to lower numbers of returning megalopae. During the past roughly 30 years, the size of the commercial catch of Dungeness crab is significantly and negatively correlated with the date of the spring transition throughout the California Current system. Long-term variation in the date of the spring transition may explain a major crash in the Dungeness crab fishery in central California, which began in the late 1950s. The data suggest that Dungeness crab population size is determined by variation in larval success and that a significant portion of this variation is due to the timing of the spring transition, a large-scale climatic forcer.

Circulating Ecdysteroid Concentrations in Alaskan Dungeness Crab (Cancer Magister)

The currently accepted reproductive cycle of Alaskan female Dungeness crab (Cancer magister) is that females molt and mate during summer and fall, extrude eggs in the fall, and incubate eggs through the winter until larval hatching in May and June. In this cycle, molting and mating are coincident with ovarian maturation. In this study, we quantified circulating concentrations of hemolymph ecdysteroids (molting hormones) in laboratory and field sampled crabs using an enzyme-linked immunosorbent assay (ELISA), for molt status assessment of individuals. Captive female crabs from southeastern Alaska (mean |${\rm{CW = 134}}{\rm{.8\, mm}}$|⁠; |${\rm{n = 48}}$|⁠) had a typical crustacean molt cycle profile of circulating ecdysteroids. Concentrations of ecdysteroids were low during intermolt (⁠|${\rm{20}}{\rm{.3 }} \pm {\rm{ 0}}{\rm{.7 ng/ml}}$|⁠), maximal during premolt (⁠|${\rm{1886}}{\rm{.5 }} \pm {\rm{ 186}}{\rm{.2 ng/ml}}$|⁠) 15 days before ecdysis and precipitously declined to low levels |$\left( { \lt {\text{ 9}}0{\text{ ng}}/{\text{ml}}} \right)$| 5 days prior to ecdysis. The duration of premolt was 150 days. Crabs held in captivity either molted (67%), extruded eggs (7%), or demonstrated no reproductive or molting activity (26%). Peak molting occurred in November for crabs held in the laboratory. Most (98%) of the female crabs sampled (⁠|${\rm{n = 579}}$|⁠) in Port Frederick, Alaska during the expected premolt period (May-July), had intermolt ecdysteroid concentrations. The capability to predict ecdysis 150 days prior to ecdysis by measuring ecdysteroid concentrations provides a useful tool to assess the molt status and timing of ecdysis in Alaskan Dungeness crab populations.

Seasonal movements and distribution of Dungeness crabs Cancer magister in a glacial southeastern Alaska estuary

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 214:167-176 (2001) - doi:10.3354/meps214167 Seasonal movements and distribution of Dungeness crabs Cancer magister in a glacial southeastern Alaska estuary Robert P. Stone*, Charles E. O¹Clair Auke Bay Laboratory, Alaska Fisheries Science Center, National Marine Fisheries Service, National Oceanic and Atmospheric Administration, 11305 Glacier Highway, Juneau, Alaska 99801, USA *E-mail: bob.stone@noaa.gov ABSTRACT: The movements of 10 female and 8 male adult Dungeness crabs, Cancer magister (Dana, 1852), were monitored biweekly to monthly with ultrasonic biotelemetry for periods ranging from 73 to 555 d. Female and male crabs had different seasonal patterns of habitat use, depth distribution, and activity. The general pattern for female crabs was: (1) a relatively inactive period between November and mid-April at depths below 20 m; ovigerous crabs were typically buried during this period in a dense aggregation; (2) abrupt movement into shallow water (<8 m) in late April and residence there until early June; this movement was coincident with the spring phytoplankton bloom and initiation of larval hatching; (3) increased activity beginning in July with movement back to deeper water, presumably to forage. Females that molted prior to oviposition did so in June and July. Male crabs occupied deep water (>40 m) from November to April, then concentrated in shallow water (<25 m), segregated from females, until late July. Males were most active in late summer and moved into deeper water (>30 m) near the mouth of the cove in fall. The range of depths were -0.5 to -61.3 m for females and +0.1 to -89.0 m for males. Female crabs showed fidelity to the head of the cove and typically ranged only 1.5 km from there. Male crabs, however, moved up to 7.2 km from the head of the cove, but did not move close to the nearest established population of Dungeness crabs 9.6 km distant. The discrete population of adult Dungeness crabs in Fritz Cove may be representative of most Dungeness crab populations inhabiting shallow embayments of the coastal fjord system of southeastern Alaska. KEY WORDS: Dungeness crab · Biotelemetry · Essential habitat · Migration Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 214. Online publication date: April 26, 2001 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2001 Inter-Research.

Comparison of Intertidal Habitat Use and Growth Rates of Two Northern Puget Sound Cohorts of 0+ Age Dungeness Crab, Cancer magister

Density, habitat use, and growth of intertidal 0+ age Dungeness crabs, Cancer magister, were examined at five northern Puget Sound (Washington, USA) sites between June 1984 and September 1987. Sampling was conducted biweekly during settlement, from June to September, and approximately monthly or bimonthly thereafter. Northern Puget Sound Dungeness crab populations appear to be largely supported by recruitment from inland parental stocks, but a smaller proportion of recruits originate from coastal or oceanic stocks, as evidenced by earlier settlement and larger size of the first instar. Settlement of Dungeness crabs in inland waters typically peaked in August, and interannual variation in year-class strength at settlement (measured as intertidal density) was low relative to that reported for coastal crab populations. Spatial and interannual differences in settlement densities were mediated by high postsettlement mortality, which varied inversely with habitat complexity. Seasonal densities were highest in mixed sand and gravel with an overstory of attached or drift macroalgae, intermediate in eelgrass (Zostera marina), and lowest on open sand. Postsettlement growth rates corresponded to seasonal water temperatures and were greatest for the coastal cohort that settled in May and June. This cohort was larger as first juvenile instars (7.2 mm carapace width, CW) and grew rapidly at summer temperatures in excess of 15°C to a size (>30 mm CW) that allowed emigration from intertidal to subtidal areas by September. The late summer cohort settled in August at 5.3 mm CW and soon after was subjected to decreasing autumn water temperatures. There crabs experienced little growth while over wintering in the intertidal, but growth rates increased in March, and the crabs emigrated in April and May, approximately 10 mo after settlement.

Interannual variability in coastal Washington Dungeness crab (Cancer magister) populations: larval advection and the coastal landing strip

ABSTRACTSubstantial and unexplained variations in abundance characterize US west coast populations of Dungeness crab, Cancer magister. It is likely that this variability reflects oceanic advection of larvae given the dynamics of the California Current system, the protracted (4–5 months) pelagic larval phase and the restrictive nature of juvenile habitat requirements. We compare 40 years of Ekman transport vectors during January‐May (1947– 1986) with time‐lagged and stratified commercial landings for coastal Washington (1951–1990). Persistent landward and net northward flow characterized the circulation of near‐surface waters during the larval periods studied. A mechanism for progressive seaward transport of larvae through ontogeny, as proposed by others, was not apparent. Overall, above‐ (below‐) average year classes were associated with relatively weak (strong) northward transport and, to a lesser degree, strong (weak) landward transport. Based on these analyses and in consideration of the prevailing coastal circulation, we propose that the relative magnitude of C. magister juvenile recruitment and, hence, incoming year class strength, reflects: (1) the proportion of larvae retained within a relatively narrow (≤ 15 km) and heterogeneous ‘coastal landing strip’, (2) the availability of suitable substrate within the coastal landing strip at time of settlement and (3) the magnitude of downstream advective losses (export) of larvae from the California‐Oregon‐Washington coastal system. Based on a consideration of the physics of the California Current system and the larval biology of the species, significant linkage of Dungeness crab populations along the west coast of North America is likely.

A trematode metacercaria encysted in the nerves of the dungeness crab, Cancer magister

Of 188 Dungeness crabs, Cancer magister, 8 contained unidentified trematode metacercariae encysted in various components of the nervous system, including the thoracic ganglion, brain, lamina ganglionaris of the eyestalk, and major nerves arising from the thoracic ganglion. A single cyst was present in the available tissue sections of 7 of the crabs and no behavioral abnormalities were exhibited. One crab with multiple cysts in major nerves near the thoracic ganglion was markedly ataxic. The cysts and included worms distorted, compacted, and destroyed nervous tissue, and occupied most of the nerves where present. Host response was minimal, but some cysts invoked massive hemocytic accumulations near the infection site. Infections probably seriously affect nerve impulse transmission and accounts for the lethargic behavior of the crab with multiple cysts in major nerves. The present report is the first record of digenetic trematode infections in the Dungeness crab and the apparent restriction of the worm to nervous tissue is unusual if not unique in the Digenea. Because of the absence of grossly recognizable lesions and the small samples excised at necropsy, both the incidence and intensity of infection in Dungeness crab populations are almost certainly higher than indicted by our data.

ASPECTS OF THE LIFE HISTORY OFCARCINONEMERTES ERRANS(NEMERTEA: CARCINONEMERTIDAE), AN EGG PREDATOR OF THE CRABCANCER MAGISTER

The nemertean genus Carcinonemertes has been known for over a century as an epibiont of brachyuran crabs. Even though symbiotic life styles are highly unusual for nemerteans, only a limited amount of research has been devoted to the study of Carcinoneniertes. Worms of this genus infest crabs after a planktonic larval stage. They next remain dormant, encysted on the host's exoskeleton. Worms on female hosts migrate into the host egg clutch when it is oviposited for brooding under the abdomen. There they grow, mature, and lay their own eggs. While the basic outline of the life history of Carcinonemertes has long been known, it was only recently confirmed as a predator of its hosts' eggs (Wickham, 1978) . Earlier workers had suspected that Carcinoneniertes was an egg predator, but never actually observed feeding ( Humes, 1942 ; Kuris, 1971 ) . The worms have been considered generalists in terms of host specificity. But little is known of the extent to which Carcinoneniertes is capable of affecting host population dynamics. Carcinoneniertes errans Wickhani is the first of these worms found to be specific to a single host species : the commercially important Dungeness crab, Cancer niagister Dana. This worm recently was found to exist at epidemic levels on the Central California Dungeness crab population (Wickham, 1979a) . The egg mortality it has caused in this region is high enough (averaging over 50% of the eggs produced annually) to implicate this worm in the collapse of the Central California Dungeness crab fishery. Carcinonemertes occurs on a large number of ecologically and economically important brachyuran crabs, and is now known to be capable of having a significant effect on its hosts' reproductive output. The following study on the life history of C. errans was initiated for this reason. Aspects of the life history of C. errans were studied in both the laboratory and the field. Worm abundance in the field was investigated using crabs caught in trawls and traps at Bodega Bay and Eureka, California. These crabs were dissected and all worms observable were counted and their locations on the exoskeleton noted. Samples also were obtained from the egg clutches of ovigerous female crabs by fishermen in the field. Sampling was done by inserting forceps into the egg clutch approximately at the middle of the exposed portion of the clutch. Care was taken to insert the forceps deeply enough to remove complete egg-bearing setae. The egg samples obtained contained an average of approximately 100 egg-bearing setae which each held about 200 eggs. Dungeness crabs carry up to 2,500,000 eggs in a clutch, so these samples were approximately 1% of the clutch. The egg samples were placed in vials with a 5 % formalin sea water solution and examined later with a dissecting microscope.

Predation by the nemertean Carcinonemertes errans on eggs of the Dungeness crab Cancer magister

The nemertean Carcinonemertes errans is an important predator on the eggs of its host, the Dungeness crab Cancer magister. Each nemertean consumes an average of approximately 70 crab eggs during the host brooding period. The nemerteans, which apparently consume no other food during the host brooding period, convert crab egg tissue to worm tissue with an efficiency of 28.9% on a caloric basis. Epidemic levels of Carcinonemertes errans on the central California Dungeness crab population have resulted in the direct mortality of an average of 55% of the eggs produced by that population over the last 5 years, making C. errans probably the most numerically significant predator on these crabs.