Geology topics

Charles E. O’Clair

Publications and source records attributed to Charles E. O’Clair.

17 recordsLinked to original sources

USGS2004

Dramatic increase in the relative abundance of large male dungeness crabs Cancer magister following closure of commercial fishing in Glacier Bay, Alaska

The size structure of the population of the Dungeness crab Cancer magister was studied at six sites in or near Glacier Bay, Alaska, before and after the closure of commercial fishing. Seven years of preclosure and 4 years of postclosure data are presented. After the closure of Glacier Bay to commercial fishing, the number and size of legal-sized male Dungeness crabs increased dramatically at the experimental sites. Female and sublegal-sized male crabs, the portions of the population not directly targeted by commercial fishing, did not increase in size or abundance following the closure. There was not a large shift in the size-abundance distribution of male crabs at the control site that is still open to commercial fishing. Marine protected areas are being widely promoted as effective tools for managing fisheries while simultaneously meeting marine conservation goals and maintaining marine biodiversity. Our data demonstrate that the size of male Dungeness crabs can markedly increase in a marine reserve, which supports the concept that marine reserves could help maintain genetic diversity in Dungeness crabs and other crab species subjected to size-limit fisheries and possibly increase the fertility of females. ?? 2004 by the American Fisheries Society.

American Fisheries Society Symposium↗

USGS2004

Estimating Dungeness crab ( Cancer magister ) abundance: Crab pots and dive transects compared

Dungeness crabs ( Cancer magister ) were sampled with commercial pots and counted by scuba divers on benthic transects at eight sites near Glacier Bay, Alaska. Catch per unit of effort (CPUE) from pots was compared to the density estimates from dives to evaluate the bias and power of the two techniques. Yearly sampling was conducted in two seasons: April and September, from 1992 to 2000. Male CPUE estimates from pots were significantly lower in April than in the following September; a step-wise regression demonstrated that season accounted for more of the variation in male CPUE than did temperature. In both April and September, pot sampling was significantly biased against females. When females were categorized as ovigerous and nonovigerous, it was clear that ovigerous females accounted for the majority of the bias because pots were not biased against nonovigerous females. We compared the power of pots and dive transects in detecting trends in populations and found that pots had much higher power than dive transects. Despite their low power, the dive transects were very useful for detecting bias in our pot sampling and in identifying the optimal times of year to sample so that pot bias could be avoided.

Alaska↗

USGS2003

Dungeness crab, Cancer magister , do not extrude eggs annually in southeastern Alaska: An in situ study

The reproductive biology of female Dungeness crabs was studied with crab-pot and dive-transect sampling in five bays within or near Glacier Bay National Park and Preserve, southeastern Alaska, in April and September yearly from 1992 to 1998. A large percentage of nonovigerous, mature females was found in April, a time when females were expected to be brooding eggs that hatch in May and June. Our study examined differences between ovigerous and nonovigerous females collected in April and September samples to corroborate our previous laboratory study in which we found nonannual egg extrusion among Dungeness crabs. Seasonal differences in the catches of ovigerous and nonovigerous females, crab sizes, shell condition, and appendage injury were examined. Additionally, all crabs collected from two bays were tagged beginning in the fall of 1995; tagging was conducted twice annually. Our pot and dive data indicate that females, particularly larger ones, do not extrude eggs annually. Larger females have lower molting probabilities, which limits mating potential and increases reliance on stored sperm. The tagging study confirmed that at least some females do not extrude eggs in one year and then extrude eggs at a later time without molting, thus skipping at least one reproductive season. A reproductive cycle of Dungeness crabs in Alaska is introduced which includes earlier egg extrusion by larger females and nonannual egg extrusion.

Journal of Crustacean Biology↗

USGS2002

Sea otter population status and the process of recovery from the 1989 'Exxon Valdez' oil spill

Sea otter Enhydra lutris populations were severely affected by the 1989 'Exxon Valdez' oil spill in western Prince William Sound, AK, and had not fully recovered by 2000. Here we present results of population surveys and incorporate findings from related studies to identify current population status and factors affecting recovery. Between 1993 and 2000, the number of sea otters in the spill-area of Prince William Sound increased by about 600 to nearly 2700. However, at Knight Island, where oil exposure and sea otter mortality in 1989 was most severe, no increase has been observed. Sea otter reproduction was not impaired, and the age and sex composition of captured otters are consistent with both intrinsic reproduction and immigration contributing to recovery. However, low resighting rates of marked otters at Knight Island compared to an unoiled reference area, and high proportions of young otters in beach cast carcasses through 1998, suggest that the lack of recovery was caused by relatively poor survival or emigration of potential recruits. Significantly higher levels of cytochrome P4501A (CYP1A), a biomarker of hydrocarbons, were found in sea otters at Knight Island from 1996 to 1998 compared to unoiled Montague Island, implicating oil effects in the lack of recovery at Knight Island. Delayed recovery does not appear to be directly related to food limitation. Although food availability was relatively low at both oiled and unoiled areas, we detected significant increases in sea otter abundance only at Montague Island, a finding inconsistent with food as a principal limiting factor. Persistent oil in habitats and prey provides a source of continued oil exposure and, combined with relatively low prey densities, suggests a potential interaction between oil and food. However, sea otters foraged more successfully at Knight Island and young females were in better condition than those at Montague Island. We conclude that progress toward recovery of sea otters in Prince William Sound is evident, but that in areas where initial oil effects were greatest, recovery may be constrained by residual spill effects, resulting from elevated mortality and emigration. It is evident that internal reproduction and immigration of juveniles has been the primary means of population recovery, as opposed to broad scale redistribution of adults from outside affected areas. The result is a recovery period protracted by long-term spill effects on survival and emigration and intrinsic limits to population growth.

Alaska↗

USGS2002

Food limitation and the recovery of sea otters following the 'Exxon Valdez' oil spill

We examined the potential role of food limitation in constraining the recovery of sea otters Enhydra lutris in Prince William Sound, Alaska, following the 'Exxon Valdez' oil spill. The spill resulted in the removal of a large number of sea otters in 1989, and as of 1998, the portion of the population in the heavily oiled northern Knight Island region had not fully recovered. Between 1996 and 1998, prey consumption rate was higher and the condition of sea otters was better at northern Knight Island than in an unoiled area of the sound (Montague Island). Estimates of prey energy available per unit mass of sea otter were about 4 times higher at Knight than Montague Island, albeit not significantly different between the 2 areas. Over this same period, the number of sea otters remained constant at northern Knight Island but increased at Montague Island. These data suggest that food was at least as abundant at Knight than at Montague Island, and that recovery of sea otters via intrinsic population growth was limited by factors other than food. However, the availability of food, the prey consumption rate, and the condition of sea otters were all much lower at both Knight and Montague Islands than in areas newly occupied by sea otters where the population growth rate was near the theoretical maximum. It is possible that the relatively short supply of food (compared to areas where sea otter population growth rate was high) may have inhibited immigration or interacted with other factors (e.g. oil-induced mortality or predation) to restrict sea otter population growth. Nonetheless, these data suggest that impacts of anthropogenic disturbances on large, often food-limited vertebrate predators can persist in spite of the availability of food resources that are sufficient for intrinsic population growth.

Alaska↗

USGS2002

Harlequin duck population recovery following the 'Exxon Valdez' oil spill: Progress, process and constraints

Following the 1989 'Exxon Valdez' oil spill in Prince William Sound, Alaska, we studied the status of recovery of harlequin duck Histrionicus histrionicus populations during 1995 to 1998. We evaluated potential constraints on full recovery, including (1) exposure to residual oil; (2) food limitation; and (3) intrinsic demographic limitations on population growth rates. In this paper, we synthesize the findings from our work and incorporate information from other harlequin duck research and monitoring programs to provide a comprehensive evaluation of the response of this species to the 'Exxon Valdez' spill. We conclude that harlequin duck populations had not fully recovered by 1998. Furthermore, adverse effects continued as many as 9 yr after the oil spill, in contrast to the conventional paradigm that oil spill effects on bird populations are short-lived. These conclusions are based on the findings that (1) elevated cytochrome P450 (CYP1A) induction on oiled areas indicated continued exposure to oil in 1998; (2) adult female winter survival was lower on oiled than unoiled areas during 1995 to 1998; (3) fall population surveys by the Alaska Department of Fish and Game indicated numerical declines in oiled areas during 1995 to 1997; and (4) densities on oiled areas in 1996 and 1997 were lower than expected using models that accounted for effects of habitat attributes. Based on hypothesized links between oil contamination and demography, we suggest that harlequin duck population recovery was constrained primarily by continued oil exposure. Full population recovery will also be delayed by the time necessary for intrinsic population growth to allow return to pre-spill numbers following cessation of residual oil spill effects. Although not all wildlife species were affected by the 'Exxon Valdez' oil spill, and some others may have recovered quickly from any effects, harlequin duck life history characteristics and benthic, nearshore feeding habits make them susceptible to both initial and long-term oil spill effects.

Alaska↗

USGS2002

Sea otter ( Enhydra lutris ) perspective: Part B. Food limitation and the recovery of sea otters following the Exxon Valdez oil spill

We examined the potential role of food limitation in constraining recovery of sea otters in Prince William Sound, Alaska, following the <i>Exxon Valdez</i> oil spill. The spill resulted in the removal of a large number of sea otters in 1989, and as of 1998, the portion of the population in the heavily oiled northern Knight Island region had not fully recovered. Between 1996 and 1998, prey consumption rate was higher and the condition of sea otters was better at northern Knight Island than in an unoiled area of the sound (Montague Island). Estimates of prey energy available per unit mass of sea otter were about 4 times higher at Knight than Montague Island, albeit not significantly different between the two areas. Over this same period, the number of sea otters remained constant at northern Knight Island but increased at Montague Island. These data suggest that food was at least as abundant at Knight than at Montague Island, and that recovery of sea otters via intrinsic population growth was limited by factors other than food. However, the availability of food, the prey consumption rate, and the condition of sea otters were all much lower at both Knight and Montague Islands than in areas newly occupied by sea otters where the population growth rate was near the theoretical maximum. It is possible that the relative short supply of food (compared to areas where sea otter population growth rate was high) may have inhibited immigration or interacted with other factors (e.g., oil-induced mortality or predation) to restrict sea otter population growth. Nonetheless, these data suggest that impacts of anthropogenic disturbances on large, often food-limited vertebrate predators can persist in spite of the availability of food resources that are sufficient for intrinsic population growth.

Alaska↗

USGS2002

Sea otter ( Enhydra lutris ) perspective: Part C. Trophic linkages among sea otters and bivalve prey in Prince William Sound, Alaska, in the aftermath of the Exxon Valdez oil spill: Implications for community models in sedimentary habitats

We exploited the Exxon Valdez oil spill in Prince William Sound (PWS), Alaska, to evaluate effects of reduced sea otter densities on prey populations in sedimentary habitats. We considered the need for and characteristics of new models for trophic effects of sea otters on coastal marine benthic communities. We viewed evidence for nonlinear or uncertain patterns of prey response to varying sea otter density as particularly significant for new model structure. We specifically examined responses of densities and size distributions of populations of mussels and clams (several taxonomic and habitat categories), all important sea otter prey in PWS, to reduction in sea otter density caused by the oil spill. We utilized two primary criteria for determining the consistency of prey demographic responses to reduced sea otter densities as predicted by null hypotheses consistent with existing published models. First, prey populations subject to reduced influence by sea otters should be denser and contain proportionately more large individuals than prey populations strongly influenced by sea otter predation. Second, response times of prey demography to reduced otter densities should be similar to response times of prey to increased otter densities, the latter as indicated in existing published models. Results were disparate with regard to expectation for the six categories of prey evaluated. With few exceptions, density data indicated nonconformance with demographic expectations. In contrast, size data for prey indicated conformance with expectation in about half the categories evaluated. We suggest that lingering effects of the oil spill, nonlinear relationships of sea otters and prey that involve thresholds in otter density, uncertainties in prey recruitment patterns, spatial differences in natural disturbance rate, and differences between areas in effects of competing predators are the main factors possibly accounting for patterns in our data. Recruitment and disturbance effects in particular may include significant stochastic components, especially in a temporal context. We suggest that recovered sea otter populations and their prey do not necessarily exist in long-term stable equilibria, and that development of new models incorporating both trophic thresholds and trophic stochasticity will be important in understanding community-level responses to variable sea otter numbers.

Alaska↗

USGS2002

Harlequin duck ( Histrionicus histrionicus ) perspective: Harlequin duck population recovery following the Exxon Valdez oil spill: Progress, process, and constraints

Following the 1989 Exxon Valdez oil spill in Prince William Sound, Alaska, we studied the status of recovery of harlequin duck ( Histrionicus histrionicus ) populations during 1995-1998. We evaluated potential constraints to full recovery, including (1) exposure to residual oil, (2) food limitation, and (3) intrinsic demographic limitations on population growth rates. In this paper, we synthesize the findings from our work and incorporate information from other harlequin duck research and monitoring programs to provide a comprehensive evaluation of the response of this species to the Exxon Valdez oil spill. We conclude that harlequin duck populations had not fully recovered by 1998. Furthermore, adverse effects continued as many as 9 years after the oil spill, in contrast to the conventional paradigm that oil spill effects on bird populations are short-lived. These conclusions are based on the findings that (1) elevated cytochrome P450 induction on oiled areas indicated continued exposure to oil in 1998, (2) adult female winter survival was lower on oiled than unoiled areas during 1995-1998, (3) fall population surveys by the Alaska Department of Fish and Game indicated numerical declines in oiled areas during 1995-1997, and (4) densities on oiled areas in 1996 and 1997 were lower than expected using models that accounted for effects of habitat attributes. Based on hypothesized links between oil contamination and demography, we suggest that harlequin duck population recovery was constrained primarily by continued oil exposure. Full population recovery also will be delayed by the time necessary for intrinsic population growth to allow return to pre-spill numbers following cessation of residual oil spill effects. Although not all wildlife species were affected by the Exxon Valdez oil spill, and some others may have recovered quickly from any effects, harlequin duck life history characteristics and benthic, nearshore feeding habits make them susceptible to both initial and long-term oil spill effects.

Alaska↗

USGS2002

Sea otter ( Enhydra lutris ) perspective: Part A. Sea otter population status and the process of recovery from the 1989 Exxon Valdez oil spill

Sea otter ( Enhydra lutris ) populations were severely affected by the 1989 Exxon Valdez oil spill in western Prince William Sound, AK, and had not fully recovered by 2000. Here we present results of population surveys and incorporate findings from related studies to identify current population status and factors affecting recovery. Between 1993 and 2000, the number of sea otters in the spill-area of Prince William Sound increased by about 600 to nearly 2700. However, at Knight Island, where oil exposure and sea otter mortality in 1989 approached 0.90, no increase has been observed. Sea otter reproduction was not impaired and the age and sex structure of animals captured are consistent with both intrinsic reproduction and immigration contributing to recovery. However, low resighting rates of marked animals at Knight Island compared to an unoiled reference area, and a high proportion of young animals in beach cast carcasses through 1998, suggest that the lack of recovery was caused by relatively poor survival or emigration of potential recruits. Significantly higher levels of cytochrome P4501A (CYP1A), a biomarker of hydrocarbons, were found in sea otters at Knight Island in 1996-98 compared to unoiled Montague Island, implicating oil effects in the lack of recovery at Knight Island. Delayed recovery does not appear to be directly related to food limitation. Although food availability was relatively low at both oiled and unoiled areas, we detected significant increases in sea otter abundance only at Montague Island, a finding inconsistent with food as a principal limiting factor. Persistent oil in habitats and prey provides a source of continued oil exposure and, combined with relatively low prey densities, suggests a potential interaction between oil and food. However, sea otters foraged more successfully at Knight Island and young females were in better condition than those at Montague Island. We conclude that progress toward recovery of sea otters in Prince William Sound is evident, but that in areas where initial oil effects were greatest, recovery may be constrained by residual spill effects, resulting in elevated mortality and emigration. It is evident that internal reproduction and immigration of juveniles has been the primary means of population recovery, as opposed to broad scale redistribution of adults from outside affected areas. The result is a recovery period protracted by long-term spill effects on survival and emigration and intrinsic limits to population growth.

Alaska↗

USGS2001

Critical habitat for ovigerous Dungeness crabs

The Dungeness crab, Cancer magister , supports an important fishery in the northeastern Pacific Ocean, yet there is limited knowledge of ovigerous female brooding locations and brooding behavior. Our earlier research suggests that ovigerous crabs aggregate at the same brooding locations for many years. Within these locations, ovigerous females occur in high densities, with the majority of the aggregation buried within the sediment. These locations often have similar water depths and sediment types and appear to be critical for this life history stage. Our study was designed to examine the bathymetric distribution of Dungeness crabs in bays with and without sea otters at eight locations within the Glacier Bay area by conducting transects with a video-equipped manned submersible. Two of the bays investigated contained large aggregations of ovigerous females. At both sites the substrate was composed primarily of sand. However, only a small percentage of the 33 km of transects were classified as sand. These data suggest that sand substrate may be a limiting resource. Since crab brooding aggregations represent a large portion of the crab population within a small area, and because they are a critical component of Dungeness life history, areas with these characteristics need to be investigated, mapped, and protected from development or exploitation. The areas requiring protection from the impact of anthropogenic wastes, fishing, and logging activities could be quite small, thus limiting conflict with alternative users.

Conference Paper↗

USGS2000

Correlates of Harlequin Duck densities during winter in Prince William Sound, Alaska

We evaluated relationships of Harlequin Duck (Histrionicus histrionicus) densities to habitat attributes, history of habitat contamination by the 1989 Exxon Valdez oil spill, and prey biomass density and abundance during winters 1995-1997 in Prince William Sound, Alaska. Habitat features that explained variation in duck densities included distance to streams and reefs, degree of exposure to wind and wave action, and dominant substrate type. After accounting for these effects, densities were lower in oiled than unoiled areas, suggesting that population recovery from the oil spill was not complete, due either to lack of recovery from initial oil spill effects or continuing deleterious effects. Prey biomass density and abundance were not strongly related to duck densities after accounting for habitat and area effects. Traits of Harlequin Ducks that reflect their affiliation with naturally predictable winter habitats, such as strong site fidelity and intolerance of increased energy costs, may make their populations particularly vulnerable to chronic oil spill effects and slow to recover from population reductions, which may explain lower densities than expected on oiled areas nearly a decade following the oil spill.

Condor↗

USGS2000

Densities of Barrow's goldeneyes during winter in Prince William Sound, Alaska in relation to habitat, food, and history of oil contamination

We evaluated variation in densities of Barrow's Goldeneyes ( Bucephala islandica ) during winter at 214 sites within oiled and unoiled study areas in Prince William Sound, Alaska in relation to physical habitat attributes, prey biomass, and history of habitat contamination by the 1989 Exxon Valdez oil spill. Using general linear model analyses, we found that goldeneye densities were positively associated with occurrence of a stream within 200 m, lack of exposure to wind and waves, and mixed (versus rocky) substrate. We speculate that these associations relate to habitat profitability via selection of beneficial attributes and avoidance of detrimental features. We also determined that biomass of blue mussels ( Mytilus trossulus ), the primary prey, was not related to Barrow's Goldeneye densities; we suggest that mussel standing stock exceeds predation demands in our study areas and, thus, does not dictate goldeneye distribution. After accounting for habitat effects, we detected no effect of history of oil contamination on Barrow's Goldeneye densities, suggesting that populations have recovered from the oil spill. Although other studies documented hydrocarbon exposure in Barrow's Goldeneyes through at least 1997, either the level of exposure did not affect populations via reductions in survival, or effects of oil exposure were offset by immigration.

Alaska↗

USGS1996

Activity and feeding of ovigerous Dungeness crabs in Glacier Bay, Alaska

The activity and feeding behavior of ovigerous Dungeness crabs, Cancer magister , were examined as part of a long-term study. Part of this study surveyed Bartlett Cove in Glacier Bay, Alaska, during the spring of 1994 where 50 commercial pots were soaked for 24 hours. Catch per unit effort (CPUE), dispersion, and stomach samples of male, nonovigerous female, and ovigerous female crabs were compared. A laboratory experiment compared feeding rate, foraging response, and survival between starved and fed ovigerous and nonovigerous crabs over a six-month winter period (December 1994-May 1995). CPUE differed significantly for crabs of different reproductive groups caught in pots during the spring 1994 survey in Bartlett Cove (ANOVA, P< 0.01). CPUE averaged 3.7 ± 0.5 (standard error) for males, 1.3 ± 0.3 for nonovigerous females, and 0.4 ± 0.1 for ovigerous females. Male and nonovigerous female Dungeness crabs were homogeneously dispersed throughout the pot transect area; however, ovigerous crabs were aggregated. A significant difference in dry weight of stomach contents and stomach fullness between reproductive groups existed. Ovigerous females had significantly lower feeding rates than nonovigerous females in the laboratory (two way ANOVA, P< 0.01). The foraging responses of ovigerous crabs were significantly lower in comparison to nonovigerous females (Mann-Whitney U test, P< 0.01). Seasonal differences were also found for foraging within the two groups over the course of the study period (Kruskal-Wallis one-way ANOVA, P< O.OI). No mortality occurred in treatments of fed and starved ovigerous and nonovigerous crabs over the six-month experiment, demonstrating Dungeness crabs can survive overwinter without feeding in laboratory conditions.

Alaska↗

USGS1996

Dispersion of adult Cancer magister at Glacier Bay, Alaska: Variation with spatial scale, sex, and reproductive status

Patterns of micro- to mesoscale distribution of Dungeness crabs ( Cancer magister ) in nearshore habitats at five locations in and near Glacier Bay National Park were revealed using subtidal transects. Sampling was conducted in April and September 1992 and 1993 and April 1994. Divers censused crabs by sex and reproductive status (ovigerous/nonovigerous females) along belt transects (2 m x 100 m) perpendicular to shore in the depth range 0 m (mean lower low water) to 18 m. A sample estimator of Morisita's index ( Î * Δ ) was used to quantify crab dispersion at 10 scales of measurement ranging from 20 m 2 to 200 m 2 at each location during each sampling period. Values of Î * Δ in ovigerous female C. magister deviated significantly (P < 0.05) from 1.0 (random distribution) toward contagion more frequently than did Î * Δ for nonovigerous female and male crabs. Ovigerous crabs also usually had higher Î * Δ than did nonovigerous female and male crabs, especially at smaller measurement scales (20-80 m 2 ). Morisita's index for all three groups of crabs decreased more frequently than it increased with an increase in measurement scale. We observed no relationship between t and crab density in nonovigerous female and male crabs, whereas Î * Δ was positively correlated with the density of ovigerous crabs. A total of 13 dense aggregations of ovigerous C. magister were observed nearshore (depth range 0-10 m) at the five study locations. About half of these were repeatedly observed at the same microsite over the course of this study. Ovigerous Dungeness crabs at Glacier Bay were usually aggregated, often forming dense aggregations with high site fidelity. These dense aggregations may concentrate a significant proportion of the brood stock of this species in a limited number of patches of optimal brooding habitat at Glacier Bay.

Alaska↗

USGS1996

Sea otter predation on Dungeness crabs in Glacier Bay, Alaska

Sea otter ( Enhydra lutris ) populations are increasing and expanding into areas of Southeast Alaska where Dungeness crabs ( Cancer magister ) are commercially harvested. Although sea otters are known to feed on Dungeness crabs, their effects upon crab demography have not been quantified. We censused Dungeness crabs in adjacent areas within Glacier Bay National Park and Preserve with and without sea otters in April 1993, 1994, and 1995 to investigate the effects of sea otter predation. Two different crab survey techniques were used. Commercial crab pots with sealed escape rings and baited similarly were soaked for 24 hours to measure CPUE in 0-25 m depths. A minimum of 50 pots was used for each site; 25 additional pots were used in 1994 and 1995 to sample crabs to 95 m depths. Divers censused crabs along belt transects (2 m x 100 m) laid perpendicular to shore from 0 m depth to 18 m; 15 transects were surveyed at each site on each date. Sea otter numbers were estimated from aerial surveys and boat observers. Prey types and feeding rates of sea otters were made from telescopic observations. In Outer Dundas Bay, where 46, 39, and 40 sea otters were present in 1993, 1994, and 1995 respectively, CPUE of Dungeness crabs in pots was 0.04, 0.04, and 0.2 crabs per day, respectively, at depths to 25 m, but was higher (4.0 crabs per day) at deeper depths (95 m). Only two Dungeness crabs were observed in 45 diver transects surveyed during three years in Outer Dundas Bay, however Dungeness crabs were a common prey item (15%) of sea otters. Approximately 16 km N in Inner Dundas Bay, where no sea otters were observed any year, CPUE of Dungeness in pots was 2.7, 1.8, and 2.5 crabs per day for 1993, 1994, and 1995, respectively. Adult crab density estimated from diver transects was 236, 78, and 50 crabs/ha in 1993, 1994, and 1995, respectively. Significantly lower crab densities and CPUE occurred in the site occupied by sea otters. Our data imply that sea otter predation decreases Dungeness crab abundance and may induce movement of crabs to greater depths. A depth refuge for crabs from sea otter predation may vary with local bathymetry.

Alaska↗

USGS1995

Nearshore distribution and abundance of Dungeness crabs in Glacier Bay National Park, Alaska

As part of an ongoing, multi-agency study to determine the effects of closure of the commercial fishery for Dungeness crabs, Cancer magister , on crab population structure we examined patterns of distribution and abundance of crabs in nearshore habitats at five locations in and near Glacier Bay National Park. Sampling was conducted in April and September 1992 and April 1993 prior to the anticipated closure of the fishery in the park. Divers censused crabs by sex and reproductive state (ovigerous/nonovigerous females) along belt transects (2m x 100m) laid perpendicular to shore in the depth range 0 m (mean lower low water) to 18 m. Preliminary results from the first three sampling periods revealed that the average densities of Dungeness crabs at the five locations ranged from 78 to 2012 crabs/ha. Crab densities differed between populations depending on sex, reproductive state of females and sampling period. Male crabs showed reduced densities at Gustavus Flats in April 1992 (P<0.01) and 1993 (P<0.001). Ovigerous females had greater density at Bartlett Cove in April 1993 (P<0.001). Sex ratios were frequently skewed toward females. At Bartlett Cove and Gustavus Flats females outnumbered males in April 1992 and 1993 (P<0.001). Most of the females at Bartlett Cove and Gustavus Flats in April 1992 and 1993 were ovigerous (P-0.001). Males tended to occupy greater depths than females in April 1992 (P<0.05) but not April 1993 (P-005). The mean depth of males shifted from deeper to shallower water between April and September 1992 (P<0.001). The depth distribution of ovigerous crabs did not differ from that of nonovigerous female crabs. Future research prior to the anticipated closure of the commercial Dungeness crab fishery in Glacier Bay will include a tagging study to determine the extent of crab movement and further study of the temporal as well as the spatial variability observed in the structure of these populations.

Alaska↗