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Shellfish Assessment Program - Habitat Research: Past Research


Juvenile King Crab Habitat

Red king crab, Paralithodes camtschaticus, and blue king crab, P. platypus, are two commercially important, federally managed species in Alaska. Juveniles of both species settle on complex habitats, such as cobble, shell-hash, and structure forming organisms such as hydroids. Although there is no evidence to suggest that adult red and blue king crabs compete for resources, juveniles might.

One potential mechanism that would allow coexistence is habitat partitioning. Limited field surveys suggest that red king crab prefer cobble habitat whereas blue king crab prefer shell hash.

This study was designed to test the hypothesis that habitat type can reduce negative competitive interactions between red and blue king crabs. In this experiment, year-0 crabs were reared in small tubs for 13 weeks with either cobble or shell hash as the habitat. Each tub had either red king crabs alone, blue king crabs alone, or a mixture of red and blue king crabs.

Every week all of the surviving crabs were counted in each tub. The mortality rate in each treatment for each species was calculated and compared among the treatments.

Both habitat and species composition affected mortality rates. Both species had better survival in shell hash than in cobble. Red king crabs however, survived better in the presence of blue king crabs, especially in shell hash where the mortality rate was almost cut in half.

In contrast, blue king crab survival was much poorer when reared with red king crabs, particularly in cobble where all of the blue king crab died within 9 weeks when red king crabs were present. Most of the mortality in the trials was probably due to cannibalism and predation as no dead crabs were removed from the tanks.

The data from this study suggest that habitat cannot mediate interactions between red and blue king crabs. Although shell hash habitat did reduce blue king crab mortality compared to cobble, adding red king crabs to the trials substantially increased mortality regardless of habitat type.

This suggests that red king crab juveniles may have an advantage over blue king crabs in most circumstances and that further investigations into the role competition and predation between the species is warranted.

Graph of year-0 red king crab in cobble 
			and shell hash reared alone and with blue king crabs. Click image to enlarge
Survival of year-0 red king crabs in cobble and shell hash habitats reared alone (RKC) and with blue king crabs (RKC/BKC). Points represent average 1 SE, and lines represent the best-fit trend lines. Click image to enlarge.
Graph of year-0 blue king crab in cobble
			and shell hash reared alone and with red king crabs. Click image to enlarge.
Survival of year-0 blue king crabs in cobble and shell hash habitats reared alone (BKC) and with red king crabs (RKC/BKC). Points represent average 1 SE, and lines represent the best-fit trend lines. Click image to enlarge.

Effects of Ghost Fishing on Red King Crab Populations

Photo of a red king crab with an acoustic tag stuck to his shell
A red king crab with an acoustic tag stuck to his shell hangs out on a pier piling in Womens Bay. The tag emits an ultrasonic beep which allows divers carrying a hydrophone to locate the crab on the bottom.

Ghost fishing, the capture and killing of marine organisms by lost or abandoned fishing gear, is a serious ecological and economic problem confronting fisheries. In a long term study (1991-2008), Kodiak researchers determined the effect of ghost fishing on the population of the red king crab (Paralithodes camtschaticus) in Womens Bay, Kodiak Island, Alaska. 192 late juvenile and adult crabs were tagged with acoustic tags which allowed researchers to track them using hydrophones (underwater microphones). Research divers could determine when a crab molted or died and, in many cases, the cause of death. During this study 13 of the 192 crabs were killed in ghost fishing gear (12 in ghost crab pots and 1 in a ghost gill net) and 20 were captured in ghost pots and released by divers. An additional 13 died of other causes, including predation by sea otters, an octopus and by human poaching. We used the results to estimate how many crabs were killed each year by ghost fishing. We calculated 2 estimates, one assuming the crabs released alive from pots would have escaped by themselves anyway and one assuming all those crabs would have died. Based on those assumptions, between 16% and 37% of the population of red king crabs in Womens Bay were killed by ghost fishing per year during the period of this study, making ghost fishing a substantial source of mortality. These results indicate that reducing ghost fishing in Womens Bay would be helpful to the population.


Cannibalism in Red King Crab

Photos of year-1 red king crab hunting for
			a year-0 crab in experimental containers: A) sand habitat, B) shell habitat and C) shell hash habitat.
			Click image to enlarge.
Year-1 red king crab hunting for year-0 crab in experimental containers in A) Sand habitat, B) Shell habitat, and C) Shell hash habitat. Click image to enlarge.

The red king crab, Paralithodes camtschaticus, is an important fishery species in Alaska. Although much is known about the adult life-history stage, juveniles are small and difficult to find or study, and thus much less is known about them. As predation is likely the most important source of mortality for juvenile crabs, understanding the predator-prey dynamics is important to help understand what factors allow them to survive to adult stages. Cannibalism in crab species can be an important determinant of how many juveniles survive to adulthood, and this might be especially important in king crab because year-0 and year-1 crab occupy the same habitat types in the wild.

An important part of predator-prey relationships is the predator functional response which shows how the prey density affects predation. It can be linear where density does not affect predation risk (Type I), inversely density-dependent where predation risk decreases with density (Type II), or density-dependent where predation risk increases with density (Type III).

In one study, we conducted experiments to see how predator size, year-1 (small) and year-2 (large) juvenile red king crabs, and habitat, sand habitat and macroalgae mimic, affect predation on newly settled year-0 crabs. The functional response of small predators changed from a Type II in sand to a Type I in macroalgae mimic. Large predators, on the other hand, exhibited a Type I functional response in sand and Type III in macroalgae mimic. Smaller predators ate fewer prey and the macroalgae mimic reduced the number of prey consumed. Large predators ate nearly all the prey in sand, but the reduction in predation, with the addition of macroalgae mimic, was much greater than for smaller predators. This suggests that smaller predators have an easier time hunting prey than do large predators in complex habitat.

These studies help us understand what kinds of habitats baby crabs need in order to survive and how habitat protect them from predators. The studies also help us to understand how habitat can alter predation risk for crabs and provide insight into how juvenile red king crab forage.





Video of foraging by year-1 red king crab foraging for year-0 crab in different habitat types. Video by Jessica Popp.




Gulf of Alaska Seamounts Exploration of Deepwater Crab

Photo of DSV Alvin launching from R/V Atlantis

Scientists at Shellfish Assessment Program participated in two ventures using the Deep Submergence Vehicle (DSV) Alvin to explore seamounts in the Gulf of Alaska.

The first was a 1999 exploration of the Patton Seamount (54.66 N, 150.50 W) to investigate the depth distribution of commercially important deep water crab species. During the study, eight dives were completed to depths of 200-3000 meters. Several crab species were observed that had previously not been known to exist in this area.

Click HERE to learn more about crabs observed during the 1999 Patton Seamount exploration.

Dr. Stevens revisited the Patton seamount, as well as the Murray seamount, when he joined the crew of the R/V Atlantis during the 2002 Gulf of Alaska Seamounts Expedition, where seven seamounts (five of which were previously unexplored) were visited.

Funding was provided by the NOAA Office of Exploration and the West Coast & Polar Regions Undersea Research Center.

Click HERE to learn more about the 2002 Gulf of Alaska Seamounts Expedition.



Shellfish Assessment Program - Habitat Research CURRENT RESEARCH


Related Publications

  • LONG, W. C., S. B. VAN SANT, and J. A. HAAGA. 2015. Habitat, predation, growth, and coexistence: Could interactions between juvenile red and blue king crabs limit blue king crab productivity? Journal of Experimental Marine Biology and Ecology, 464: 58-67.

  • LONG, W. C., P. A. CUMMISKEY, and J. E. MUNK. 2014. Effects of ghost fishing on the population of red king crab (Paralithodes camtschaticus) in Womens Bay, Kodiak Island, Alaska. Fishery Bulletin, 112: 101111. (.pdf, 465 KB). Online

  • LONG, W. C., and L. WHITEFLEET-SMITH. 2013. Cannibalism in red king crab: Habitat, ontogeny, and the predator functional response. Journal of Experimental Marine Biology and Ecology, 449: 142-148.

  • LONG, W. C., J. POPP, K. M. SWINEY, and S. B. VAN SANT. 2012. Cannibalism in red king crab, Paralithodes camtschaticus (Tilesius, 1815): Effects of habitat type and predator density on predator functional response. Journal of Experimental Marine Biology and Ecology, 422-423: 101-106.


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