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Shellfish Assessment Program

Cannibalism in Red King Crab: Effects of Habitat, Predator Density, and Prey Density

figure 1, see caption
Figure 1.  Proportional predation rates as a function of prey density for type I, type II, and type III functional responses.  Note that in a type II functional response, as the prey density decreases, the predation rate increases, leading to possible local extinction, whereas in a type III functional response the predation rate decreases as the prey density decreases, leading to a low density refuge from predation.

Red king crab, Paralithodes camtschaticus, an important fishery species in Alaska, exhibits cannibalism both within and among age groups. Cannibalism in crab species can be an important determinant of recruitment success, and this might be especially important in king crab because year-0 and year-1 crabs occupy the same habitat types in the wild. An important aspect of the predator-prey relationship is the predator functional response, which describes how the predation rate of predators varies with prey density.

Three common types of functional responses are type I or density independent, type II or inversely density dependent, and type III or density dependent. Type II functional responses are destabilizing and can lead to local extinction of prey species, whereas type III functional responses are stabilizing with a low-density refuge from predation (Fig. 1). The functional response can be changed by differences in habitat, spatial arrangement of prey, predator density, or the presence and density of alternative prey.

The red king crab population in the Gulf of Alaska crashed in the 1980s and has not recovered despite a closure of the commercial fishery. In response, researchers are exploring the possibility of enhancing the population through the release of hatchery-reared crabs into the wild. It is critical to understand the predator-prey relationships between newly released crabs and their predators in order to determine the ideal habitat and density that will maximize survival of the crabs.

figure 2, red king crabs
Figure 2.  Year-1 red king crabs hunting year-0 red king crabs in A) sand, B) shell, and C) shell hash habitat types during experimental trials.  Photos by Chris Long and Jessica Popp.

In this study we used laboratory experiments to determine the predator functional response of year-1 crabs to year-0 crabs in three different habitat types: sand, which was unstructured soft sediment; shell, which was whole clam valves; and shell hash, which were smaller pieces of crushed shell (Fig. 2). Experiments were performed in plastic containers 31 x 20 x 24 cm (L x W x H). Five densities of prey were used in the experiment: 2, 5, 10, 18, and 25 crabs per container. We also examined how predator density (1 or 2 predators) affects the functional response using prey densities of 2, 5, 10, 25, and 50 crabs per container.

The trials were run as follows. At 3 p.m. the day before the trials, the appropriate habitats were established in each container. Then the prey crabs (carapace width 1.45.0 mm) were removed from the holding tank, placed at the appropriate density in each of the containers, and acclimatized overnight. At 9 a.m. the day of the trial, predator crabs (carapace length 1523 mm) were introduced into the containers to initiate the trial. Predators were starved for 2448 hours prior to use in trials to standardize hunger levels. The predators were allowed to feed for 2 hours and then were removed and the number of surviving prey counted. Proportional predation was calculated for each trial.

The data was fitted to type I, type II, and type III functional response models and the best fit model was chosen. The functional response was a type II in all habitat types; however, the predation rate was lower at all prey densities in the shell habitat than in shell hash and sand (Fig. 3). This indicates that shell habitat provides a good refuge from predation, but that shell hash and sand do not. The functional response was a type II at both predator densities as well. The presence of a second predator decreased both the attack rate and the handling time, resulting in slightly lower predation rates at low prey densities and slightly higher at high prey densities when compared to the single predator treatment (Fig. 4). This indicates that predators may be interfering with each other's foraging.

figure 3, see caption
Figure 3.  Functional response of year-1 red king crabs to year-0 red king crabs in three different habitats in terms of A) proportional predation and B) number eaten.  Symbols represent the mean + SE.  Sand points are offset slightly.  Lines represent the type II functional response estimated by maximum likelihood.  One line is plotted for sand and shell hash because there was no difference in the functional response between those habitats.
  figure 3, see caption
Figure 4.  Functional response of year-1 red king crabs to year-0 red king crabs at two predator densities in terms of A) proportional predation and B) number eaten per predator.  Symbols represent the mean + SE, and the two predator points are offset slightly.  Lines represent the type II functional response estimated by maximum likelihood.

This work has implications for potential stock enhancement activities, as year-1 crabs could inhibit enhancement success though cannibalism on introduced year-0 crabs, especially given the destabilizing nature of the type II functional response.

Given these results, a moderate to high stocking density in complex habitat is the most likely to maximize survival of the crabs. However, more research needs to be done to examine whether the presence of alternative prey changes the functional response to a type III, for this would suggest that a low stocking density would be best. Additionally, as year-1 crabs have the potential to strongly influence the success of the next year class, it may be advisable to stock a given area only once every 2 years. Larger crabs are less effective predators on year-0 crabs, and crabs start to pod and move out of the most complex habitats during the second year.

By W. Christopher Long

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