The Effects of Holding Space on Juvenile Red King Crab (Paralithodes camtschaticus) Growth and Survival
Figure 3. The experimental rearing tank consists of three treatment holding cells, 20, 40, and 77 mm diameter and 30 replicates per treatment. At the beginning of the experiment, one juvenile red king crab was randomly placed in each holding cell.
Figure 4. Red king crab carapace removed from the molt. The red line illustrates how carapace length is measured.
Alaskan red king crab (Paralithodes camtschaticus) historically supported one of the most valuable crab fisheries in the United States; however, populations declined in the early 1980s and have not recovered despite closures of some areas to commercial fishing. The population decline and continued low population levels in the Gulf of Alaska have spurred early life-history research in order to better understand recruitment processes, habitat requirements, predator/prey interactions, effects of environmental change, and the feasibility of rehabilitating the population via stock enhancement.
The advancement of research and stock enhancement necessitates the development of reliable techniques for producing adequate numbers of juvenile king crab in captivity. When crustaceans, including juvenile red king crab, are reared communally, high rates of mortality due to cannibalism result in low yields. One potential strategy to reduce loss from cannibalism is to rear crustaceans in individual cells. Small holding cell size, however, can result in decreased growth or increased mortality. Therefore it is essential to identify the optimal holding cell size, both for mass culturing efforts and for experimental design purposes.
To determine if space limitation affects juvenile red king crab growth and survival, we reared red king crab (3.67- 8.30 mm carapace length), in 20-, 40-, and 77-mm diameter holding cells (Fig. 3) for 274 days. Holding cells were checked daily for mortalities and molts which were recorded and removed. To examine growth, carapaces were carefully removed from all molts and mortalities and photographed under a dissecting scope. Carapace length was measured using image analysis software (Fig. 4). At the termination of the experiment, all surviving crab were sacrificed and measured.
Crab size did not differ among holding cell sizes after the first, second or third molts; however at the end of the experiment, crabs reared in small holding cells were significantly smaller (17%) than those reared in the large holding cells (Fig. 5). Carapace length did not differ between crabs reared in small and medium holding cells or medium and large holding cells.
Figure 5. Mean carapace lengths after each molt and at the end of the experiment and mean inter-molt period in degree days for crabs reared in small, medium, and large holding cells. Reported percentages are the percent of crabs alive at the end of the experiment in each holding cell size that underwent a third molt.
Figure 6. Percent survival by experimental day of juvenile red king crab reared in small, medium, and large holding cells. We modeled survival with the following equation S = e-mT, where S is proportional survival, m is the mortality rate and T is the time in days. Mortality rates estimated by maximum likelihood are 0.0039 (SE = 0.000068) for small holding cells, and 0.00037 (SE = 0.000013) for the medium and large holding cells.
The inter-molt period in days did not differ among crab reared in different sized holding cells between the beginning of the experiment and the first molt or between the first and second molt (Fig. 5). The inter-molt period was not compared between the second and third molt as only 84% of crabs reared in large holding cells and 40% of the crabs in the small and medium holding cells underwent a third molt by the termination of the experiment; however, these data suggest that the inter-molt period is shorter for juveniles reared in large holding cells, as a higher percentage of these crabs underwent a third molt compared to those reared in small and medium holding cells.
Overall survival at the end of the experiment was 83% in the large and medium holding cells and 17% in the small holding cells (Fig. 6). This corresponds to mortality rates in the small holding cells being an order of magnitude larger than those in the medium and large holding cells.
Small holding cell size reduced both growth and survival of juvenile red king crab. For rearing individual juvenile crab, the optimal holding cell size is as small as possible without substantially reducing growth and survival. Based upon these criteria and the cell sizes examined in this study, the medium holding cell is ideal to rear juvenile red king crabs of the sizes examined; mortality rates were no higher than those in the large holding cells while growth rates were, only minimally lower.
Our results provide laboratories and hatcheries the information to determine if achieving increased survival outweighs the increased labor and space requirements associated with rearing juveniles individually and to determine the appropriate size of container to use in experiments.
