Effects of stocking density on a typical crab-clam polyculture system: Behavioral mechanisms of predation and competition in swimming crab (Portunus trituberculatus)

Abstract Stocking density determines the performance of polyculture. However, the underlying mechanisms of how density affects the behavior of aquaculture animal are still unclear, especially the predation and competition. Here we selected a typical polyculture system containing predator swimming crabs (Portunus trituberculatus) and prey Manila clams (Ruditapes philippinarum), and examined the influence of stocking density on crab predation, competition, and their interactions. Specifically, we calculated the predation rate, and quantified the foraging behavior and encounter behavior between crabs. Results show that, with increasing stocking density, the predation rate significantly decreased as a result of aggressive interactions between crabs (e.g., fighting and threatening). Increased crab density promoted their competition, which was manifested as an increase in the proportion of time spent on aggression. However, the probability of aggressive behavior upon encounter was reduced to avoid time loss and the risk of injury during aggression. Nevertheless, the prolonged aggressive behavior negatively affected the foraging efficiency, as indicated by an increase in the proportion of handling time and a decrease in the probability of consumption upon capture. While the crabs attempted to compensate for the reduced foraging efficiency by significantly increasing the probability of capture upon encounter, the impacts of aggressive interactions outweighed their coping capacity, rendering a reduction in observed predation rates. The revealing of behavioral mechanisms not only provides an improved understanding of how the stocking density affects the behavior of crabs, but also helps to optimize the management of crab-clam polyculture.