Growth and survival of recently settled snow crab Chionoecetes opilio in relation to intra- and intercohort competition and cannibalism: a laboratory study

Abstract

A laboratory experiment was conducted on snow crab Chionoecetes opilio to test the hypothesis that abundance of recently settled crabs is more likely to be regulated by cannibalism between than within cohorts. Growth and mortality of postlarval Instar I crabs (3 mm carapace width, forming the junior cohort) at different densities (7, 14, 28 or 56 by tray), or at constant density (56 by tray) but in the presence of 1, 3, 6, 9, 12 or 24 postlarval Instar V crabs (14 mm carapace width, form- ing the senior cohort), was recorded over 104 d. With the junior cohort alone, mean proportional sur- vival of crabs was high (64 to 81%), independent of density, and similar to that of individually reared crabs (68%). However, the first and second molts occurred increasingly late as density increased, possibly reflecting growing levels of agonism and competition. Molting crabs were not particularly vulnerable to cannibalism. In treatments mixing the junior and senior cohorts, mean proportional sur- vival of junior crabs decreased from 31% in the presence of 1 Instar V to 0% in the presence of 9 or more Instar V and the prevalence and intensity of limb loss for survivors increased with density of the senior cohort. Cannibalism was the additional or sole source of mortality for junior crabs in treatments with Instar V crabs and it occurred swiftly in the first 5 d of the experiment. Moreover, the presence of any number of larger conspecifics resulted in a severe delay in molting of junior crabs compared to control (no Instar V crabs). Therefore, intercohort predation is more important than intracohort predation or competition as a process effecting year class strength and condition during the early benthic stages of snow crab.