Long-term measures of predation and cannibalism in juvenile blue crabs.
This study reports an unusual 25-year record of field experiments indicating seasonal and annual variation in predation on juveniles of an important estuarine predator. Blue crabs (Callinectes sapidus) are dominant epibenthic predators forming valuable commercial fisheries in estuaries of the eastern and gulf coasts of North America. We used replicated tethering of multiple sets of 10 individuals to provide an index of predation on intermolt crabs in specific size ranges and habitats. Tethering consisted of attaching a 1-m light-weight wire line to a figure-8 harness glued across the back a crab, allowing it free movement within the radius of the site. The other end of the line was attached to a large spike pressed into the bottom sediment, and a surface float also tied by a light line to the spike showed the location. Mortality and damage was checked after 24 hr. Lab tests with known predatory fish and crabs showed that cannibalism by large adult crabs left distinct pieces of carapace or punctured crab bodies; whereas predation by fish resulted in catching the fish on the tether. Field tests using Dual Frequency Imaging Sonar (DIDSON) and tethers with hooks showed that only large crabs attacked juvenile crabs. Tethering can provide a good comparative index of relative, but not absolute, rates of predation rate.
Tethering experiments in the Rhode River in central Chesapeake Bay indicated cannibalism by large adult crabs accounted for >95% of mortality tethered crabs in the field; fish predation was rare. Predation/ cannibalism was size-dependent: <70mm juveniles incurred high levels of mortality; 90-110 mm pre-adults had low mortality; and >120mm adults no mortality. Mortality was also depth-dependent: mortality was high at depths >1m; intermediate at 70 cm; and lowest at 20-40 cm shallows. Cannibalism rates peaked in July-Aug during warmest temperatures when adults were most abundant and active. Long-term tethering showed that summer predation rates on juveniles varied among years and declined markedly from 70%/day in 1989-1991 to 8% in 2000-2004, then increased to 35-40% in 2008-2010, followed by another decline. Predation/cannibalism on juveniles was positively and linearly correlated with adult crab abundance over the 25-yr record. Cannibalism by adult blue crabs provides a density-dependent regulation of juveniles.