Temporal Variation in Juvenile Blue Crab Mortality: Nearshore Shallows and Cannibalism in Chesapeake Bay

Adult abundance is determined by both recruitment and survival of juveniles. Blue crabs exhibit a distinct population size-structure reflecting seasonal cycles of recruitment and growth in the Rhode River, a subestuary lacking submerged vegetation in central Chesapeake Bay. Seasonally shifting modes in the size-structure, corresponding to juvenile, prepubertal and mature crabs, allowed trawl catches to be partitioned into 0+ and 1+ year classes of the crabs' 2-3 year life span. Fluctuations in the annual abundance of the year classes during ]988-1993 indicated that abundance of 0+ juveniles was not a good predictor of 1+ yearclass abundance. We used tethering techniques to assess sources of, and temporal variation in, juvenile blue crab mortality as a function of water depth in the nearshore shallows of the subestuary. Laboratory experiments and analysis of remains of crabs tethered in the field indicated that cannibalism by large blue crabs was the source of 75-97% of mortality of juveniles. Crabs 30-50 mm CW suffered significantly higher mortality than 50-70 mm crabs. Mortality of tethered juveniles was significantly lower in shallow (15 cm) than deep (>70 cm) water. Mortality rates did not differ between day and night periods. Mortality of tethered juvenile blue crabs was high (40-90% per day) and exhibited significant seasonal and annual variation from 1989-1993; and water depth showed significant interaction effects with these components of temporal variability. Multiple regression analysis indicated that water temperature, abundance of 1+ year-class crabs in deep-water trawls, and abundance of 0+ yearclass crabs in nearshore seines accounted for 82% of the annual variation in mortality of juveniles tethered at medium (40 cm) depth. Together, these results emphasize the importance of the nearshore shallows and body size as a crucial refuges for juvenile blue crabs encountering intense, but temporally variable cannibalism by large crabs. Juvenile mortality may be an important variable regulating population dynamics of blue crabs. Variation in recruitment is a key component of population dynamics (Rothschild, 1986; Roughgarden et al., 1988). Fluctuations in abundance of adult populations reflect variation in both larval settlement (Phillips, 1986; Gaines and Roughgarden, ]987) and juvenile survival (Caputi and Brown, 1986; Herrnkind and Butler, 1986). Juvenile mortality may obscure the effects of variation in larval settlement through both density-independent and density-dependent mechanisms (Murdoch and Bence, ]987; Richards and Lindeman, 1987). Predation is often a dominant component of density-dependent mortality for juveniles (Beverton and Holt, 1957), and cannibalism can comprise a major source of predation upon juveniles (Elgar and Crespi, 1992) that regulates recruitment success in size-structured populations typical of many aquatic ecosystems (Tschumy, 1982; Werner and Gilliam, 1984; Ebenman and Persson, 1988; Stein et aI., 1988). Predation rates are often mediated through prey utilization of refuges (Gilliam and Fraser, 1987; Persson, 1993), which may include spatial (habitat) and/or temporal (diurnal, seasonal, annual) components that reduce access or effectiveness of predator impact on prey. Spatial and temporal aspects of refuges may interact so that juvenile mortality may exhibit significant temporal variation within a refuge habitat (Heck and Wilson, 1987), especially when a critical habitat provides only partial refuge from predation (Eggleston et aI., 1992; Dittel et aI., 1995). Thus, temporal patterns of juvenile mortality within refuge habitats may provide insight into variation in recruitment success regulating population dynamics.