Importance of low salinity areas for juvenile blue crabs, Callinectes sapidus Rathbun, in river-dominated estuaries of southeastern United States

Abstract

Early juveniles represent a critical stage for population dynamics of the blue crab, Callinectes sapidus. Periodic high predation pressure on small juveniles emphasizes the importance of suitable habitat for refuge. Seagrass beds are an important nursery habitat in some areas, providing refuge from predation as well as possible greater growth rates. However, seagrass beds and certain other subtidal structural habitats, such as subtidal oyster beds, are largely absent from many estuaries along the southeast Atlantic coast from southern North Carolina to southern Georgia. Understanding limitation of blue crab populations in these areas requires knowledge of alternative juvenile habitats. On smaller spatial scales, marsh channels and woody debris may provide refuge and foraging habitat. However, low salinity regions of these estuaries may also provide critical nursery habitat over broader spatial scales. We examined juvenile blue crab use of low salinity areas through a combination of monthly monitoring of distribution patterns across a salinity gradient in two estuaries, examination of growth and survival within single-crab enclosures at sites of varying salinity, and examination of predation pressure from oligohaline to river mouth locations using tethering studies and sampling of predator abundances. Juveniles <13 mm CW were initially more common at higher salinity sites in spring, but exhibited a seasonal change and were more common at lower salinity locations in summer and fall. Juveniles 13–24 mm CW were negatively correlated with salinity during spring and summer. Larger crabs (>24 mm CW) were uncorrelated with salinity or were more common at higher salinity sites. Lower salinity sites were associated with greater survivorship, more rapid molting, and greater crab dry weight compared to higher salinity areas. Tethering indicated a trend towards lower predation mortality at lower salinity areas and abundances of larger conspecifics, a potential predator, were positively associated with increasing salinity in these estuaries. These results suggest that lower salinity regions of the small, river-dominated estuaries of southeastern United States may be utilized as important nursery areas, may be associated with increased growth and reduced predation, and may represent a vital habitat for maintenance of local blue crab populations.