Early blue crab recruitment to alternative nursery habitats in Mississippi, USA

Journal of Shellfisheries Research, Jan, 2005 by Chet F. Rakocinski, Donna Drury McCall

ABSTRACT Although seagrass in the lower segments of estuaries provides good nursery habitat for blue crabs, the role of alternative inshore habitats for early blue crab recruitment is not well-known. Using suction sampling, we examined the recruitment dynamics of early blue crabs (Callinectes spp.) over a 10-wk study period at 7 sites representing potential nursery habitats, including 2 marsh edge sites, 2 subtidal unvegetated sites adjacent to salt marsh, 2 subtidal unvegetated sites adjacent to developed marsh, and 1 upestuary subtidal vegetated site. Abundances of small (<6.0 mm CW) and large ([greater than or equal to] 6.0 mm) juvenile crabs varied over the study period, mainly reflecting two monthly pulses of small crabs coinciding with the new moon phase. Early crabs were significantly more abundant from structured habitats than from subtidal unvegetated habitat; although small crabs were moderately abundant at subtidal unvegetated sites situated lower in the estuary. Large crabs were not abundant from subtidal unvegetated sites; whereas large crabs were abundant from sites with structured habitat. Subtidal unvegetated sites were characterized by crab size-distributions with single modes representing postsettlement crabs and with few large crabs; whereas structured habitats contained relatively more large crabs. However, crab size-distributions varied among structured habitats. Spatio-temporal variation in the early recruitment dynamics of blue crabs reflected temporal changes, such as lunar periodicity; landscape effects, such as proximity to currents; and habitat effects.

KEY WORDS: blue crab recruitment, Callinectes, nursery habitat

INTRODUCTION

Early blue crab recruitment involves an estuarine-dependent bipartite life-cycle consisting of a larval-supply phase as well as a postsettlement phase. High densities of postsettlement blue crabs commonly occur in vegetated and other structured habitats (Williams et al. 1990, Pile et al. 1996). For example, many studies of early blue crab recruitment focus on the role of submerged aquatic vegetation (SAV) as a nursery habitat. However, the blue crab postsettlement phase entails a sequence of ontogenetic shifts in movements and habitat use that are not fully understood (Orth & van Montfrans 1987, Pile et al. 1996, Pardieck et al. 1999). For example, Mense and Wenner (1989) found high densities of early juveniles in unvegetated sandy-mud substrate; early stages may even burrow in soft-sediments or seek refuge among the interstices of large particles in unvegetated sediments. Later juveniles also undertake movements from vegetated to unvegetated habitats. Movements of early juveniles from unvegetated sediments may even subsidize the supply of crabs to structured habitats (Rakocinski et al. 2003). In addition, upestuary movements of early crabs may also influence blue crab recruitment dynamics (Pardieck et al. 1999).

In Mississippi coastal waters, structured habitats such as seagrass and emergent fringing marsh are rapidly declining, whereas areal coverage by subtidal unvegetated habitat is high. Indeed, the estimated coverage by submerged aquatic vegetation (SAV) in Mississippi Sound has declined by ~85% over the last 30 years (Moncreiff et al. 1998). In this region, fringing emergent vegetation is also rapidly declining due to shoreline development. In Chesapeake Bay, the historical decline in SAV has resulted in shifts by early crabs into shallow unvegetated habitats (Ruiz et al. 1993).

While SAV in the lower segments of estuaries clearly provides an important nursery habitat for early blue crabs, little is known about how alternative nursery habitats function in early crab recruitment within the US Gulf coast region. In a previous study in Mississippi Sound, we found considerable numbers of early postsettlement blue crabs from subtidal unvegetated sediments, suggesting that this habitat might subsidize nearby structured habitat (Rakocinski et al. 2003). Other patterns of postsettlement movement among different nursery habitats at various spatio-temporal scales may also influence early blue crab recruitment. Clearly, not enough is known about blue crab recruitment dynamics on the landscape scale in the US Gulf coast region.

One approach to evaluating nursery value is to measure abundances of early stages occurring in different habitats over time. The value of a particular habitat is also dependent on its physiographic context. For example, the same kind of habitat may vary greatly in nursery potential depending on contextual features, such as position with respect to supply rates of early stages, hydroperiod, substrate, salinity, etc. The overall objective of this study is to examine variation in blue crab recruitment among seven inshore sites representing potential nursery habitats, including 2 marsh edge sites, 2 subtidal unvegetated sites adjacent to salt marsh, 2 subtidal unvegetated sites adjacent to developed marsh, and 1 upestuary subtidal vegetated site within the Mississippi Sound estuary over a 10-wk period during late summer and early autumn 1999. Specific objectives involved comparisons of: (1) abundances of postsettlement crabs (i.e., small crabs <6 mm CW) and early juvenile crabs (i.e., large crabs [greater than or equal to] 6 mm CW) across six biweekly sample periods; (2) abundances of postsettlement crabs and early juvenile crabs among sites representing nursery habitats; (3) size distributions of early stages of blue crabs among sites representing nursery habitats.