How Have Other People Tried to Conserve Blue Crabs Natural Resource or Ecosystem
Food availability and growth of the blue crab in seagrass and unvegetated nurseries of Chesapeake Bay
Abstract
Variation in habitat quality and resource availability can affect the distribution and growth of animals. Thin-shelled clams dominate many benthic communities in Chesapeake Bay, both in numbers and in biomass, and they can comprise up to 50% of the blue crab ( Callinectes sapidus ) diet. Our objective was to determine which habitats were optimal for juvenile crab growth and how growth related to food availability. We experimentally examined benthic infaunal food availability (primarily bivalves) and concurrent growth of juvenile blue crabs at 30–40 sites along 50 km of the York River during fall 2000 and spring 2001. Each year, 4–10 replicate sites along the York River were established in each of five habitats: (1) Seagrass, (2) Mud at the river mouth, (3) Sand at the river mouth, (4) Mud upriver, and (5) Sand upriver. Food availability inside and outside of 0.43-m 2 crab growth cages was examined, along with crab growth after 3–6 months inside cages. In both years, after 3–6 months, the Baltic clam, Macoma balthica , was abundant inside and outside the cages, whereas the soft-shell clam, Mya arenaria , was only abundant inside cages. Densities of Macoma were greatest in upriver mud and sand, while those of Mya were greatest in upriver sand. Crab growth was significantly greater in spring-summer than fall-winter and was significantly higher in upriver mud and sand, where clam densities were highest, than at the river mouth. The upriver region was near the turbidity maximum, which may enhance pelagic and benthic productivity and thereby provide more food for clams and therefore for blue crabs. Crab growth in seagrass was intermediate between that upriver and at the mouth, suggesting that upriver, unvegetated, subtidal habitats adjacent to salt marshes serve as valuable nursery habitats rivaling seagrass beds.
Introduction
The blue crab in Chesapeake Bay is important both economically and ecologically. This species supports the largest commercial fishery in the state (Lipcius and Stockhausen, 2002) and serves as both predator and prey in a complex food web with multiple linkages to benthic and pelagic species (Baird and Ulanowicz, 1989, Chesapeake Fisheries Ecosystem Plan Technical Advisory Panel, 2004). Blue crab stocks in Chesapeake Bay have been declining in recent years (Lipcius and Stockhausen, 2002), prompting action by scientists and managers to determine optimal juvenile habitats to target for preservation or restoration. It is becoming apparent that the combination of fishing pressure, environmental disturbances, and destruction of coastal nursery habitats has driven the Chesapeake Bay's blue crab population to unprecedented low levels. Long-term management of blue crab populations in the Chesapeake Bay and other ecosystems depends on a complete understanding of the blue crab's basic biology, natural history, and ecology, including the importance of habitat-specific population dynamics.
Variation in habitat quality can affect the densities of both juvenile and adult crabs, with highest densities of juveniles typically found in seagrass beds or unvegetated mud habitats (Hines et al., 1990, Pile et al., 1996, Perkins-Visser et al., 1996, Seitz et al., 2003). A habitat of high quality (i.e., nursery) can be characterized as one in which growth and survival of juveniles is enhanced compared to other habitats (Beck et al., 2001).
Seagrass beds typically have been considered the principal and optimal nursery grounds for juvenile blue crabs in Chesapeake Bay (Heck and Thoman, 1984, Eggleston et al., 1998, Pardieck et al., 1999, Hovel and Lipcius, 2001, Heck et al., 2001) because of their structural complexity, which offers protection to juvenile crabs from predation by fish and cannibalism by conspecifics (Ruiz et al., 1993, Pile et al., 1996, Moksnes et al., 1997, Ryer et al., 1997, Orth and van Montfrans, 2002). In addition, seagrass beds may enhance growth rates of juveniles through the provision of invertebrate prey that inhabit seagrass (Perkins-Visser et al., 1996).
Bivalves serve as prey for various predators and they are an important component of benthic community assemblages because they are biomass dominants, although polychaetes may be numerical dominants (Boesch, 1977, Virnstein, 1977, Hines and Comtois, 1985, Hines et al., 1990, Dauer et al., 1989). The thin-shelled infaunal bivalves Mya arenaria and Macoma balthica are common and abundant throughout Chesapeake Bay (Boesch, 1977, Holland, 1985), and, along with conspecifics, polychaetes, and other crabs, comprise a large percentage of the blue crab diet (up to 50% during certain seasons–Laughlin, 1982, Hines et al., 1990, Mansour and Lipcius, 1991, Mansour, 1992). These species are exceptionally abundant in shallow, unvegetated mud and sand habitats that adjoin coastal and estuarine salt marshes (Seitz et al., in review), suggesting that these habitats may also serve as valuable nurseries for the blue crab.
The objectives of this study were therefore to determine, through a series of field experiments: (1) juvenile blue crab growth in various potential nursery habitats of lower Chesapeake Bay (seagrass beds, unvegetated sand, unvegetated mud, upriver, and near the river mouth), (2) bivalve prey availability in relation to crab growth, and (3) the impact of predation by juvenile blue crabs on the bivalve prey. Ultimately, we sought to delineate the optimal habitats for juvenile crab growth in lower Chesapeake Bay and to identify the underlying mechanism producing the growth patterns.
Section snippets
Research location
This study was conducted in the York River, a 50-km-long tributary of Chesapeake Bay located at approximately 76°N latitude, 37°W longitude (Fig. 1). In the York River, two river zones (upriver and mouth) separated by ∼45 km were compared in terms of bivalve density, clam survival, and crab growth. Within the zones, three habitats were compared: unvegetated mud, unvegetated sand, and seagrass (only present at the mouth). The studies were conducted in the fall–winter of 2000 (deployed September
Crab growth
We recovered 56% of the juvenile crabs deployed in field growth cages. In 2000, we recovered 18 of 30 crabs, while in 2001 we recovered 21 of 40 crabs (Table 1). In 2000, crab growth was similar among the five habitats, 16–20 mm CW increases over the 3+ months for all habitats, except Mouth sand where it was only 9 mm. Crab growth did not differ significantly by Habitat in 2000 (one-way ANOVA, log-transformed data, p=0.721). Interestingly, crab growth was similar in Seagrass to that in mud (
Growth
Juvenile blue crab growth was 9–20 mm carapace width (CW) per 3–8 months and differed considerably between the 2 years of our experiment, likely due to seasonality. The 2000 experiment was conducted during fall and winter, when temperatures and growth rates are low, whereas the 2001 experiment was conducted in summer and fall, when temperatures and growth rates are elevated. There was no difference in crab growth by sediment type (i.e., sand and mud) in either year; concomitantly, prey (i.e.,
Conclusions
The evidence presented herein demonstrating increased growth of juvenile crabs and increased densities of clams in upriver sand and mud habitats indicates the importance of such unvegetated habitats as nurseries. Ongoing studies aim to identify critical nursery habitats for the blue crab to aid in preservation and conservation of this species. Given the results of this study, combined with knowledge that clams comprise up to 50% of the blue crab diet (Hines et al., 1990), we conclude that
Acknowledgements
We thank Nancy Olmstead, Buck Stockhausen, Marcel Montane, Kristen Delano, Paul Gerdes, Jill Fox, and Katy Dannenberg for their help with field collections and data analysis. Funding was provided by NSF, Virginia Commonwealth, NOAA Chesapeake Bay Program (grant #NA17FU2841 to the Blue Crab Advanced Research Consortium), and National and Virginia Sea Grant. This is contribution number 2651 from the Virginia Institute of Marine Science. [SS]
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For juvenile red king crabs, predation is likely the most important factor in determining success, similar to other species, such as the blue crabs, Callinectes sapidus (Hines and Ruiz, 1995). For example, although juvenile blue crab growth may vary by a factor of 2 among habitat types (Seitz et al., 2005), predation risk can vary by a factor of 5 among habitat types without any significant change in growth (Long et al., 2011). It is therefore important to understand the predator–prey dynamics of a system in order to release crabs in the optimal habitat and at the best time, density, and size to minimize predation and maximize enhancement success (Hines et al., 2008; Johnson et al., 2008).
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