Tropical Storm and Environmental Forcing on Regional Blue Crab (Callinectes Sapidus) Settlement
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FISHERIES OCEANOGRAPHY Fish. Oceanogr. 19:2, 89–106, 2010 Tropical storm and environmental forcing on regional blue crab (Callinectes sapidus) settlement DAVID B. EGGLESTON,* NATHALIE B. ment events generally associated with ‘onshore’ storm REYNS, LISA L. ETHERINGTON,à GAYLE R. tracks that made landfall from the ocean and moved PLAIA AND LIAN XIE inland along a southeasterly ⁄ northwesterly path, or Department of Marine, Earth & Atmospheric Sciences, North ‘coastal’ storms that followed a path roughly parallel to Carolina State University, Raleigh, NC 27695-8208, USA the coastline and were located <300 km offshore of the coast. Key words: blue crab, larval dispersal, hurricanes, ABSTRACT recruitment, settlement, tropical storms Global climate change is predicted to increase the frequency and magnitude of hurricanes, typhoons and other extreme cyclonic disturbance events, with little INTRODUCTION known consequences for recruitment dynamics of marine species that rely on wind-driven larval trans- Extreme variation in recruitment of many finfish and port to coastal settlement and nursery habitats. We invertebrate fisheries (Caddy and Gulland, 1983; Sis- conducted a large-scale settlement study of the blue senwine, 1984; Hare and Able, 2007) often masks the crab (Callinectes sapidus) in the Croatan-Albemarle- effects of overexploitation, thereby hampering man- Pamlico Estuarine System (CAPES) in North agement efforts (Ludwig et al., 1993). The need to Carolina, the second largest estuary in the US, during understand seemingly chaotic fluctuations in popula- a 10-yr period that encompassed 35 tropical storms of tion sizes of animals and plants fuels efforts to identify varying magnitudes and tracks, to determine the the relative roles of intertwined external forcing and effects of hurricane track, wind speed and direction as internal feedbacks on population dynamics (May, well as lunar-associated explanatory variables on 1981; Higgins et al., 1997; Cowen et al., 2000; Hare spatiotemporal variation in settlement. The results and Able, 2007 and references therein). External suggest that much of the spatiotemporal variation in environmental forcing has been linked to fluctuations blue crab settlement within the CAPES is due to a in fish and invertebrate distribution and abundance combination of: (i) stochastic, meteorological events patterns through several mechanisms, including such as the number of tropical storm days during the change in decadal-scale temperatures influencing fall recruitment season (28% of the monthly varia- spawning and recruitment success, and change in wind tion explained), (ii) the frequency and duration of regimes and subsequent oceanographic currents wind events blowing toward the southwest and, to a determining successful delivery of larvae to coastal lesser degree, (iii) periodic events such as hours of dark nursery habitats (Ma, 2005; Criales et al., 2006; Hare flood tide. Tropical storms and hurricanes expand the and Able, 2007 and references therein). One impli- blue crab nursery capacity of the CAPES. The benefits cation of global climate change is the predicted of hurricane-forcing to megalopal settlement was increasing frequency and magnitude of hurricanes, dependent upon the storm track, with highest settle- typhoons and other extreme cyclonic disturbance events (Webster et al., 2005), with little known con- sequences for recruitment dynamics of marine species *Correspondence. e-mail: [email protected] that rely on wind-driven larval transport to coastal Present addresses: Department of Marine Science and settlement and nursery habitats. Environmental Studies, University of San Diego, 5998 Alcala´ The blue crab Callinectes sapidus supports the Park, San Diego, CA 92110, USA. world’s largest fishery for crabs (Lipcius and Eggleston, àNOAA, Cordell Bank National Marine Sanctuary, PO Box 2001) and represents North Carolina’s most valuable 159, Olema, CA 94950, USA. commercial fishery (NC Blue Crab FMP 2004). Blue Received 4 August 2008 crab population size and commercial landings are Revised version accepted 28 September 2009 declining in many areas throughout the East and Gulf Ó 2010 Blackwell Publishing Ltd. doi:10.1111/j.1365-2419.2009.00530.x 89 90 D.B. Eggleston et al. coasts of the USA (Lipcius et al., 2007). In this study, However, recent research in Pamlico Sound demon- we use results from a 10-yr recruitment study of blue strates that recently settled early juvenile blue crabs crab conducted in the Croatan-Albemarle-Pamlico exhibit pelagic emigration (Etherington and Eggle- Estuarine System (CAPES) to assess the effects of ston, 2000, 2003; Blackmon and Eggleston, 2001; hurricane track, wind speed and direction, as well as Reyns et al., 2007). This pelagic emigration is pre- lunar-associated explanatory variables, on spatiotem- dominantly undertaken by the first juvenile stages poral variation in megalopal settlement, to test and is density-dependent (Reyns and Eggleston, hypotheses (elaborated below) about environmental 2004). influences, especially tropical storm forcing, on large The blue crab exhibits considerable variation in scale blue crab settlement success. abundance between different regions along the Atlantic and Gulf coasts of the USA, as well as over Blue crab time (Lipcius and Van Engel, 1990; van Montfrans Like many marine invertebrates, the blue crab has a et al., 1995). This variation in abundance is particu- complex life cycle. Estuarine-dependent females larly evident in North Carolina, where average annual migrate to estuary mouths to spawn such that larvae population abundance can vary 20-fold between suc- are advected seaward to high salinity continental shelf cessive years or between local regions within a year waters (Provenzano et al., 1983). Larval development (Eggleston et al., 2004). Variable recruitment may be proceeds through a series of seven to eight zoeal (lar- due to varying spawning stock biomass (Higgins et al., val) stages over 30 days, followed by metamorphosis 1997; Lipcius and Stockhausen, 2002), physical to the post-larval or megalopal stage (Van Engel, transport (Roughgarden et al., 1988; Eggleston et al., 1958). Blue crab megalopae are likely transported 1998), and survival of settled individuals in nursery shoreward by wind-driven Ekman circulation and into habitats (Eggleston and Armstrong, 1995; Pile et al., estuaries by means of barotropic flow moving in from 1996). For the blue crab in NC, although recruitment the shelf (reviewed by Epifanio, 2007). Post-larval dynamics of early juvenile blue crabs can be strongly influx into Chesapeake Bay, USA, and Pamlico density-dependent at the local scale, the population Sound, NC, USA, is correlated with wind speed and size of early juvenile blue crabs at the scale of Pamlico direction (Goodrich et al., 1989; Mense et al., 1995), Sound appears to be limited by post-larval recruitment whereas post-larval influx in the Newport River near when overall population size is relatively low (Ethe- Beaufort Inlet, NC, was not correlated with cross-shelf rington and Eggleston, 2003; Eggleston et al., 2004). or along-shore wind speed; rather, settlement occurred Approximately 30% of the variation in blue crab with semi-lunar periodicity with highest settlement at settlement can be explained by variation in spawning the times of neap tides during quarter phases of the stock biomass (Eggleston et al., 2004). Moreover, moon and increased with hours of dark flood tides megalopal settlement has been correlated with blue (Forward et al., 2004). The proposed explanation for crab landings 2 yr later (Forward et al., 2004). Thus, in the Newport river example was that (i) megalopae North Carolina, recruitment dynamics of early juve- undergo flood tide transport for entrance into estuaries nile blue crabs, and possibly adults, appears to depend and up-estuary movement, and (ii) the behavior upon spawning stock biomass in a given year and underlying flood tide transport is most effective when subsequent recruitment of post-larval blue crabs. all the nocturnal flood tide occurs in darkness (For- Spatiotemporal variation in abundance of juvenile and ward et al., 2004). Blue crab megalopae may also use a adult crabs may be due, in part, to settlement patterns combination of flood-tide currents and wind-driven by postlarvae. currents in micro-tidal estuaries such as Pamlico Sound to reach shallow settlement habitats (Reyns Study site et al., 2007). Once within the estuary, blue crabs settle CAPES is the largest lagoonal system and second in beds of submerged aquatic habitat (e.g., seagrass or largest estuarine system (after Chesapeake Bay) in the other complex habitats) where they undergo meta- USA (Fig. 1). The vast shallow waters of this system morphosis to the first benthic instar (J1) (Heck and and its tributaries serve as nursery and adult habitat for Thoman, 1981; Orth and van Montfrans, 1987; numerous estuarine-dependent species, and support Etherington and Eggleston, 2000). It is generally North Carolina’s annual $1 billion fishing industry. accepted that juveniles remain in these habitats until Extensive seagrass beds line the sound-side of the they reach the fifth to seventh instar (J5–7) stage and barrier island chain and serve as primary nursery begin to migrate benthically into non-vegetated hab- habitats for newly settled and early juvenile blue crabs itats (Orth and van Montfrans, 1987; Pile et al., 1996). (Etherington and Eggleston, 2000, 2003). The western Ó 2010 Blackwell Publishing Ltd, Fish. Oceanogr., 19:2, 89–106. Tropical storm