Elsevier

Estuarine, Coastal and Shelf Science

Volume 214, 15 December 2018, Pages 89-97
Estuarine, Coastal and Shelf Science

Spatial and temporal settlement patterns of blue crab (Callinectes sapidus and Callinectes similis) megalopae in a drought-prone Texas estuary

https://doi.org/10.1016/j.ecss.2018.09.017Get rights and content

Highlights

  • Small fraction of blue crab larvae in nearshore Gulf of Mexico recruit to estuary.

  • Seasonal settlement peak of Callinectes sapidus occurs in fall.

  • Seasonal settlement peak of Callinectes similis occurs in winter.

Abstract

The Mission-Aransas Estuary is the wintering ground for the only sustained wild population of the endangered whooping crane (Grus americana), and blue crabs (Callinectes sapidus) are an important component of their diet as well as being a major food source for important sport fishes such as the red and black drum. Blue crabs also support a commercial crabbing industry, and fisheries data indicate that blue crab populations have been declining since the 1980s. Possible factors leading to decline in blue crab populations include overfishing, increased populations and predation by regulated sport fishes, reduced freshwater inflows into estuaries, and reduced larval recruitment. Little is known about blue crab recruitment dynamics in this region, but restricted passes between coastal estuaries and the Gulf of Mexico along with extended periods of drought that often lead to hypersaline conditions in coastal bays may limit larval recruitment from the Gulf into the bays. To investigate blue crab larval recruitment patterns, citizen scientist volunteers used hogshair settlement collectors to sample five monitoring sites over a four year period. Results show that large numbers of blue crab megalopae are common in nearshore waters of the Gulf of Mexico, but only a small fraction (∼1%) recruit into the estuary. Peak periods of ingress into the estuary occur during fall and winter months, with C. sapidus primarily contributing to the fall peak and C. similis dominating the winter peak. Increased salinity in the estuary during droughts may reduce the ability of blue crab larvae to detect and enter passes into the estuary.

Introduction

Blue crabs (Callinectes sapidus and C. similis) are an important food source for the migratory endangered whooping crane (Gus americana, Linnaeus, 1758) population which overwinters in or near the Aransas National Wildlife Refuge in Texas (Westwood and Chavez-Ramirez, 2005). It is also a major food source for sport finfishes such as black drum (Pogonias cromis, Linnaeus, 1766), red drum (Sciaenops ocellatus, Linnaeus, 1766), and spotted seatrout (Cynoscion nebulosus, Cuvier in Cuvier & Valenciennes, 1830) in Texas bays and estuaries (Scharf and Schlicht, 2000, Vanderkooy 2013). The Atlantic blue crab (C. sapidus) is also regarded as an important commercial fishery throughout its range including Texas (Sutton and Wagner, 2007). Picariello and Rosenberg (2015) reported that 1.9 million pounds of blue crab, valued at 2.3 million dollars, were landed in Texas in 2013. However, the Texas Parks and Wildlife Department (TPWD, 2007) has reported declining commercial landings of Atlantic blue crab in Texas waters since 1987. Many factors could be contributing to the downward population trends such as limited freshwater inflow into the estuarine system (Guillory et al., 2001: Picariello and Rosenberg, 2015), habitat alteration and/or loss (Guillory et al., 2001), reduced larval recruitment (Longley, 1994), and increased predation by regulated sportfishes (Guillory and Prejean, 2001; Picariello and Rosenberg, 2015).

Interest in blue crab population dynamics in South Texas has increased due to their importance in the diet of the endangered whooping crane (Nelson et al., 1996). The whooping crane is the tallest bird in North America and nearly went extinct in the middle of the 20th Century (Urbanek and Lewis, 2015). In 2008, after years of steady population increases, 28 birds died in the winter of 2008–2009 and it was suggested that these deaths were due in part to reduced blue crab populations that resulted from drought conditions and diversions of freshwater from the Guadalupe and San Antonio Rivers (Gulley, 2014).

Atlantic blue crabs undergo a complex life cycle as they transition from larval to adult stages and utilize a variety of habitats including the lower, middle, and upper estuary as well as adjacent nearshore coastal waters of the Gulf of Mexico (Perry and McIlwain, 1986). Zoeae (first larval stage) hatch in the higher salinity waters of the Gulf of Mexico and drift among other plankton for several months undergoing 5–7 zoeal stages until metamorphosing into the megalopae postlarval stage (Epifanio, 2007). Megalopae are then transported into the estuary by nearshore currents, flood tides, and wind driven processes (Tilburg et al., 2009; Epifanio and Garvine, 2001) where they settle into a primarily benthic existence and metamorphose a final time into the juvenile crab stage (Lipcius et al., 1990). As juvenile blue crabs grow and molt to maturity, they tend to utilize less saline shallow waters of the estuary, occupying areas of structured habitats such as seagrass beds, salt marshes, and oyster reefs as well as soft muddy and sandy non-structured substrates (Lipcius et al., 2005). As adults, males prefer less saline waters of the upper estuary whereas female crabs usually occupy the middle to lower estuary with higher salinities. Mating usually occurs in the lower saline waters of the upper estuary, then female blue crabs migrate to the higher saline waters of the lower estuary and adjacent coastal waters of the Gulf of Mexico when ready to release their larvae (Perry and McIlwain, 1986).

Although reduced recruitment of blue crab at the megalopae stage may be a factor contributing to their declining populations in the Mission-Aransas Estuary, very little is known about their recruitment patterns in the area. Larval recruitment may be an especially important component of blue crab population dynamics on the South Texas coast, since connections between local estuaries and the Gulf of Mexico are limited by nearly continuous barrier islands with widely separated narrow passes. The behavioral adaptation that allows weakly swimming planktonic blue crab larvae to be transported from the coastal ocean to estuaries is known as selective tidal-stream transport (Forward et al., 2003). By responding to environmental variables including light, changes in salinity, and turbulence, blue crab larvae move into the estuary by swimming up into the water column during nocturnal flood tides of increasing salinity and remain on the bottom during ebb tides with decreasing salinity. Freshwater inflows into South Texas estuaries are often reduced due to extended periods of drought, increased demand for freshwater by agriculture and municipal purposes, and capture of water in reservoirs (Montagna and Kalke, 1992). These factors lead to increased salinity in South Texas estuaries, and experimental and modeling studies indicate that increased salinity can lead to reduced transport of blue crab larvae by selective tidal-stream transport (Bittler et al., 2014).

A simple but labor-intensive method for estimating the recruitment of blue crab larvae involves the deployment of standardized settlement collectors constructed of an artificial substrate (air-conditioning filter) in a cylindrical design over a 24 h period (Metcalf et al., 1995). A citizen science larval blue crab monitoring project was started in 2012 to better understand the potential role of larval recruitment in the population dynamics of blue crabs in the winter feeding grounds of the whooping crane, and to investigate whether reduced freshwater inflows and resulting hypersalinity in estuaries of south Texas affected larval recruitment. Using settlement collectors this study gained insight into the proportion of blue crab larvae that recruit into the estuary from the Gulf of Mexico, how far these larvae travel into the estuary before metamorphosing into juveniles, and the seasonal pattern of larval recruitment in subtropical south Texas.

Section snippets

Study area

The Mission-Aransas National Estuarine Research Reserve (NERR), located along the south-central coast of Texas, encompasses 751.5 sq. km of terrestrial, wetland and marine habitats characteristic of western Gulf of Mexico estuaries (Diener, 1975; Mission-Aransas NERR, 2015) and includes the Aransas National Wildlife Refuge, winter home to the last wild whooping crane flock. The extensive shallow bays within the reserve boundaries are diverse with an array of complex habitats such as seagrass

Data summary

The complete daily time-series of megalopae settlement for each of the 5 sites analyzed in this study differed greatly in length and temporal coverage (Fig. 3; see also Methods). There were also distinct differences in megalopae settlement between sites (Fig. 3, Table 2). Overall, the average number of megalopae settling on collectors in the open Gulf of Mexico at HC was anywhere from 3 to 5 orders of magnitude higher than the average settlement at the estuary sites. Also, the relative

Discussion

One goal of this study was to gain insight into the recruitment dynamics of blue crab larvae in arid South Texas, examining a hypothesized relationship between periods of drought, high salinity in estuaries and the decline in adult and juvenile blue crab populations, which in turn might reduce food supply to winter flocks of whooping cranes (Gulley, 2014). Due to an extended period of drought during this study and the high variability in crab larvae collected a clear relationship between

Author contribution statement

T.F.W. carried out identification and analysis of crab larvae, assisted with data collection, and contributed to writing and editing the manuscript. L.P.S. performed data analysis and contributed to study design, writing and editing the manuscript. E.J.B. conceived and designed this study, supervised data collection, assisted with data analysis, and contributed to writing and editing the manuscript.

Role of the funding source

The funding sources had no role in study design, collection, analysis and interpretation of data; in the writing of the manuscript or the decision to submit the manuscript for publication.

Declarations of interest

None.

Acknowledgments

This research was funded by the National Oceanic and Atmospheric Administration (NOAA), USA through operations grants to the Mission-Aransas National Estuarine Research Reserve, USA (NA12NOS4200110, NA13NOS4200119, NA14NOS4200129, NA15NOS4200133) and by additional funding provided by the Texas State Aquarium, USA (UTA14-001260). Dr. Zack Darnell and Cammie Hyatt provided technical assistance; Colleen McCue Simpson and Nicole Poulson organized the volunteers. We would especially like to

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