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OAS accession Detail for 0305475
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Title: NOAA RESTORE Science Program: Gulf-wide assessment of habitat use and habitat-specific production estimates of nekton in turtlegrass (Thalassia testudinum): Blue crab growth and mortality rates, 2018-05-14 to 2018-09-14 (NCEI Accession 0305475)
Abstract: This dataset consists of blue crab growth and mortality data from caging and tethering experiments in turtlegrass-dominated seagrass beds of the northern Gulf of America, including Lower Laguna Madre, TX; Coastal Bend, TX; Chandeleur Islands, LA; St. George Sound, FL; Cedar Key, FL, and Charlotte Harbor, FL.
Date received: 20250523
Start date: 20180514
End date: 20180914
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West boundary: -98
East boundary: -82
North boundary: 31
South boundary: 26
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Submitting institution: US DOC; NOAA; NOS; National Centers for Coastal Ocean Science
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Supplementary information: Methods:
Study sites
Blue crab growth and mortality experiments were conducted at six sites spanning the range of turtle grass distribution in the Northern GOA: two sites in Texas (Lower Laguna Madre and the Texas Coastal Bend), one site in Louisiana (Chandeleur Islands), and three sites in Florida, (St. George Sound, Cedar Key, and Charlotte Harbor).

Growth experiments
To quantify relationships between blue crab growth and seagrass complexity, a field caging experiment was conducted. Sampling stations at each of the six sites were selected using a stratified random sampling method of hexagonal tessellation in which a grid of hexagons (500 or 750 m edge) was overlaid on the mapped areal extent of known seagrass cover at each site. At each site, 10–15 hexagons with > 50% turtle grass cover were randomly selected and a randomly generated station was chosen within each selected hexagon to conduct each experiment. In cases where no turtle grass was found at a station, or stations were inaccessible, alternative hexagons were chosen and new stations were randomly generated.
A single mesocosm was deployed at each station 24–48 hrs prior to the start of the experiment to reduce effects of disturbance. Mesocosms each consisted of a collapsible, bottomless cylinder (1.07 m in diameter and 0.76 m tall) with 3.2 mm nylon mesh around the circumference and top connected by two fiberglass rings, PVC pipe, and rebar. A small closable (11.4 cm diameter, 15 cm long) sleeve sewn in the top allowed access to inside the mesocosm. During deployment, rebar was driven into the three PVC pipes supporting the mesocosm frame to anchor it in place, and the bottom edge of the mesocosm was driven ~5 cm into the sediment using a rubber mallet. Care was taken to ensure there were no gaps between the cage and the sediment and that macroalgae was cleared from the experimental area to remove potential confounding variables. Prior to deployment, the mesocosm deployment area was swept with dip nets to remove potential predators or competitors, as this could bias experimental results.

Juvenile blue crabs 11–44 mm in carapace width were collected 24–96 hrs prior to the beginning of the experiment from seagrass habitat using trawl, benthic sled, throw trap, and dip net techniques, and transferred to the lab in aerated containers. Because of logistical constraints, all blue crabs used in both the growth and mortality experiments for CB, LM, and CH were collected at CB and then transported to their respective sites. To differentiate between individuals, crabs were tagged using visible implant elastomer (VIE) tags (Northwest Marine Technologies) injected into the basal segments of the right or left swimming leg, the abdomen, or the body; each crab had a unique combination of tag placement location and color to enable identification of individuals. After VIE implantation, all crabs were held overnight to monitor survival and tag retention prior to deployment.

Approximately 24 hrs after VIE tagging, the blue crabs were transferred in aerated buckets to mesocosms and eight individuals were randomly deployed to each mesocosm to start the experiment. During the experiment blue crabs were not fed, but instead relied on natural prey items within the mesocosm in order to assess natural growth rates. Mesocosms were deployed for approximately 30 days. Timing of mesocosm deployment was staggered across sites between 2 June and 19 July, 2018 and experiments were terminated between 27 June and 13 August, 2018.

Environmental characteristics that may affect blue crab growth were measured throughout the experiment. Water depth was measured to the nearest cm, and salinity, water temperature (°C), and dissolved oxygen (mg L-1) were measured using a handheld meter (Pro 2030, YSI Inc., Yellow Springs, OH) at the beginning, middle, and end of the experiment.

At the end of the experiment, blue crabs were removed from the mesocosm using throw traps, bar seines, and dip nets and measured for carapace width (mm) in the field. After the conclusion of the experiment, turtle grass structural complexity was measured at all stations. Percent cover of seagrass by species and bare sediment were quantified in 1 m2 PVC quadrats sectioned into 100 10-cm2 squares placed directly over the area where the cage was located. The presence of drift and attached macroalgae was also noted within each quadrat. Species-specific seagrass shoot density was quantified in a randomly pre- selected cell within the quadrat, and total seagrass shoot density (# shoots m-2) was calculated by multiplying species-specific percent cover and shoot count then summing those values. If seagrass was not present in the pre-selected cell, shoot density was quantified in a second (or third, if needed) randomly pre-selected cell. In each quadrat, seagrass leaf length (mm) was also measured on three replicate plants of each species, and maximum canopy height was calculated as the maximum leaf length of all measured seagrass blades.

Additionally at the end of the experiment, a single core (15 cm diameter x 10 cm deep) was collected in an undisturbed area near the periphery of each cage. Cores were sieved in the field using either a 508 μM sieve or a 2.5 mm mesh bag, stored on ice and frozen for subsequent processing in the laboratory. Within each core, the number of seagrass shoots was counted for each species, leaves were scraped with a dull razor blade to remove epiphytes, and leaf lengths (mm) and widths (mm) were measured. Aboveground biomass (leaves) and belowground biomass (roots and rhizomes) were separated, and seagrass leaves, roots/rhizomes, and epiphytes were dried separately in a drying oven at 60°C for a minimum of 48 hours, after which they were weighed for dry weights (g). Leaf area index (LAI) was calculated as the total surface area of all leaves (length x width) in a seagrass core multiplied by two and divided by the total surface area of the core bottom (0.018 m-2); seagrass shoot density (# shoots m-2) was calculated by adding the total number of shoots in a core and dividing by the core area; and epiphyte density (g g-1 m-2) was calculated as total epiphyte biomass divided by total dried seagrass aboveground biomass per square meter. Seagrass canopy height was calculated as the maximum seagrass blade length in each core. All seagrass core morphology metrics were calculated separately for each seagrass species then combined for total seagrass complexity measurements.

Tethering experiments
To assess the role of turtle grass structural complexity on blue crab survival, a field experiment was conducted to assess mortality due to predation. Tethering experiments were conducted at a subset of 10 hexagons from the 10–15 hexagons used for the growth experiment, with 6–12 crabs tethered in each hexagon at each site for a total of 79–120 tethers at each site. All tethering experiments took place between 2 June and 20 July, 2018. Tethered crabs were spaced at least 20 m away from one other and from mesocosm experiments to ensure independence and minimize disturbance. Juvenile crabs 9.7–38.8 mm cw were collected in nearby adjacent seagrass habitats using the same collection methods as described for the mesocosm experiments, and a tether (75–100 cm long) of 20 lb test clear monofilament line was attached to each crab’s carapace using a drop of cyanoacrylate glue. Prior to placement in the field, blue crabs were acclimated to tethers for at least 24 hrs in aerated seawater. The free end of the tether was tied to a small PVC stake (3.3 cm in diameter and 60 cm tall) that was inserted into the sediment with the turtle grass-dominated meadows at each station. Tethered blue crabs had the freedom to move anywhere within a ~1.0-m diameter area around the PVC stakes, but were limited to the assigned habitat. Blue crabs were tethered at each station for ~24 hrs and then retrieved by hand. On retrieval, crabs were categorized as live, missing, or molted (entire carapace remaining on line). Previous field and laboratory studies have indicated that crabs cannot easily escape from tethers so it was assumed that all crabs missing from tethers after 24 hrs were eaten.

Prior to blue crab tethering, turtle grass structural complexity was measured near each tethering pole using the same seagrass quadrat survey techniques described for the growth experiments, and at the conclusion of the experiment one seagrass core (15 cm diameter x 10 cm deep) was collected close to each tether location, which was processed using the same methods described above. Water temperature (°C), salinity, and dissolved oxygen (mg/L) were measured at the beginning of the experiment using a handheld meter (Pro 2030, YSI Inc., Yellow Springs, OH).
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Metadata version: 8
Keydate: 2025-06-18 19:45:43+00
Editdate: 2025-06-27 14:15:03+00