We utilized tethering trials as a proxy of predator-driven mortality of blue crabs and pinfish within edge and interior regions of the seagrass meadow at Jack’s Island. Our design consisted of 21 "edge" (0-1 m from seagrass-sandflat boundary) and 21 "interior"(>3 m from seagrass-sandflat boundary) plots, with each plot defined by two 1x1-m subplots separated from each other by 0.5 m (but with the entirety of each plot being at the suitable distance for edge/interior designations). Each of the 42 total plots were separated from one another by >2 m. For both the edge and interior treatments, seagrass shoot density was reduced by 50% in a third of the plots, seagrass shoot density was reduced by 80% in another third of the plots, and seagrass shoot density was left at ambient in the final third of plots (all randomly assigned). This resulted in a 2x3 experimental design in which meadow location and shoot density were fully crossed. Reduction of shoot densities was achieved by deploying a 1x1-m quadrant with a 10x10 grid (with each grid cell = 0.01 m2). We then removed all seagrass in 50 or 80 of the cells for the 50% and 80% reduction treatments, respectively. The resultant shoot densities were as follows: ambient treatments had a mean of 575 shoots m-2, 50% reduction treatments had a mean of 283 shoots m-2, and 80% reduction treatments had a mean of 124 shoots m-2.
We deployed 126 tethered blue crabs (5.2 ± 0.1 cm carapace width) and 168 tethered pinfish (5.1 ± 0.1 cm total length) in plots over three and four trials, respectively. We ran one less trial with blue crabs due to the availability of specimens within our preferred size range during our experimental window. All crabs and pinfish were collected via small trawl on the day before deployment. During each trial, a tethered blue crab was randomly assigned to one of the subplots within each plot, while a tethered pinfish was placed in the remaining subplot (i.e., 42 juvenile blue crabs and 42 pinfish were deployed in a trial). Each tethering device consisted of a lawn staple as an anchor placed in the center of a subplot, connected to a 30-cm long section of 3.6-kg clear monofilament fishing line. For blue crabs, the free end of the monofilament was glued to the center of the crab’s carapace after making a lasso around the crab’s body. Blue crabs had each of their claws glued shut using Loctite super glue gel to prevent them from cutting the tether. Pinfish were tethered through the soft tissue immediately behind their lower jaw bone by piercing this tissue, threading the line through the piercing, and the tying an overhand knot in the line. As a method check, we individually tethered >40 blue crabs and >20 pinfish in laboratory tanks outfitted with artificial seagrass. Over a 4-day period, none of the tethered animals became free, tethered pinfish did not behave noticeably different that untethered pinfish also in the tank, and tethered animals did not become entangled in artificial seagrass blades.
Tethered blue crabs and pinfish were deployed in our field experiment ~3 hours before daytime high tides. Following deployment, each tethered animal was checked after 1 hour, 2 hours, 3 hours, and 24 hours to assess loss rates (presumably via predation). Individual blue crabs or pinfish missing at the 1-, 2-, 3-, and 24-hour checks were randomly assigned a survival time ranging between 0-1, 1-2, 2-3, and 3-24 hours, respectively, to acknowledge that we could not be sure within check intervals when predation occurred. Furthermore, this approach insured that we did not artificially reduce variances among replicates. Any animal remaining on its tether after 24 hours was assigned a survival time of 24 hours, and then released.
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