Methods:
Three trips were made to Northeastern University’s Marine Science Center in Nahant, MA on October 15th and 27th and November 12th, 2004. All trips were planned to coincide with low tide. Each time, the intertidal was sampled for all crab species along the same transect line.
A transect was run from the high intertidal to the water and a laser-sighted
level was used to measure heights from the low tide mark.
Click here for details on the laser-sighted level.
Starting at 3 m above low tide, a .5 m2
quadrat was placed on the
transect. This was used as the center transect. Another quadrat was placed one
meter to the left/right of the center quadrat’s leftmost/rightmost edge,
creating three quadrats for the title height.
Quadrat in mid intertidal
The transect line used for all crab density assessments in this study can be seen here.
The area determined by each quadrat was examined and picked through to locate and remove any crabs found. Crabs were then placed in a plastic container. After each quadrat was searched, any crabs found were identified and measured with digital calipers. Crabs were replaced as found (usually under the Ascophyllum nodosum canopy) a few meters away from the study area.
The laser-sighted-level was used to determine intertidal heights of 3.0, 2.5, 2.0, 1.5 and 1.0 m above the low tide mark. As long as conditions permitted and sufficient daylight was available, measurements were made at each of these tidal heights, starting from the highest and working towards the water. This essentially sampled from the upper mid-intertidal to the mid lower-intertidal. The center quadrat was placed on the transect and the two side quadrats were placed one meter away from the center quadrat’s edge, yielding three quadrats at every tidal height. In total, 37 quadrats were sampled.