Wednesday, August 17, 2011

Crab Count: The Results are In

After a summer of hard work pulling up traps, turning over rocks, and measuring fiesty crabs, it's now time for the fun part: putting all of our data together and looking at the results!
First, some basic numbers:
  • In total, we caught and measured 493 individual crabs this summer!
  • 270 were Asian shore crabs
  • 130 were green crabs
  • 89 were red rock crabs
  • 4 were spider crabs
  • 300 were male and 193 were female
  • They ranged in size from a tiny 4mm to a whopping 12.04 cm
From our total numbers, it certainly appears as if invasive Asian shore crabs and green crabs are the dominate species in Boston Harbor, but let's take a closer look at our individual sites.

Asian shore crabs, the most dominant species in our total, are actually only the most common species at two of our six sites: Camp Harbor View on Long Island and the Harry McDonough Sailing Center in South Boston. These sites are both rocky and gravely beaches where we searched for crabs in the intertidal zone (the area between the high and low tide marks which is sumberged for only part of the day). Asian shore crabs do very well in the intertidal zone because they are better than other crab species at tolerating the dryer conditions of this habitat.
Black's Creek in Quincy is also a gravely beach where we searched for crabs in the intertidal zone, but we found almost exclusively green crabs at this site. Why no Asian shore crabs?
Black's Creek is a Salt Marsh Estuary where fresh water mixes with ocean water and lowers the salinity. Green crabs are very tolerant of a wide range of different salinities, but Asian shore crabs can't survive as well in the low salinity of Black's Creek.
Green crabs are dominant at Piers Park Sailing Center in East Boston, Courageous Sailing Center in Charlestown, and in Fort Point Channel at the Boston Children's Museum as well. These crabs were introduced to New England in the early 1800's; their quick reproduction and adaptability to a wide range of conditions have allowed them to easily outcompete native crabs, so it is no surprise that we found so many of them.
At Piers Park, Courageous, and the Children's Museum, we caught crabs from the subtidal zone (the area that is always submerged, even at low tide) using traps. At these sites, we saw two additional spcies of crabs that are not as tolerant of the dryer intertidal zone. Red rock crabs were common at Piers Park and Courageous, and spider crabs were found at Piers Park and the Children's Museum. Although we never saw the sea floor at these locations, the species of crabs that we found at each of them can actually tell us a little bit about what the bottom is like! Red Rock crabs (as you can probably guess from the name) like habitats with lots of rocks, while spider crabs (a type of mud crab), prefer muddy bottoms. Based on the data we collected, Courageous probably has a more rocky bottom, Fort Point Channel has a more muddy bottom and Piers Park likely has a mixture of both.
This is only a small example of the kinds of things we can learn from the crab count information we've collected this summer. We'll be sending all of our data along to Dr. Judy Pederson at MIT who will use it to study the spread of invasive species. Hopefully, she'll keep us posted on what she finds!

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