Lisa Ayers Lawrence & Dave Rudders, Virginia Institute of Marine Science
1 - 1.5 hrs.
Scallops are bivalve molluscs and can be found worldwide. They have two almost circular shells attached by a hinge, and a powerful adductor muscle that opens and closes the shell. This large muscle is what enables scallops to swim, a unique ability for a bivalve. By contracting the adductor muscle, the scallop snaps its shell closed pushing water out and propelling it forward. This large muscle or "meat" is also the part of the scallop that is eaten.
There are hundreds of species of scallops, but only a few are commercially harvested. The largest of these is the sea scallop, Placopecten magellanicus, which is found along the continental shelf of the northwestern Atlantic Ocean from the Canadian Maritimes to North Carolina. Sea scallops grow rapidly during the first several years of life, quadrupling their meat weight between ages 3 and 5. Sea scallops generally grow to about 6.5 inches shell height, though some have been known to grow as large as 9 inches. Scallops reach reproductive maturity at age 2, but it is not until they reach age 4 that they contribute significantly to total egg production. Spawning occurs primarily in late summer and early autumn, and scallops in the mid-Atlantic may also spawn in the spring. After spawning, the larvae drift as plankton in the water column for 1 to 2 months before settling on the bottom.
Scallops are harvested using two different types of gear, dredges and otter trawls. A dredge is a heavy steel frame with a collection bag consisting of 3.5 in. diameter steel rings which is towed across the bottom. An otter trawl consists of a large mesh net pulled along the bottom with otter boards at the mouth of the net to help keep the mouth open.
Commercial sea scallop fishing began around 1880 in the inshore areas of the Gulf of Maine. The catch or "landings" remained low until large offshore scallop beds were discovered off the mid-Atlantic and on Georges Bank (off of New England) in the 1920s and 30s. Since 1953, annual landings by the U.S. and Canada have fluctuated around 10,000 metric tons, with a record high of 17,500 metric tons (worth $149 million) in 1990, making sea scallops the second most valuable seafood species after the American lobster. In 1994, three large areas of Georges Bank were closed to all bottom fisheries in response to depleted groundfish (cod, haddock, flatfish) stocks that shared the same habitat as the sea scallop. Subsequently, drastic increases in sea scallop numbers and size were observed in these areas, so in 1998 two additional areas were closed to scalloping, one off the coast of New Jersey and the other off the coast of Virginia. Scientists along the northeast are studying the scallop populations in these closed areas to see how effective this management plan is.
This month's data activity will use scallop data from the Virginia Institute of Marine Science to plot size frequency distributions in order to compare the number and size of scallops taken from closed fishing areas to those of open fishing areas. A size frequency distribution graph is a simple bar or line graph with the size of the animal (in our case shell height) on the X axis and the sum of them number of individuals of that size (or the frequency) on the Y axis.
The VIMS scallop data show average frequency distributions inside and outside of two closed scallop areas, one off of Virginia Beach and one in the Hudson Canyon. The first column of the data is shell height which will be our X axis. The second column shows the number of scallops for each size class in an area outside of the closed area (in other words not closed to scallop fishing) off of Virginia Beach. The third column is for inside the Virginia Beach closed area (in other words, closed to scallop fishing). The fourth and fifth columns have similar data for the Hudson Canyon closed area.
Print out the data and create a line or bar graph for the Virginia Beach area scallops and a second one for the the Hudson Canyon area scallops.