DAY 8: Thursday August 21, 2003
Our task today was to analyze spat bags that have been retrieved from several spat lines located at three study sites in the Great Salt Pond. The use of spat bags is somewhat of an art. To use the spat lines with any success, you must know when to deploy the spat lines and where to place them. Karin did her graduate work in scallop recruitment to spat collectors and she approached the where and when questions with methods she employed in another estuary in Massachusetts.
Bay scallops spawn during the late spring and early summer months. This is also when many other bivalves are spawning. The trick with the spat lines is to get them into the water as close to the time when the scallop veliger is looking for settlement substrate. If the lines go in too late, you miss the spat fall completely. If the lines go in too early, there's a chance that the bags become completely covered in biofouling organisms and the veligers are not able to enter the bags and set on the netron in the bags. There is also the possibility of collecting anything else that is spawning at the same time, including predators. Juvenile stages of predators may get caught in the bags and would appreciate the buffet of scallop spat.
One way of making sure that your spat lines are deployed when scallop veligers are abundant is to monitor the water and deploy when veliger densities are high. This requires filtering a known quantity of water through a plankton net on a weekly basis. The only problem with this method is that all bivalve veligers look alike. It is almost impossible to distinguish a bay scallop veliger from a quahog veliger.
Karin selected the three study sites in the salt pond based on two criteria; proximity to historically productive scallop beds and sites with abundant eelgrass. Spat lines containing twenty spatbags were deployed weekly at the three sites beginning in mid June. Once a line had been in the water for one month, ten of the bags were retrieved for analysis. Using this method weekly monitoring for spat fall could be accomplished. For example, if lines were put in June 7, 14, 21 and 28 and no spat were found in the bags that went out on the 7th and 14th, but loads of spat were found in the bags that went in the water on the 21st, you would know that during this particular season, the scallops had spawned ten days to two weeks before the 21st. If monitoring takes place over several seasons you can really get a handle on when the bay scallops spawn.
In order to analyze the bags they are first hosed off over a fish tote. They are then opened and the contents are spilled into the tote. The bags are turned inside out and because the scallops use byssal threads to attach themselves you have to look very carefully at the bag and the netron for clinging scallops. The contents of the tote are then poured into a large screen. The screen is then combed over looking for the tiny scallop spat; some of the ones that we found were smaller than a peppercorn. Each bag is evaluated for fouling and assigned a number based how much the bag covered. The number and type of predators present in each bag is noted and every scallop spat is measured.