Written by Hallie Arno, former aquaculture research intern and current student at College of the Atlantic.
The life cycle of a scallop. Scallops float into the bags as larvae, around 30 days after spawning. They then settle on the blue mesh in the bags to become juveniles. Image from Stewart and Arnold 1994.
Spat bags are the names of the green mesh bags we put in the ocean every fall to collect young scallops, called spat. When scallops are in their larval stage, they are free-floating through the water. When they find a suitable settlement habitat, they settle and metamorphose into the familiar form we know. The spat bags are filled with a rigid blue mesh, which mimics a habitat for larval scallops to land, or settle. They then grow in the bags and can’t escape the outer fine green mesh.
What the spat bags look like before we put them in the water last fall. We mark the buoy with our phone number in case it gets lost over the winter.
We retrieve the spat bags in the spring, usually in April and May. We just finished retrieving 12 of our spat bags that we deployed in Fall 2020 - we got back 11 of them and are gearing up to start sorting them! With the help of students and visitors we spend the summer counting how many scallops have settled in each bag and the other kinds of organisms we find along with the scallops. This involves sorting out the tiny 0.5-2 centimeter scallops from everything else in the spat bags, such as mussels, nudibranchs, and tunicates, which can be a time-consuming process. Luckily, there are often students and island visitors to help! We then count the number of scallops and record the presence and absence of other species.
We can use this data to estimate the number of larval scallops in the water column. Since many of the spat bags were in the same place every year, we can compare changes between years. We can also compare between different locations. For example, the Lower Muscle Ridge area has been closed to fishing since 2013 as part of the Midcoast Maine Collaborative Scallop Project, so we might expect to see more spat in that area that is closed than the area that is fished. Learn more about the MMCSP here. The goal of a closure zone is to be a larval source for not only the closed area, but areas open to fishing nearby; a higher population of adult scallops in a small area could provide larvae for a much larger area and benefit both fishing and conservation goals. The Hurricane Island Research Team has been investigating the effect of the Muscle Ridge closure on spat in the area since 2013.
Here is a graph of the average number of scallops seen per bag annually, in the closure zone, north of the closure zone, and south of the closure zone:
We monitor changes in spat availability in the area from year to year by deploying spat bags inside the closure zone as well as directly north of the closure and directly south of it. This site is about a 45-minute boat ride from Hurricane Island.
Spat bags in Muscle Ridge have been at similar sites inside and outside of the closure every year since 2013, so it is a good place to compare year to year.
The first year of data collection occurred in 2014 and spat was counted from bags that were deployed in 2013, before the area was closed to fishing. This could explain why we see such a low count of spat compared to other years. After the lower Muscle Ridge closed to fishing in 2013, spat abundance increased in both the closure and around it, suggesting that the closure could be contributing to an increased larval supply throughout the entire area. However, after a few years, spat was lower. This could be part of a natural cycle, or there could be other factors at play. Spat can travel 300-600 kilometers over 40-60 days, so spat in the Muscle Ridge area could be influenced by spat beyond Nova Scotia or Cape Cod. Local current patterns could carry spat from Mussel Ridge all over the Gulf of Maine, and spat from other areas could end up settling in this area.
One way to tell if the scallop population on Muscle Ridge is enhancing the overall larvae supply in the local area would be to have a dive survey to assess the adult population, or plankton tows in the area while scallops are in the larval stage. We are also putting spat bags in other areas, such as around Hurricane Island, to see if these trends are the same throughout other areas of Penobscot Bay. There are many hypotheses as to why scallop spat isn’t increasing over time, and Hurricane is excited to learn more about the scallop populations in the Gulf of Maine!
Sources:
Batchelder, J. P. 2017. Temporal changes in the larval Placopecten magellanicus population in a small-scale fishery closure area in coastal Maine, USA.
Stewart, P.L. and S.H. Arnold. 1994. Environmental requirements of the sea scallop (Placopecten magellanicus) in eastern Canada and its response to human impacts. Can. Tech. Rep. Fish. Aquat. Sci.2005: 1-36.
