Summary
DEI studied soft-shell clam recruitment levels in the St. George River (three coves in Thomaston) and the Weskeag River (one site in South Thomaston) in 2017. In 2018 we continued the study at one site, “Neil’s”, in the upper St. George River (Thomaston) that had a lot of settlement in 2017.
The studies found that clams are still settling in areas of the St. George River, but green crabs are limiting the amount of clams that survive their first year.
Clams in a recruitment box from the lower intertidal gradient in the St. George River (Thomaston).
2017 Methods:
On June 16, 2017, a series of soft-shell clam recruitment boxes were deployed at four coves in Thomaston and South Thomaston (three in the upper St. George River; one in the upper Weskeag River). Five boxes were placed in the upper, mid, and lower intertidal areas (for a total of 15 boxes) in one of the coves located adjacent to Georges River Land Trust (GRLT) on the upper St. George River. At all other sites, five boxes were placed at the upper intertidal of the respective cove.
Sites for the 2017 St. George and Weskeag Rivers Recruitment Study.
Upon deployment, five sediment cores were taken (surface area 0.2 ft sq., depth of 6 inches,) at each location. These samples were processed through a 1-mm mesh sieve to estimate density of soft-shell clams present at the time of deployment (June 17, 2017).
The only places that clams were found were an upper intertidal site on the upper Georges River (with upland owned by Neil Krane) and in the upper intertidal site at GRLT. The clam survey results are detailed here.
The 30 boxes remained at the sites passively collecting settling clams during the year. Boxes were retrieved and analyzed in the late fall of 2017. On Oct. 13, 2017 the unprotected sediment was again sampled in the same way, with the following results:
All the clams found in October were less than one-quarter inch in size.
2017 Results
Chart showing number of clams in each of the five 2017 recruitment boxes deployed at each site in the upper St. George and Weskeag River.
The 2017 study concluded that:
- In the upper part of the St. George River, soft-shell clam populations are not limited by the lack of juveniles that settle onto intertidal flats.
- As many as 2,296 clams occurred in one box! This was the second highest amount ever found in a single box (after 6,009 clams in a box in Freeport in 2015).
- At sites where average clam abundance was relatively high, there was also a large difference in the number of juvenile clams between boxes. This variability can be attributed to the presence and size (biomass) of green crabs in boxes.
- No juvenile clams were found in boxes at two sites at the upper and mid-intertidal of Georges River Land Trust. For sites with clams, the highest average number was 867 per sq. ft., and the lowest was 4 per sq. ft.
- Clam sizes in boxes ranged from 1.9 mm to 21.3 mm (≈ 1/12 to 3/4 inches).
- Green crabs settled into boxes at every site and ranged in size from 3.3 mm to 37.3 mm.
- At sites where average clam abundance was greater than zero, there were fewer green crabs in a box.
- Green crabs are likely responsible for a large percentage of clam mortality in the Upper St. George River.
Chart showing the size of green crabs found at each site.
2018 Methods:
We continued the study at one site, Neil’s, in the upper St. George River in South Thomaston that had excellent clam settlement in 2017. DEI scientists and landowner Neil Krane deployed five recruitment boxes at each tidal height. The boxes (n=15) passively collected clams in the low, mid, and high intertidal gradients from June to December.
2018 Results
The 2018 results showed far more clams per square foot in the predator protected boxes located in lower intertidal than the mid or high intertidal areas. We again found very high numbers of clams in this area. We found:
- An average of 1,096 clams per sq. ft in the low intertidal!
- An average of 116 clams per sq. ft in the mid intertidal
- An average of 115 clams per sq. ft in the high intertidal
At the same time, core samples of sediment adjacent to the boxes and unprotected from predators had very few surviving clams per square foot:
- 4 clams per sq. ft in the low intertidal
- 2 clams per sq. ft in the mid intertidal
- 3 clams per sq. ft in the high intertida
These results show, once again, how soft-shell clam predators are limiting the amount of clams that survive their first year, similar to results of our Freeport study that were published in the Journal of Shellfish Research in 2018.
Resident Neil Krane and Dr. Brian Beal measure clams found in the 2018 Thomaston recruitment box study.
