In 2015, we began by overwintering clams off the South Freeport docks using methods that we have used at DEI since the early 1990's. Clams that had been reared in the nursery upweller in 2014 had attained sizes of 12-20 mm in length, and these juveniles were added to Pet screen bags at densities of three to five thousand per bag (18-inches x 18 inches) in late November 2014.
Approximately 95% of clam juveniles survived during the overwintering process, and these were used in various field trials that were initiated in April and May 2015.
The six components of the 2015 field trials
A. Green crab trapping
For the third year-in-a-row, we established a green crab trapping program to better understand the dynamics of this predator in the Harraseeket River. As in the 2014 trials (see the first and second progress reports), we divided the river into an Upper and Lower section with five study sites located in each section -- 2 subtidal and 3 intertidal. Unlike the previous two years, however, we set and fished the traps (five at each site) every four days and switched bait upon each haul: one time salted herring, similar to that used by lobster fishers, and the next time crushed adult soft-shell clams, similar to that used in the previous two years. The first traps were hauled on May 10 and the final haul date was October 29.
B. Spatial variation in cultured clam growth and survival (Harraseeket River)
In 2014, we observed dramatic differences in cultured clam survival, and in wild clam recruitment, from one side of the Harraseeket (near Collins Cove) to the other (directly across the river on the Wolfe Neck shore). We were curious whether these differences (less than 10% survival in the 40 plots near Collins Cove vs. > 90% survival at the site across the river) were a result of some random event, or whether these differences could be attributed to some intrinsic biological or environmental factor. Beginning in mid-April 2015, we established ten study sites on the east and west side of the Harraseeket River by deploying four plots (14-ft x 22-ft) that were seeded with overwintered hatchery seed at a planting density of 10.5 clams/ft2. Core samples were taken from each of the 40 nets on the east side of the river and 40 nets on the est side of the river in November. Each sample was washed through a 1 mm sieve, and the number of live and dead clams counted. All live clams also were measured to estimate growth rates. Wild soft-shell clam recruits (0-year class individuals) from each core sample also were counted and measured.
C. Sediment buffering experments
One hypothesis that has been proposed for the recent declines of soft-shell clams along the shores of Casco Bay is that ocean and coastal acidification has resulted in acidic mud and seawater, and that these environmental assaults result in the dissolution of clam shells that ultimately leads to widespread mortalities. The Friends of Casco Bay has sounded the alarm on ocean acidification indicating that acidic mud could wipe out clam harvesting in Casco Bay. In 2015, a study was released that linked ocean acidification to declining clam numbers.
In 2014, we investigated the relative importance of ocean acidification versus predation on soft-shell clam recruitment at an intertidal mud flat that had low pH (ca. 7.2) as measured by the Friends of Casco Bay (Staples Cove, Freeport, Maine). We added crushed soft-shell clam shells (at two rates - 13 lbs or 26 lbs) to 10-ft x 10-ft plots. In addition, we incorporated control (no shell) plots into the design. Those three treatments examined the effect of shell (to help buffer acidic muds) in plots that did not deter predators. Another three treatments (the same as above, but all covered with a flexible netting - 1/6th-inch aperture) also were incorporated into the design. Results demonstrated no effect due to shell addition or netting on soft-shell clam recruits; however, nearly 40x more hard clam recruits were observed in plots covered with netting compared to those without netting. No differences in hard clam recruits could be attributable to the addition of shells to plots.
To examine the same hypothesis, two study sites (Little River and Winslow Park) were chosen for 2015, and the same experimental design used.
D. Spatial variability in wild soft-shell clam recruitment (Harraseeket River)
It is possible to measure variation in wild soft-shell clam recruitment by taking bottom core samples under nets seeded with cultured soft-shell clam juveniles, and processing the samples using a small-mesh sieve. One problem with this analysis is that cultured clams may have some effect (positive or negative) on wild clam recruitment. To examine natural recruitment without the influence of cultured clam seed, we initiated a study at the same twenty sites in the Harraseeket River that were described in Part B (above).
We deployed a series of six wooden boxes (1-ft x 2-ft x 3-inches deep) that were placed directly on the surface of the mud at each of the ten sites on the east and west side of the Harraseeket River. Two boxes had Pet screening (1.6 mm aperture) on both top and bottom, and contained no sediment initially. Two boxs had Pet screening on both top and bottom, and contained 1,000 ml of play sand. Two boxes had an extruded polyethylene netting (6.4 mm aperture) on both top and bottom, and contained no sediment initially. We assumed that small clams would either wash into the boxes or settle directly into the boxes as larvae (as they do when they settle to the mudflats naturally). The boxes with Pet screen excluded large crabs and nemertean worms (milky ribbon worms - Cerebratulus lacteus), but the larger mesh boxes would not exclude small crabs or milky ribbon worms from crawling through the netting. Each end of each box had a piece of heading twine that formed a loop. A wooden lath with a notch that hooked the twine was forced into the mud, and this kept the boxes in place for the seven months that the experiment was in the field.
In November, the contents of each of the 120 boxes (2 sides of the river x 10 sites/side x 6 boxes/site) were washed through a 1 mm sieve and all clams counted and measured.
E. Spatial variability in cultured clam growth and survival in wooden boxes (raised beds)
We deployed the same experiment at five sites in Freeport during late April to early May 2015. The experiment used a series of thirty 4-ft x 2-ft wooden boxes that were placed directly on top of the mudflat surface. The bottom of each box was lined with a flexible netting (1/6th-inch aperture). The tops of one-half of the boxes were covered with 1/4-inch flexible netting while the other half were covered with 1/6th-inch flexible netting. One-third of the boxes were seeded with cultured clam seed at a density of 30 clams/ft2, one-third with 45 clams/ft2, and one-third with 60 clams/ft2. Both factors (protective netting on the box top; stocking density) were combined to make a total of six treatments, each with five replicates. Boxes were placed on the flat in a 6 x 5 array.
In November 2015, two core samples were taken from each of the 150 boxes, and the contents of each washed through a 1 mm sieve. All live and dead clams were counted and measured to estimate survival and growth.
An additional experiment was initiated in mid-May to examine the effects of infaunal predators (milky ribbon worms and green crabs) on the survival and growth of cultured soft-shell clam juveniles. A series of 21 boxes were arrayed near the mid-intertidal at Staples Cove, Freeport, Maine. All boxes were covered on top with a piece of flexible netting (1/6th-inch aperture). Seven boxes had a piece of extruded netting (6.4 mm aperture) on the bottom, seven boxes had a piece of flexible netting (4.2 mm aperture), and seven boxes had a piece of Pet screening (1.6 mm aperture) on the bottom. The Pet screening was used in an attempt to exclude the larger benthic predators while the larger netting would not deter milky ribbon worms or small green crabs. Two core samples were taken from each box in early November, and the contents of each core washed through a 1 mm sieve. All live and dead clams were counted and measured to estimate survival and growth.
F. Growing cultured clam seed in two bivalve nursery upwellers
With the success seen in 2014 with the first upweller, we decided to construct and deploy a second nursery upweller at the South Freeport dock in 2015. The upweller was deployed in mid-July, and doubling the capacity for growing soft-shell clam seed provided great opportunities for continued efforts in 2016.
Funding for 2015 was provided by: 1) National Marine Fisheries Service (Saltonstall-Kennedy program); 2) University of Maine System (Maine Economic Improvement Fund - Small Campus Initiative); and, 3) The Broad Reach Fund (Maine Community Foundation).
To see the third progress report for NOAA/NMFS, click here. To see the fourth progress report for NOAA/NMFS, click here. That report covers most of the results from the 2015 projects. The Final Report can be downloaded if you click here.