In 2013, Freeport’s Town Council approved a total of $170,000 to be used to help its Municipal Shellfish Program gather information about green crabs and declining clam stocks. A portion of those funds was used to conduct two field studies – green crab trapping, and green crab fencing.
2013 Green Crab Fencing
The effort began in April with a proposal to the Army Corps of Engineers (ACE) to install traditional “green crab fences” along the mouth of Recompence Cove to deter crabs with the goal of encouraging settlement/recruitment and increasing survival of wild clam spat behind the fencing. The ACE proposal also included a field experiment at nearby Little River flat that would establish six 30-ft x 30-ft x 18-inches tall fences (with 1/2-inch mesh) to closely examine the effect of crab deterrence on clam spat.
2013 Green Crab Trapping
In addition, a green crab trapping study was initiated during the last week of May 2013 that continued through early November. Information collected from the trapping studies included the effect of “soak time” (length of time the traps were fished) on catch mass, whether differences in crab abundance occurred between traps fished in the intertidal vs. subtidal, and how sex ratios, and size frequencies varied spatially and temporally.
Crushed, commercial-size soft-shell clams were used as bait for the entire study. Clams were used as bait because it was thought that other forms of bait may attract untypically high numbers of green crabs to the traps. For the experiment we wanted to record as-close-to-natural conditions as possible, and used clams because they already inhabited the areas where the traps were fished.
Traps were fished in groups of five at ten places in the Upper Harraseeket River, and the contents from all five traps were pooled into a single tote that was weighed.
MEDIA Coverage of the 2013 Freeport Shellfish Restoration Project
To read from other sources about the trapping studies conducted in 2013, see:
Portland Press Herald (27 May 2013)
Bangor Daily News (9 July 2013)
WCSH-TV report (28 August 2013)
Green Crab Exclusionary Fencing 2013
On 26-27 July 2013, we began to deploy fencing at Recompence flat and Little River flat. The wooden fences were pre-fabricated in lengths of 8-, 10-, and 12-feet, and then taken by airboat to both flats. The goal of the fencing at Recompence flat was to completely close off the mouth of a cove to green crabs in hopes that deterring these predators would result in an enhancement of wild seed clams that could be seen/measured/documented in the fall. (Soft-shell clams spawn in the spring/early summer; swimming/planktonic larvae develop for 3-4 weeks, then settle to the bottom; by October/November, clams that have survived [called spat] are typically 2-8 mm in shell length and are easily seen from bottom samples that are washed through fine mesh screens.)
Due to the threat of entrapping endangered Altantic sturgeon, Acipenser oxyrinchus oxyrinchus, and/or short-nosed sturgeon, Acipenser brevirostrum, the Army Corps permit required that every 150-ft of fencing have a 2-ft wide gap to allow entrained fish a safe passage out. In addition, the permit required one 10-ft wide gap. Although crab traps eventually were placed at/near these openings, collectively, the gaps were difficult to manage/maintain and, from time-to-time green crabs were seen crawling behind the fencing.
Unfortunately, by mid-September, routine maintenance of fencing at Little River and Recompence ceased and erosion at the fence/sediment interface occurred as well as the above-ground structural integrity.
On 16 November 2013, fenced plots, netted plots, and control plots were sampled. Unfortunately, most of the fenced plots either had collapsed, or had lost sections that allowed green crabs and other predators unfettered access to the “fenced” areas.
Most of the fences had at least one collapsed section or side by November 2013. Regardless, samples were taken from inside these “fenced” plots and the contents of each were washed through a 1 mm sieve to determine the density and size of wild clam spat.
Left: The netting is peeled back to allow samples to be taken.
Right: Netting was lost recently (within days of the 16 November 2013 sampling). Nonetheless, samples were taken from this “ghost net” and results compared to the plots that remained netted for the entire field trial.
At Recompence Flat, samples were taken on 17 November using the airboat.
While some sections of fencing at Recompence Flat were intact, many sections had eroded so that crabs and other predators could have burrowed, swum, or otherwise managed to counteract the fencing.
Sections of fencing missing from Recompence Flat. These are large enough for any clam predator to access the flats behind the fencing.
Samples were taken at three tidal heights between the fencing and the shore. Few soft-shell clam spat were found in any of the 72 samples taken on 17 November 2013.
Predator Protection Using Netting (Plant pots)
To determine the interactive effects of clam size and predator netting, we devised and set out a field experiment at both Little River and Recompence flats on 18 August 2013. Shown below are the three different methods used to deter crabs and other predators from clams seeded within 6-inch diameter plant pots that were filled with ambient sediments.
a) open pot – the rim of netting around the periphery ensures that clams remain enclosed in the six-inch diameter area – the netting does nothing to deter predators; b) a pot that is covered in two types of netting – a piece of 6.4 mm extruded (hard) plastic netting is closest to the pot and this is held in place with a piece of 4.2 mm flexible netting, similar to the type used in the larger experiment (see above);
c) a pot that is covered with a piece of 4.2 mm flexible netting.
Not shown are the clams originally planted at 12 per pot. Three sizes of cultured clams were used: 8.2 mm, 14.2 mm, and 19.4 mm. Every combination of clam size (a = 3) and predator exclusion (b = 3) were used to form nine treatments. Five replicates of each treatment were used at each site. Pots were removed from Little River flat on 16 November, and from Recompence flat the following day.
Maine Clammer’s Association President, Chad Coffin, inspects a matrix of 45 pots at Little River Flat on 12 October 2013.
These are the “Large” clams (initially about 20 mm in shell length) that were planted in protected pots at Little River flat. The experiment ran from 18 August to 16 November 2013. The “hatchery mark” is clearly visible at a size of around 20 mm, and all new growth can easily be measured.
To review the results of the green crab trapping, fencing, and small-scale predation/clam size experiment, see the Final Report to the Freeport Town Council that was completed in early January 2014.