HomeMy WebLinkAboutDrive Over Test 102 Bluff Point 1
TO: Conservation Commission
FROM: Natural Resources, Town of Barnstable
DATE: February 9, 2023
RE: Drive-Over Testing for Sealegs and Iguana
In an effort to have more information on the direct impacts of the amphibious vessels
“Sealegs” and “Iguana”, Natural Resources conducted an experiment directly driving
over planted softshells, Mya arenaria. “Sealegs” was not in operation for this experiment.
Over 600 softshell clams were dug up from the Welo Property in Cotuit Bay on June 13,
2022. They were separated based on size into two categories: over one inch and less than
one inch. These animals were then transplanted to 102 Bluff Point under six, two foot by
four foot predator exclusion nets in the intertidal zone (figure 1). Under each net, 60
softshells over one inch were planted and 40 softshells under one inch were planted. In
theory, the larger the animal, the longer the neck is for the clam and therefore the deeper
down it can dig for protection leaving the smaller ones closer to the surface.
Figure 1: Schematic of Plots
Town of Barnstable
Marine and Environmental Affairs Department
1189 Phinney’s Lane, Centerville, MA 02632
Derek Lawson, Director
508-790-6273 / Fax 508-790-6275
www.townofbarnstable.us
Animal Control 508-790-6274 Natural Resources 508-790-6272
Harbormaster 508-790-6273 Sandy Neck 508-790-6272
Marinas 508-790-6273 Sandy Neck Gatehouse 508 -362-8300
Moorings 508-790-6273
Control
ll
Drive-Over
Bulkhead
Ocean
1
2
3
4
5
6
2
Plot Planted >1"Planted <1">1" Recovered <1" Recovered Broken By Drive Broken By Staff % Recovered % Broken Drive %Broken By Staff
1 60 40 40 12 Control 2 52 0 3.8
2 60 40 19 3 Control 5 22 0 22.7
3 60 40 50 24 Control 2 74 0 2.7
4 60 40 41 16 6 1 57 10.5 1.8
5 60 40 31 5 2 2 36 5.5 5.6
6 60 40 43 18 5 1 61 9.8 1.6
Plots 1-3 were used as a control since it is notoriously difficult to successfully transplant
softshell clams and we don’t want to attribute mortality incorrectly to driving over them.
Plots 4-6 were driven over once down to the ocean and once back up. The animals were
allowed to acclimate for about 6 weeks. On August 3, 2022 Natural Resources Staff
proceeded to dig up the softshells, by hand, and count the number of animals retrieved,
separated by size, the number of animals found broken, and the other animals we broke
by digging them up. The chart below (figure 2) gives the information on each individual
plot.
Figure 2: Results from Drive-Over Test
Results and Discussion:
As one can tell from the above chart, recovery was a challenge. Softshell clams are
difficult to transplant and survival is usually minimal which is why growers tend to put
down netting and hope to catch a wild set (Landry, 2021). The larger the softshell
transplanted, the higher chance of survival. Natural Resources has attempted a few times
to transplant softshells and grow them from seed with minimal long-term success. This is
even more so why it is imperative to prot ect softshell habitat because growers are having
trouble even coming close to replacing a wild softshell clam set. Staff broke a fair
number of softshells harvesting them for counting which is typical with any softshell
clam harvest. Between 5 and 10% of softshells were broken from driving over them, most
of which were seed. Regardless of how many were broken by harvest, this is still an
additional percentage that could be expected to be broken from “Iguana”.
It was surprising that more animals were not crushed and that is great news!
Unfortunately, those that are being affected most are the seed population which can affect
the future shellfishery. There is not enough evidence from this experiment to suggest that
the Iguana should not be able to operate as it has been within a designated corridor.
However, there are ideas that need to be discussed based on the shortcomings of this
experiment.
This experiment has allowed us to better visualize the direct impacts on the current
softshell clam population on site. It does nothing to tell us if the depressions from the
treads, movement, and compaction of sand over time will prevent softshell clam
settlement and recruitment in what is now healthy, highly suitable habitat. This area in
question is in a protected shellfish relay area because it was deemed as habitat that should
be set aside for the shellfishery and protected from development. One drive over is not
sufficient for any experimental conclusions and there would have to be several sample
3
sizes over years to get any sort of real idea what is happening here. We are not going to
know the extent of damage, if any, these vehicles are c ausing until it happens and even
so, it will be challenging to ever attribute it to the vehicles alone. Driving over a softshell
clam population has proven that it will kill some shellfish. The continual driving over
them will not show more promising results. As a result, if these vehicles are to continue
to operate, the corridor habitat is essentially being sacrificed and will have some
unknown impacts on the shellfishery at this location. It just needs to be recognized that
the more of these situations that happen, the more area that is being sacrificed and it will
have a cumulative effect.
Sources:
Landry, Shantelle, "Recruitment And Post-Settlement Mortality Of The Soft-Shell Clam,
Mya Arenaria" (2021). Dissertations, Theses, and Masters Projects. William & Mary.
Paper 1627407484. http://dx.doi.org/10.25773/v5-hnta-5775