Northeast Fisheries Science Center Reference Document 07-07
Evaluation of a modified scallop dredge’s ability
to reduce the likelihood of
damage
to loggerhead sea turtle carcasses
by Henry O. Milliken1, Lisa Belskis2, William DuPaul3, Jeff Gearhart4,
Heather Haas1, John Mitchell4, Ron Smolowitz5, Wendy Teas2
1National Marine Fisheries Serv, Woods Hole Lab, 166 Water St, Woods Hole MA 02543-1026
2National Marine Fisheries Serv, Southeast Fisheries Science Cent, 75 Virginia Beach Dr, Miami FL 33149-1003
3Virginia Institute of Marine Science, Gloucester Point VA 23062
4National Marine Fisheries Serv, Southeast Fisheries Science Center, 705 Convent Ave, Pascagoula MS 39567
5Coonamessett Farm, 277 Hatchville Rd, East Falmouth, MA 02536
Print
publication date April 2007;
web version posted May 9, 2007
Citation: Milliken HO, Belskis L, DuPaul W, Gearhart J, Haas H, Mitchell J, Smolowitz R, Teas
W. 2007. Evaluation of a modified scallop dredge’s ability to reduce the likelihood of
damage to loggerhead sea turtle carcasses. US Dep Commer, Northeast Fish Sci
Cent Ref Doc. 07-07; 31 p.
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ABSTRACT:
In 2003 and 2004 the scallop industry, Coonamessett Farm, and the Virginia Institute of
Marine Fisheries, with funding from the NOAA Fisheries Service, successfully tested a series of
chains that excluded turtles from entering the dredge bag. After further consultation with the
Northeast Fisheries Science Center and with additional funding, Coonamessett Farm was
awarded a contract to redesign the dredge frame to reduce the probability of a turtle on the
seafloor going under the dredge frame. In collaboration with the Harvesting Systems and
Engineering Branch of NOAA Fisheries Service, divers videoed the results of placing turtle
carcasses in the path of a modified scallop dredge. Twelve trials were completed, and turtle
carcasses went over the dredge in all but three trials. Damage was assessed as “slight or none” in
all valid assessments of damage to a carcass following an encounter.
INTRODUCTION
Sea scallop dredge gear fished in the mid-Atlantic region incidentally
captures sea turtles (
Murray
2004a, 2004b, 2005, 2007). NOAA National
Marine Fisheries Service’s (NMFS) Northeast Fisheries Science Center (NEFSC)
has been working collaboratively with the scallop industry, Coonamessett Farm,
Virginia Institute of Marine Science (VIMS), and the Southeast Fisheries
Science Center (SEFSC) to mitigate the bycatch of sea turtles in the sea scallop
dredge fishery. A study with twenty-two experimental
cruises and 3,248 paired hauls assessed the effectiveness of “turtle chains” affixed
to sea scallop dredge gear in reducing sea turtle bycatch (DuPaul et al. 2004).
On August 25, 2006, NMFS issued a final
rule to require sea scallop dredge vessels fishing south of 41°9.0′ N.
latitude May 1–November 30 each year for dredges to be equipped with turtle
chains (NMFS 2006).
Despite the implementation of the chain mat rule, there is
concern that some turtles may sustain injuries if they encounter the dredge on
the seafloor and go under the dredge. Questions
have also been raised about injuries turtles might sustain if, after they
interacted with the dredge, they go over the dredge rather than under the
cutting bar.
In response to these concerns, the NEFSC funded the
development of a modified dredge designed to act as a wedge and guide turtles
over the top of the dredge. A prototype was
evaluated in 2005 in
Panama City
FL using fiberglass turtles and
turtle carcasses. This work achieved
some success at increasing the probability of turtle carcasses going over the
dredge as opposed to under the cutting bar, and resulted in several ideas for additional
modifications.
This report summarizes (1) the gear modifications that
occurred after the 2005
Panama City gear trials,
(2) the 2006
Panama City
gear trials, (3) the documentation of damage that occurred to turtle carcasses
after interacting with the redesigned dredge, and (4) suggestions for future
study.
METHODOLOGY
The primary objectives of this project were to (1) modify
the 2005 prototype dredge, (2) provide qualitative assessments of the efficacy
of the modified scallop dredge to cause turtle carcasses to go over the dredge
while fishing on the seafloor, and (3) document any carcass damage associated
with such interactions.
Modifications to the dredge design
As a result of the 2005 testing, a new modified dredge
prototype was conceived and constructed under contract to Coonamessett Farm for
evaluation during 2006. The experimental
dredge was a modification of a standard
New
Bedford style sea scallop dredge (Figure 1A). The modifications consisted of moving the
cutting bar forward, removing all brace bars in the bale section, and adding
several vertical round stock bars along the face of the dredge between the
depressor or “pressure” plate and the cutting bar (Figure 1B). Moving the cutting bar forward changes the
geometry of the dredge frame, and the new wedge shape was thought to increase
the probability of turtles going over the frame rather than under the cutting
bar. Removing the brace bars from the
bale section eliminates barriers that might hold a turtle under the bale rather
than letting it pass through the bale and escape over the dredge frame. The round stock bars were added to keep
contact with the seafloor and to add a structure that might help turtles move up
and over the cutting bar.
Assessment of modified dredge design
The experiment was designed to simulate the “worst case
scenario” of a dredge overtaking and hitting a motionless turtle lying on the
bottom. This scenario was achieved by
having two NOAA divers place either a turtle carcass or turtle model in the
path of the towed dredge. Turtle
carcasses were deployed at predetermined locations and orientations along the path
of the dredge to help identify potential flaws in the dredge design. Each interaction was documented by
videotaping the event from three different perspectives. The team of divers deploying the turtles
videotaped the initial portion of the interaction, while a team of divers on
the dredge videotaped the entire interaction. An additional camera was placed at different locations on the dredge
bale, depending on the predetermined interaction location, to document the
entire tow including the interaction.
Due to time and cost constraints, turtles were not randomly
placed in the path of the dredge. Divers
were instructed to place turtles in specific predetermined orientations and
locations along the face of the dredge in an effort to identify design flaws. Because of the methodology, the ratio of carcasses
that went over the dredge might be higher than what was recorded.
This project used the F/V Capt. Wick, a 60 gross ton, 65-ft
shrimp trawler. The vessel was modified by adding an ‘A’ frame off the stern of
the vessel to tow the 13’ modified dredge. This vessel also served as the
platform for staging the dive operations while an inflatable boat was used to
transport the divers between the vessel and the tow path. Gross necropsies of
the turtle carcasses were performed on the vessel, while the finer scale examinations
were completed onshore.
Based on preliminary tows to determine optimum scope and towing
speed, all tows were conducted using a 3:1 ratio of tow wire to depth and a towing
speed at 3.0 knots. This was a compromise to achieve a dredge towing angle that
closely matched that of commercial dredges while allowing divers to safely ride
the dredge. This speed was slower than typical commercial towing speeds of 4-5
knots.
Assessment of post-interaction
damage to the carcass
Five separate turtle carcasses were used in 14 field trails
(Table 1). The carcasses were found dead
on beaches (stranded) and varied in freshness, size, and levels of
emaciation. All of the carcasses were
inspected thoroughly prior to deployment to document existing external
damage. Placement of turtle carcasses in
the path of the dredge was not random. Placement for most trials was
predetermined to assess the effectiveness of the design in various areas of the
dredge. After each interaction, the carcasses were recovered and damage
assessments were performed by trained NMFS staff. Each carcass was used multiple times. A successful deployment was defined as a
trial where a carcass was placed in the path of the dredge and the carcass
interacted with the dredge. Damage assessments were performed after each interaction
between the carcass and the dredge, but only the damage assessment after the
first carcass deployment was used to assess potential damage due to gear
interactions because the carcasses could become more easily damaged if their
structure was compromised during the initial interaction. The fiberglass turtle model (tow 17) used to
mimic how a motionless turtle might interact with dredges was of a similar size
to turtles incidentally captured by commercial sea scallop dredge vessels, but it
had inflexible appendages (head, flippers). Because of the dissimilarities to carcasses, it was not included in the
assessments.
RESULTS
Assessment of modified dredge design
Seventeen repeated dredge tows were completed and twelve
successful trials were achieved (Appendix 1). Two test tows were used to determine proper towing configuration, two
tows failed to interact with the turtle carcass, and one tow used the fiberglass turtle. In eight
of the twelve trials, the carcasses went over the dredge (n=7)
or were deflected to the side (n=1; tow 3). In one of the remaining four trials (tow 14), the turtle carcass was
outfitted with weights because it was buoyant, started to go over the dredge
but was constrained from passing completely over by the dredge by the weights
which caught the frame of the dredge. Additionally, the front flippers may have
been caught (see Appendix 1). In the remaining three trials, the bale held the
carcass from going over the dredge. There were no instances of any carcass
going under the dredge frame.
In addition to the twelve trials with carcasses, one trial (tow
17) used a fiberglass turtle. The
fiberglass turtle became trapped under the bale by its rigid flippers (see
Figure 12 in Appendix 1). Because of the rigidity of the appendages, it is difficult
to infer very much from this interaction.
Assessment of post-interaction
damage to the carcass
As previously indicated, damage assessments to the carcasses
were limited to the first trial in which the turtle carcass was used, even
though turtle carcasses were used multiple times (Tows 3,6,8,10 and 14: Table 1). Each of the five carcasses was successfully deployed in the path of the
dredge. Carcass damage from the dredge interaction for each in the five trials
was slight or none (Severity score 1: Table 1).
DISCUSSION
The turtle carcass interaction trials should be considered an
exploratory scenario; the behavior of a live turtle near a dredge may change
the outcome of the interaction. The use
of turtle carcasses and a turtle model did not provide information about the
behavior of sea turtles around scallop dredges, particularly on how and where
(in the water column) interactions occur with the dredge and how turtles may be
able to avoid a dredge at or near the bottom.
There are several reasons why it is inappropriate to
conclude that the damage to turtle carcasses in this experiment are representative
of injuries that result from benthic interactions in the scallop dredge
fishery:
- this study used turtle carcasses which could not exhibit any
behavioral responses to the dredge
- physical damage to the carcasses in the experiment may have
been affected by the level of pre-test decomposition of the carcasses
- carcasses were placed in predetermined locations along the
face of the dredge which biases results
- tows were prematurely ended when turtles were trapped under
the bail to minimize damage
- the experimental sample size was small
Because of the limitations of using models and turtle
carcasses, and conducting the trials in a different geographic area where the fishery
interactions occur, this project could not assess how turtles interact with commercial
sea scallop dredges. Rather, the goal of
this project was to examine how well the modified prototype dredge minimized
the impact on motionless turtles interacting with the gear while the dredge was
fishing on the seafloor.
The experimental dredge performed well, allowing most carcasses
that passed under the bale to be deflected up and over the front of the dredge
frame. In the 12 successful interaction
trials, only three turtle carcasses ending up trapped in locations that
required removal by divers. However,
this outcome should not be interpreted as measure of the dredge design’s
performance because carcasses were not randomly placed in the path of the
dredge. Divers were instructed to place
turtles in specific predetermined orientations and locations along the face of
the dredge in an effort to identify specific design flaws. Nevertheless, the performance of the
experimental dredge design was a substantial improvement over both the
traditional New Bedford style dredge and the 2005 modified dredge.
Although the Panama City gear trials indicate that the 2006
dredge design directed more turtles over the dredge than under the cutting bar,
there are a few additional gear modifications which could further reduce
possible injuries to sea turtles on the seafloor. The dredge design might be improved to reduce
the likelihood of turtles getting blocked either under the center bale bar (Tow
16) or under the side bale bars (Tows 6 and 11). Although live turtles trapped
in these areas might be able to escape to either side of the obstructing bar,
the encounter might increase the likelihood of an injury. The utility of the round
bar strut extensions on the front of the cutting bar appeared to be equivocal
to the movement of the carcass over the dredge frame and may warrant future
examination of alternate designs or elimination of this modification.
The 2006 Panama City gear trials showed that moving the
cutting bar forward and adding round stock to the dredge frame was successful
in creating a ramp that caused turtle carcasses to go over the dredge. Further work
will incorporate the same frame design with modifications to the bale to reduce
the likelihood of entrapping turtles under these bars.
REFERENCES
DuPaul WD, Rudders DB, Smolowitz RJ. 2004. Industry
trials of a modified sea scallop dredge to minimize the catch of sea turtles.
VIMS Mar Resour Rep No. 2004-12; 31 p.
Murray KT. 2004a. Bycatch of sea turtles in the mid-Atlantic sea
scallop (Placopecten magellanicus) dredge fishery during 2003. US Dep Commer, Northeast Fish Sci Cent
Ref Doc. 04-11; 28 p.
Murray KT. 2004b. Magnitude and distribution of
sea turtle bycatch in the sea scallop (Placopecten magellanicus) dredge fishery
in two areas of the northwestern Atlantic Ocean, 2001-2002. Fish Bull
(DC) 102:671-681.
Murray KT. 2005. Total bycatch estimate of
loggerhead turtles (Caretta caretta) in the 2004 Atlantic sea scallop (Placopecten
magellanicus) dredge fishery. US Dep Commer, Northeast Fish Sci
Cent Ref Doc. 05-12; 22 p.
Murray KT. 2006. Estimated average annual
bycatch of loggerhead sea turtles (Caretta caretta) in U.S. Mid-Atlantic bottom
otter trawl gear, 1996-2004. US Dep Commer, Northeast Fish Sci
Cent Ref Doc. 06-19; 26 p.
Murray KT. 2007. Estimated
bycatch of loggerhead sea turtles (Caretta caretta) in U.S. Mid-Atlantic
scallop trawl gear, 2004-2005, and in sea scallop dredge gear, 2005. US Dep
Commer, Northeast Fish Sci Cent Ref Doc 07-04; 30 p.
NMFS. 2006. Endangered and threatened wildlife; sea turtle
conservation. Final Rule. Fed Regis. 71(165): 50361-53373.
