Design Evolution of a Turtle Excluder Dredge for the Northwest

Design Evolution of a Turtle Excluder Dredge for the Northwest Atlantic
US Sea Scallop Fishery
Ronald Smolowitz*
Coonamessett Farm, Inc,
277 Hatchville Road, East Falmouth, Massachusetts, 02536, USA
Matthew Weeks
Coonamessett Farm, Inc,
277 Hatchville Road, East Falmouth, Massachusetts, 02536, USA
Henry O Milliken
NOAA / NMFS / NEFSC,
166 Water Street, Woods Hole, Massachusetts 02543, USA
*E-mail: [email protected]
Keywords: Sea turtle, Scallop dredge, conservation engineering
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Abstract
Design modifications to a traditional New Bedford style scallop dredge to prevent loggerhead
sea turtles (Carretta carreta) from snagging on top of the dredge frame or becoming trapped
under the dredge bale, while maintaining efficiency for dredging sea scallops (Placopecten
magellanicus), were evaluated. The final design, the Cfarm turtle excluder dredge, proved
effective at guiding turtle carcasses over the top of the dredge by eliminating most of the bale
bars and forming a ramp with a forward positioned cutting bar and closely spaced struts leading
back at a forty-five degree angle.
From May 2006 until November 2009, thirty-three trips on thirteen different commercial scallop
vessels tested five sequential dredge modifications for impacts on scallop catch, fish bycatch,
turtle bycatch, and frame durability. The modified dredge and an original New Bedford dredge
(as the control) were compared by conducting paired, side-by-side tows, using identical tow
parameters. A total of 4,059 paired tows were conducted in which tow data, scallop catch, and
bycatch were recorded; data from slightly less than half of the tows (44%) were sampled
sufficiently for analysis. Summarized dredge catches showed a significant 3% increase in
scallop catch, and significant decreases in bycatch in many species, including yellowtail flounder
(46%), winter flounder (69%), barndoor skate (18%) and winter skate (20%).
The final dredge frame design tested in this study held up to the rigors of commercial fishing on
most scallop grounds, maintained commercially-acceptable levels of scallop catch, and decreased
fish bycatch compared to the control. Flow characterizations in a flume tank provided insight
into cutting bar and frame hydrodynamics that may explain the field trial results. Features that
could reduce injury to loggerheads can be applied successfully, and still be found to be readily
acceptable by fishers with no increase in costs or labor.
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INTRODUCTION
Historically, fisheries stocks were managed by maximizing the sustainable catch of target
species, regardless of catch composition (Pikitch et al., 2004). With a more holistic approach to
management, fisheries have been migrating towards more specific control of catch composition
through technological innovation (Kennelly and Broadhurst, 2002). Over fifteen years of
continued research has gone into preventing fish bycatch in the New Bedford style scallop
dredge (Henriksen et al, 1997; Smolowitz et al, 2001, 2002, 2004; Smolowitz and Weeks, 2008,
Walsh, 2008). Only within the last ten years has an effort taken place to reduce the impacts of
dredge interactions with loggerhead sea turtles (Smolowitz et al., 2005, 2007). Initially, the
focus has been on preventing loggerheads from entering the dredge, through implementation of
turtle chains, which are very effective (DuPaul et al, 2004; Murray, In press).
However, even with turtle chains in place, many stakeholders were concerned that design
elements of the New Bedford style scallop dredge could still inflict injuries to loggerheads.
Initial concerns started with observers and fishermen documenting reports of loggerheads
wedged in the space between the depressor plate and the cutting bar on the dredge frame. In this
position, the loggerhead is at risk of injury if the dredge frame encounters the vessel side or if the
turtle becomes dislodged while the dredge is up in the air over the vessel’s deck. Further
potential for injury was supported by video experiments conducted in 2005, 2006 and 2009,
where turtle carcasses were placed in the path of a towed scallop dredge and interactions
recorded (Milliken et al, 2007; Smolowitz et al, 2010). These simulations of turtle and dredge
interactions indicated that turtles could be run over by the dredge and end up trapped beneath it
(Figure 1). Video analysis and direct inspection of the carcasses offered insight into the type of
injuries sustained, as well as the path or trajectory associated with that injury, highlighting the
target areas on the dredge in need of modification. Close spacing of bale bars was immediately
identified as preventing the turtle from escaping upward before encountering a flat-faced cutting
bar (Milliken et al, 2007). This observation indicated the need to find additional alternative
measures to prevent loggerhead bycatch.
Sea Turtles have long been subject to world-wide dangers from traditional and commercial
fisheries (Parsons, 1962; Witzell, 1994), including incidental capture or injury in the scallop
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fishery (Murray 2004, 2004a, 2005, 2007). Decreases in nesting abundance (Limpus and
Limpus, 2003) and uncertainties in sea turtle population stability (Committee on Sea Turtle
Populations, 1990) suggest the need to mitigate these dangers has become increasingly more
important. However, it also is important to consider the impacts of any proposed bycatch
mitigation measures on fishermen and local economies and to strive for the development of
effective measures that also maintain sustainable fisheries industries. The scallop fishery was
valued at US$385 million in 2007 (NOAA, NMFS, 2008) and is critical to communities in the
Northeast.
This paper chronicles the evolution of design changes tested to develop a sea scallop dredge that
reduces bycatch, while improving efficiency of targeted scallop harvest. Design modifications
critical to sea turtle safety focused on frame design changes that seemed likely to mitigate
documented concerns. Basic changes consisted of moving the cutting bar forward, removing all
the interior bale bars except the center bar, increasing the opening between the outer bale and
frame, and decreasing spacing between struts. As is common when developing modified fishing
equipment, the evolution of changes created new challenges (e.g., maintaining viable catch
results), all of which are addressed in this paper.
METHODS
Each of the five experimental dredge designs tested during this study represented a sequential
variation on a standard New Bedford dredge (Figure 2). Experimental and control dredges had
uniform measurements and outfitting, measured 4.6 m wide, and were configured with identical
chain bags. Modifications were based upon four major design changes to prevent interaction
with turtles, guided by previous work and observations (Smolowitz et al, 2010). First, the
cutting bar was moved forward of the pressure plate and placed at a 45-degree angle. Strut
spacing was reduced and struts were rotated from the standard 90 degrees to a 45-degree angle to
tow direction to allow easier deflection of objects. In dredge designs 3 and 5 the outer bale was
extended straight forward from the cutting bar before turning towards the tow point. Lastly, the
number of bale bars were reduced from 9 to 3 (except for dredge design #4 which had a center
truss) to allow turtles to escape upwards. Some of these elements are illustrated in Figure 3, as
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tested in the flume tanks. These changes, combined with the elimination of many of the doublers
and gusset plates used for structural reinforcement, enabled the modified dredges to be as
lightweight as possible while still being able to fish in areas with low levels of boulders. These
reductions in frame weight may reduce fuel consumption and reduce impacts to benthic habitats.
Individual experimental design models were numbered chronologically as Experimental Dredge
1 thru 5. Each model represented a minor modification of design features while the primary
modifications (as described above) remained. Dredge 5 represents the final accepted dredge
design developed from an evolutionary progression starting from Dredge 1. To validate sea trial
observations, elements of the five prototype dredges were tested in a flume tank. Cutting bar
orientation, style and position were tested to determine which resulted in the best hydrodynamic
flow while still exhibiting functional durability in the field tests.
Sea Trials
Testing occurred over a 42-month period (starting in 2006) onboard 37 trips, using 13 vessels
capable of towing two dredges simultaneously, and the results are summarized in Table 1. Each
trip compared a single experimental dredge design with a single New Bedford style dredge as the
control. Comparative fishing occurred under typical commercial operations and a variety of
weather conditions. Tow times averaged between thirty and sixty minutes for each pair of
dredge comparisons depending on location, and tow speed ranged between 4.5 and 5 knots. Gear
was switched between the vessel's sides approximately half way through each trip in order to
reduce any potential bias resulting from being fished on one particular side.
After an observed tow, the catch from each dredge was separated by species and individually
counted; scallop catches were recorded as bushels (bu = 35.2 liters). A one-bushel subsample of
scallops was picked at random from most tows, and those collected were measured in binned 5
mm incremental groups. Trained samplers recorded tow times, tow parameters (such as vessel
heading, speed, wire out, etc), and identified and counted species collected. When the trained
sampler(s) were off watch, the vessel’s crew was responsible for recording tow parameter data as
well as bushel counts of kept scallops but did not record fish bycatch.
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Sea trials were distributed opportunistically across a variety of traditional scallop fishing
grounds. Differences in seafloor characteristics make catch comparisons of the modified dredges
to each other difficult and potentially biased. Because of this, comparisons between modified
dredges are not addressed in this paper. However, it is important to characterize the seafloor
morphology and composition to understand the wide variability in commercial use of the New
Bedford style scallop dredge.
Eight different fishing grounds were sampled (Figure 4). The northern portion of Georges Bank
Scallop Access Area CAII is primarily a flat, poorly sorted sand substrate with patches of gravel
(Valentine et al, 2005), high currents, and with large concentrations of larger sized scallops and
few yellowtail flounder. The Southern part of Georges Bank CAII has a similar but mostly sand
substrate, lower currents, with high concentrations of both scallops and yellowtail. The Northern
Edge of Georges Bank, north of CAII, has mixed substrates of movable sand, gravel pavement,
cobble, and boulders, high currents, and patches of smaller scallops with few yellowtail flounder.
The Nantucket Light Ship Access Area (NLSA) has undulating sand waves with some boulders,
strong currents, and large populations of scallops and low levels of fish. Georges Bank Scallop
Access Area CAI is a complex substrate with dense patches of scallops, few yellowtail, but
larger populations of winter flounder. In the Mid-Atlantic, the Hudson Canyon Access Area
(HCAA), the Elephant Trunk Access Area (ETAA) and the Delmarva Access Area (DMAA)
have primarily flat sand bottom with dense scallop concentrations, few fish, and dense patches of
benthic organisms such as sand dollars. Skates are common in all areas.
Data Analysis
Paired Student t-tests were used to test for significance between the experimental and control
dredges in terms of catch of scallops and ten other species. The methodology of towing two
dredges simultaneously provided for the assumptions necessary to analyze the data using a
paired t-test. Since all comparisons had more than 30 samples, the assumption of normality was
justified. We also evaluated comparisons using the non-parametric Wilcoxon matched pairs test
(Wilcoxon, 1945) and found that the results were consistent with those provided by the paired
Student t-tests Catch ratios for each dredge were calculated in order to compare the total count of
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each bycatch species per a sampled scallop bushel.
Flume Tank Tests
Given the significant design changes, flume tank tests were used to validate the hydrodynamic flow
with the modified components, and provide insight into sea trial performance. To do this, a one-meter
wide section of the modified dredge was tested at the flume tank of Memorial University in
Newfoundland, Canada; testing was conducted on May 3-4, 2007. Design components were made of
aluminum and mounted by bolts to facilitate quick change and modification during the testing process.
One plate, opposite the viewing side, was fabricated out of aluminum, while the plate on the viewing
side was made of rigid, transparent material (Lexan) to facilitate observations. Flow was observed by
injecting a narrow stream of dye into the water path just ahead of the dredge components. Tank flow
speeds were set between 0.85 and 2.0 knots. Commercial dredge tow speeds can be much greater than
those tested, but 2.0 knots was the upper flow-speed limit of the flume tank Rubber disks (~75 mm in
diameter), used to simulate scallops, were placed on the flume tank belt ahead of the dredge model
during testing and observed during flow tests (Figure 5).
Ten separate tests were conducted in the flume tank (Table 2). Eight of the tests used a standard
cutting bar, moved forward and angled at either 45° or 90° to the direction of flow. One test used a
round cutting bar, forward positioned and seven cm in diameter (Test 5). A second test employed a
standard forward-positioned cutting bar with the addition of a “hat” to direct flow downward (Test 7).
Only Test 10 was rigged as a conventional dredge with the cutting bar under the pressure plate. Turtle
chain plates were affixed to the cutting bar on five of the tests (Tests 1, 2, 6, 8 and 9).
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RESULTS
Sea Trials
Results from the individual dredge tests are summarized in Table 3, and show the comparison of
each dredge to the corresponding control (New Bedford standard dredge). In the sampled tows,
Dredge 1 caught significantly more scallops with 1330 bushels as compared to 1202 bushels by
the standard New Bedford dredge (+10.7%). Dredge 1 was found to significantly decrease the
catch of Little Skate (-11.8%), Yellowtail Flounder (-28.5%), and Sand Dab (-38.1%) and Winter
flounder (63.6%). Dredge 2 also significantly increased scallop count, though not to the level
that was encountered with Dredge 1. It also decreased the bycatch of Little Skate (-13.3%),
Barndoor Skate (-37.3%), Winter Skate (-26.3%); Grey Sole (-54.5%), Yellowtail Flounder (48.2%), Winter Flounder (-71.7%), Four Spot Flounder (-43.3%), and Sand Dab (-43.8%). In
total, Dredge 3 was found to catch 11.2% more scallops, but the catch of little skate in Dredge 3
was significantly larger by 10.5%. Dredge 4 did not catch a significantly different number of
scallops, however it did significantly reduce the bycatch of little skate by (-25.8%), summer
flounder (-42.1%), and yellowtail flounder (-15.4%). It should be noted that throughout the final
trip with Dredge 4, a bend could be seen in both the face of the bale and a twist in the center
towing structure that appeared to worsen as the number of tows increased. Dredge 5 caught both
more scallops and more little skate, while bycatch of summer flounder was decreased by 16%.
Data from dredges 1, 2, 3, and 5 were aggregated to assess the overall ability of the experimental
dredge design concept (cutting bar forward of depressor plate, 45° cutting bar and strut, reduced
number of bale bars) to increase the catch of scallops while decreasing the retention of important
bycatch species. Dredge 4 was excluded from the compilation because its single bail design
differed significantly from the other versions of the excluder dredge concept. Results from the
analysis can be found in Table 4.
Of the 1,632 observed tows analyzed relative to the standard New Bedford dredge, the
experimental dredges significantly increased scallop catch by 3%, while having significant
decreases in summer flounder (-10.5%), yellowtail flounder (-46.1%), winter flounder (-68.7%),
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barndoor skate (-18%), winter skate (-20%), sand dab (-47.7%), and fourspot flounder (-20%).
Interestingly there were no significant differences in the catch of little skate (-0.3%), and
monkfish (1%) along with a significant increase in one species, the American plaice (+114%).
Hydrodynamic Testing in the Flume Tank
Dye injection tests clearly showed that the relocated-forward cutting bar bifurcated the flow and
that flow stalled behind the pressure plate (Figure 6) in the majority of the tests. This was
evidenced when at Speeds of 2.0 knots, where rubber disks were lifted up after the cutting bar
and many stalled behind the cutting bar. The cutting bar and turtle chain plate formed a crude
wing, and laminar flow was observed to accelerate over the top of the cutting bar. The horizontal
turtle chain plate is located in a position that reduces drag created by the cutting bar. The turtle
chain plate in effect acts as a splitter plate reducing the drag coefficient of the vortex-stress
producing shape of the blunt cutting bar. Without the chain plate there was pulsating vortex
shedding. Over the top, the spinning was clockwise; underneath the cutting bar, the spinning was
counter-clockwise. The stagnant water behind the cutting bar travels along with the cutting bar
so the slip stream does not go up. When the die injector is located close to the tank floor the dye
stream shows some turbulence several centimeters beyond the cutting bar. It was not possible to
determine if the angle of the cutting bar had a significant impact on the flow patterns.
When the pressure plate width was increased from 22 cm to 30 cm, the space between the bottom
of the pressure plate and the top of the cutting bar was decreased to 16.5 cm. Flow was observed
to be directed downward after passing between the pressure plate and the cutting bar. This
occurred to the flow stream when it was encountering the cutting bar directly; the water flowed
over the cutting bar and then was directed downward. The combined flow creates increased
turbulence. Rubber disks, inserted at 2.0 knots, were lifted by the turbulence behind the cutting
bar. When the pressure plate width was decreased to 10 cm, the space between the cutting bar
and bottom of the pressure plate was increased to 36 cm. The flow over the cutting bar was
directed downward but at a point further aft than with the wider pressure plates. There were also
smooth undisturbed flow streams in the space between the cutting bar and pressure plate. The
pressure plate was not observed to disturb flow to any great extent when compared to the wider
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plates.
Tests 5, 7 and 10 presented major differences in water flow due to the modifications to the type
of cutting bar. In Test 5, a 7 cm diameter piece of solid PVC round stock was used as the cutting
bar without a turtle chain plate attached. Periodic vortex shedding took place, and this resulted in
generating lift several diameters behind the round cutting bar at speeds of 0.85 to 2.0 knots. In
Test 7, a 28 cm diameter “hat” was mounted between two struts at a 30-degree angle to the tank
floor above the cutting bar. At 1.0 knots, the hat directed water flow down as it went over the top
of the hat and behind the cutting bar, creating turbulence. When the speed was increased to 2.0
knots, rubber disks demonstrated significant lift behind the unit compared to the cutting bars.
In Test 10, configuration was most like a standard dredge. The 7.6 cm cutting bar was placed
under the frame and perpendicular to the flow, and the pressure plate was mounted in the
standard position. No struts were used. The flow behind the cutting bar was very turbulent. The
flow streams from the pressure plate influenced the flow from the cutting bar. Vortexes were
being shed off the top of the cutting bar and flowing upward. The flow around the pressure plate
indicated the same stall area behind the plate as in the previous test.
DISCUSSION
The initial modifications to dredge design resulted in a raised cutting bar height. To address this
in design 1, strut extensions were added every 30 cm and wrapped around the cutting bar. We
hypothesized this would help keep a turtle from being forced under the cutting bar so they were
named turtle guards. The cutting bar was turned 45 degrees to form a ramp. This was
implemented on all the experimental dredge designs tested to reduce the vertical flat surface,
thus providing a smoother path of escape in parallel to the assumed water flow over the top of
the dredge.
By moving the cutting bar forward 38 cm, dredge 1 maintained the same depressor plate angle
(45°) as in the unmodified New Bedford dredge. Moving the cutting bar this distance nearly
doubled the necessary shoe length to 80 cm. In order to maintain consistency within other
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elements of the dredge, the distance between the bullring (tow point) and the heel of the shoe
was kept the same by shortening the bale. Lengthening the dredge frame would either require
major changes to the vessel handling systems or shortening the bag, which would affect catch
retention.
Three significant issues arose from these changes. First, the longer shoes crushed more scallops
on deck. The long shoe design increased the amount of scallops being crushed by the dredge
frame during the dumping of the catch. The extent to which this happened depended upon a
number of variables, including sea state, crew experience, catch size, presence of turtle and/or
rock chains, and the dredge’s position on deck upon retrieval. During one trip, approximately ½
of a bushel of scallops was being crushed by Dredge 1 as compared to 0 – ¼ by the control
dredge. The precise amount crushed was estimated from the mix of trash and crushed scallops
being picked over and immediately discarded by the crew.
The relocation of the cutting bar also affected the space between the outer bale and the cutting
bar, reducing egress space. Video data indicates this as an opening where turtles may escape
(Milliken et al 2007; Smolowitz et al, 2010). In addition, the shorter distance between the tow
point and the cutting bar results in the cutting bar being farther off the sea floor. This higher
positioning of the cutting bar may increase the chance for a loggerhead to get run over by the
dredge.
Dredge designs 1-3 had a common problem with the turtle guards being entangled with the chain
bag and twine top. The entanglement often occurred during the beginning of the trip, when the
crews were learning how to fish the experimental dredge in varying environmental conditions
(e.g., tide and sea state). This malfunction occurred during 2% of all the paired tows. Since
entanglement likely happened at the start of the towing, not during the towing, these
unsuccessful tows were not included in the analysis. The authors felt the lack of any catch in
these entangled tows would unnecessarily bias the catch total calculations. Similarly, in
commercial fishing these entanglements represent lost time and money, and even if uncommon
they are considered a major detriment by fishers.
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Turtle guards were ultimately eliminated from the dredge design, not because of problems
derived from operator error, but rather a design flaw noted during the testing. The turtle guards
were unnecessarily worn during towing and were reduced by roughly half thickness after
approximately 100 tows. After 200 tows, the turtle guards broke off entirely, leaving jagged
edges. Broken turtle guards were not replaced, and resulted in both mutilated fish bycatch and
an increase in gear hang-ups. Once broken, catch results were not calculated and thus were no
longer included in the catch analysis.
By the time we incorporated modifications into dredge 3, it was very evident that the longer shoe
design, in addition to crushing more scallops, was not serving any purpose. The shoe design was
modified to end at the dredge box frame; this placed the heel of the shoe in the same position
relative to the cutting bar as in the standard dredge. The shorter shoe allowed the new bale to be
extended 38 cm, at which point it bends towards the tow point. This widened the bale to cutting
bar distance, allowing for increased potential loggerhead escapement. Theoretically, this larger
opening could conceivably even allow for larger, more mature loggerheads to escape the dredge.
Minor modifications to the chain bag were necessary to attach to the new frame; however the
overall volume of the chain bag remained unchanged. The impact of chain bag modifications on
catch was not analyzed.
Dredge 4 benefitted from the shortened shoe, and was further modified to include a change in the
bale bar from a "V" shape frame to a single tow bar attached to the centerline of the dredge. This
single point bar design reduced the number of bale bars, which have the potential to trap turtles,
from nine to one and reduced both weight and hydrodynamic drag. This simpler design,
however, required heavy construction to resist large bending moments when the dredge
encountered large boulders. The design can resist a bending moment of 200,000 Newton Meters.
Which is equal to a force of 133,333 Newtons at 2/3 the distance (1.5 m) out from the centerline.
The load was chosen to equal the potential maximum operating load of the typical scallop vessel
hauling system (winch, wire, and blocks). Although simple in concept, performance was poor.
The single tow bar held up to fishing with no sign of bending, however the dredge frame showed
some twisting. The twisting is likely due to the loss of torsional stiffness supplied by the Vshaped bale. This mechanical problem coupled with poor fishing performance resulted in this
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design being discarded.
To strengthen the outer bale bar in dredge 3 and 5, a second bar of equal diameter was attached
alongside it by a continuous weld in the horizontal axis. Adding a bar was more weight efficient
than just increasing the diameter of the tow bar because the material is added in the direction of
bending. The resulting strength was increased by approximately a factor of five in the x (towed
direction) and a factor of two in the y (vertical direction) as compared to a single outer bale. Not
only is this geometry efficient in strength, it decreases hydrodynamic drag. The drag coefficients
(at Reynolds number of approximately 105) for the single bar was 1.17 and for the two welded
bars was 0.7 (Hoerner 1965, pg. 8-1).
To further reduce the risk of turtles snagging on the frame, and to strengthen the frame, strut
spacing was reduced from 46 cm to 23 cm. Because the depressor plate is supported by these
struts, a reduction in spacing reduces the unsupported beam length, which increased the
depressor plate resistance to bending by 78% for a uniform load or 33% for a point load. The
added struts also eliminated the need for the reinforcing bar across the top of the depressor plate.
While impact on catch of the decreased strut spacing is not exactly known, we do know that the
flow at this section of the dredge is high enough that a scallop should pass easily into the bag,
even with the slightly increased strut surface area. This also reduces the possibility of a
loggerhead becoming lodged in the spaces between struts.
It should be noted that all tows with Dredge 5 were conducted during times and in areas of the
mid Atlantic where turtle interactions with scallop gear have historically been observed.
Unfortunately, these areas do not have the diversity or abundance of bycatch species found
further north on Georges Bank, so the bycatch totals for dredge 5 may be slightly misleading
relative to its predecessors (when viewed alone). Testing dredge 5 in a “higher turtle” area was a
conscious effort by the authors, knowing that dredge 5 was already benefitting from the positive
results and design changes of dredges 1-3, which showed significant reductions in fish bycatch.
Dredge 5, the Cfarm turtle excluder dredge, underwent additional successful testing with turtle
carcasses proving the effectiveness of the design in minimizing injuries to loggerheads
(Smolowitz et al, 2010).
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As noted earlier, seasonal and spatial variations in bycatch species catch rates, combined with an
extremely heterogeneous and mobile bottom-type makes exact comparison of one single dredge
design to another from different trips difficult. However, aggregating the results of the modified
dredge designs highlights the importance of these design changes in reducing bycatch, while
increasing scallop harvesting efficiency at the same time. A 46% reduction in yellowtail
flounder and a 69% reduction in winter flounder bycatch illustrate just how profoundly changes
to gear designs can impact exclusion measures. The exclusion potential of the modified dredge
is documented by video footage (Figure 7) of a yellowtail flounder attempting to escape a towed
dredge by swimming up rather than forward and away. We attribute the fish bycatch reduction
of this dredge design to the forward cutting bar. A fish encountering the cutting bar, similar to a
turtle encounter, has an opportunity to escape upwards that is not available when encountering a
cutting bar located under the depressor plate. An increase in deflective surfaces, mostly achieved
by the rotation of key components from 90 degrees to 45 degrees, allows uninterrupted egress in
the direction of the most likely route.
Flume tank tests also offered some insight into the slight increase in efficiency in scallop capture.
In the standard dredge, the pressure plate overhangs the cutting bar and seems to mitigate some
of the flow effects that help create lift behind the cutting bar. Moving the cutting bar ahead of the
pressure plate places the cutting bar into an area of undisturbed flow. In this environment more
lift seems to occur behind the cutting bar and this may partly be due to slightly higher flow
disturbances behind the rotated cutting bar.
The rotated orientation of the cutting bar results in slightly higher drag, since more area is
presented in the tow direction. This impact was moderated by welding the turtle chain
attachment plate at the centerline of the cutting bar. In this configuration, the plate acts as a
splitter plate to interfere with the motions of the vortex stream and may decrease the overall drag
coefficient. Energy lost to drag can be re-directed into stronger flow streams that create the lift
behind the cutting bar. In the forward position, the lift behind the cutting bar occurs under the
depressor plate, which seems to augment the upward flow. The flume tank tests also suggest an
advantage to widening the pressure plate in the forward cutting bar design, in that increased lift
is created behind the cutting bar.
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All of these investigations can and should be developed further, especially design efforts focused
on the relationship of the cutting bar to the depressor plate in developing a strong lifting stream.
The forward cutting bar and the depressor plate can provide a strong lifting system, just as the jib
and mainsail can provide strong forward momentum to a sailing vessel. There is also great room
for other improvements in the hydrodynamic characteristics of the dredge frame. The depressor
plate is of poor hydrodynamic design with lift to drag ratio of approximately one. This ratio can
easily be increased by changing the angle and lowering the vertical profile. For example,
changing the 45 degree angle of attack to 22.5 degrees gives a lift to drag ratio of 2.4 (Blevins,
1984), which should save fuel. Drag on this plate as now implemented at 4.5 knots is 2900 N.
The strut thickness can be reduced from 1.6 to 1.27 cm, and the leading and trailing edges can be
rounded to decrease weight and drag. The resulting low-profile dredge in the future may also
offer benefits related to reduced fish bycatch and less potential impact on turtles. In conclusion,
the Cfarm turtle excluder scallop dredge has improved efficiency on the target catch,
significantly reduces fish bycatch, and minimizes risk of injury to sea turtles. Indications are that
many additional improvements can be made to the basic design to improve catch, reduce
bycatch, and minimize the carbon footprint of the dredge fishery.
ACKNOWLEDGEMENTS
We would like to acknowledge all the captains and crews of the participating vessels, the people
in the welding shop at Dockside Repairs, Kenneth Doherty for his engineering expertise, and
Cliff Goudey and the people at the Memorial University for help with the flume tank testing.
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Literature Cited:
Blevins, R.D., 1984, Applied Fluid Dynamics Handbook, Van Nostrand Reinhold Co. Inc., New
York City, 568p.
Crowder, L.B., Crouse, D.T., Heppell, S.S., Martin, T.H. 1994, Predicting the Impact of
Turtle Excluder Devices on Loggerhead Sea Turtle Populations. Ecological Applications:
Vol. 4, No. 3, pp. 437-445.
DuPaul, William D., David B. Rudders, and Ronald J. Smolowitz. 2004. Industry Trials of a
Modified Sea Scallop Dredge to Minimize the Catch of Sea Turtles. VIMS Marine
Resource Report No. 2004-12. 31pp.
Henriksen, S., E. Welch, S. Therrien, R.J. Smolowitz, P.J. Struhsaker, C.A.Goudy, and H.
Kite-Powell. 1997. Results of Gear Modification Tests to Reduce Bycatches of
Commercial Finfish in Sea Scallop Dredges. August, 1997. Final Report, NOAA Award
No NA66FD0026, NMFS, NE Region Office, Gloucester, MA
Hoerner, S. F., 1965. Fluid-Dynamic Drag, Published by the author.
Kennelly, S. J. and Broadhurst, M. K., 2002, By-catch begone: changes in the philosophy of
fishing technology. Fish and Fisheries, 3: 340–355.
Limpus CJ, Limpus DJ (2003). The biology of the loggerhead turtle in western South Pacific
Ocean foraging areas. In: Bolten AB, Witherington BE (eds) Loggerhead sea turtles.
Smithsonian Books, Washington, DC, p 93–113
Milliken, H. O., L. Belskis, W. DuPaul, J. Gearhart, H. Haas, J. Mitchell, R. Smolowitz, and W.
Teas. April 2007. Evaluation of a Modified Scallop Dredge’s Ability to Reduce the
Likelihood of Damage to Loggerhead Sea Turtle Carcasses. Northeast Fisheries Science
Center Reference Document 07-07, Woods Hole, MA.
Murray, K.T. in press. Interactions between sea turtles and dredge gear in the U.S. sea scallop
(Placopecten magellanicus) fishery, 2001-2008. Fisheries Research.
Parsons, J.J., 1962. The Green Turtle and Man., University of Florida Press, Gainesville.
Pikitch, E. K., Santora, C., Babcock, E.A., Bakun, A., Bonfil, R., Conover, D.O. Dayton, P.,
Doukakis, P., Fluharty, D., Heneman, B., Houde, E.D., Link, J., Livingston, P.A.,
Mangel, M., McAllister, M.K., Pope, J., Sainsbury K.J., 2004, Science, Vol. 305. no.
5682, pp. 346 - 347.
Seidel, Wilber R. and Charles McVea, Jr. 1995. Development of a Sea Turtle Excluder Shrimp
Trawl for the Southeast U.S. Penaeid Shrimp Fishery. In Biology and Conservation of
Sea Turtles Revised Ed. Ed. Karen A. Bjorndal. Pp 497-502.
16
Smolowitz, R., H. Haas, H. O. Milliken, M. Weeks, and E. Matzen. 2010. Using Sea Turtle
Carcasses to Assess the Conservation Potential of a Turtle Excluder Dredge. North
American Journal of Fisheries Management 30:993-100.
Smolowitz, R. J. and M. Weeks. 2008. Field Testing of a New Dredge for the Sea Scallop
Fishery. Final Report NOAA Award NA07NMF4540029. Coonamessett Farm, East
Falmouth, MA 02536
Smolowitz, R.J., M. Weeks, and K. Bolles. 2007. The Design of a Turtle Excluder Dredge for
the Sea Scallop Fishery. Final Report NOAA Award NA05NMF4541293. Coonamessett
Farm, East Falmouth, MA 02536
Smolowitz, R. J., C. Harnish, and D. Rudders. 2005. Turtle-Scallop Dredge Interaction Study.
Project Report. Coonamessett Farm, Falmouth, MA 83 pp.
Smolowitz, R. J., D. Rutecki, P.J. Struhsaker, and W. DuPaul. 2004. Comparison of Ten Inch vs.
Six Inch Twine Tops to Reduce Discard of Bycatch In The Sea Scallop Fishery. 2003
Final Report. Coonamessett Farm, Falmouth, MA.
Smolowitz, R.J., P.J. Struhsaker, and W.D. DuPaul. 2002. An Experimental Fishery to test
fishing gear for bycatch reduction in the Sea Scallop fishery of Georges Bank.
Coonamessett Farm, Inc. East Falmouth, MA 02536.
Smolowitz, R.J., P.J. Struhsaker, and W.D. DuPaul. 2001. Dredge modifications to reduce
incidental groundfish catches in the Northwest Atlantic Sea Scallop fishery. NOAA
Award No. NA16FM1032. Coonamessett Farm, East Falmouth, MA 0253
Smolowitz, R. J. and F. M. Serchuk, 1988. Developments in sea scallop gear design. Proc
World Symposium on Fishing Gear and fishing vessel design. p531-540.
Troëng S, Rankin E (2005) Long-term conservation efforts contribute to positive green turtle
Chelonia mydas nesting trend at Tortuguero, Costa Rica. Biological Conservation
121:111-116.
Walsh, S. 2008. A Review of Current Studies on Scallop Rake Modifications to Reduce
Groundfish Bycatch in the Canadian Offshore Scallop Fishery on Georges Bank.
Research Document 2008/050, Canadian Science Advisory Secretariat. 87 pp.
Witherington, B., Kubilis, P., Brost, B., and A. Meylan. 2009. Decreasing annual nest counts in a
globally important loggerhead sea turtle population. Ecological Applications. 19:30—54.
Witzell, W.N., 1994. The origin, evolution and demise of the US sea turtle fisheries. Marine
Fisheries Review 56, pp. 8–23.
17
18
Table 1: Cruises, dates of tows, sample areas, and dredge frame tested for each cruise.
Vessel
Celtic
Celtic
Westport
Celtic
Westport
Resolution
Resolution
Resolution
Nordic Pride
Nordic Pride
Westport
Celtic
Freindship
Freindship
Freindship
Freindship
Freindship
Diligence
Diligence
Celtic
Westport
Kathy Ann
Tradition
Grand Larson
Elizabeth
Araho
Celtic
Generation
Kathy Ann
Generation
Kathy
Westport
Kathy Ann
Diligence
Tradition
Celtic
Diligence
ID
2006‐1
2006‐2
2006‐1
2006‐3
2006‐2
2006‐1
2006‐2
2006‐3
2007‐1
2007‐2
2007‐1
2007‐1
2007‐1
2007‐2
2007‐3
2007‐4
2007‐5
2007‐1
2007‐2
2007‐6
2007‐2
2008‐2
2008‐1
2008‐1
2008‐1
2009‐1
2009‐1
2009‐1
2009‐2
2009‐2
2009‐4
2009‐1
2009‐7
2009‐3
2009‐2
2009‐4
2009‐4
Date Sailed
5/19/2006
5/25/2006
7/31/2006
10/6/2006
9/14/2006
111712006
11/13/2006
12/9/2006
1/612007
2/9/2007
3/28/2007
4/10/2007
5/15/2007
6/5/2007
6/27/2007
6/5/2007
8/22/2007
9/20/2007
8/20/2007
11/5/2007
11/20/2007
8/6/2008
8/6/2008
8/19/2008
10/31/2008
6/4/2009
6/11/2009
6/17/2009
6/22/2009
7/8/2009
7/17/2009
8/25/2009
9/19/2009
9/30/2009
10/9/2009
10/13/2009
10/13/2009
Area
SNE open
SE part
CAII
CAII
CAII
NLSA
CAII
Northern Edge
Northern Edge
Northern Edge
ETAA
ETAA
HCAA
HCAA
HCAA
HCAA
ETAA
CAl
CAl
ETAA
ETAA
ETAA
ETAA
ETAA
ETAA
ETAA
ETAA
ETAA
ETAA
ETAA
ETAA
ETAA
ETAA
ETAA
Delmarva
Delmarva
Delmarva
Total Tows
11
218
27
114
162
25
91
186
252
295
68
32
100
184
161
116
42
88
93
109
100
107
92
63
60
111
106
38
118
41
203
130
239
127
159
118
152
19
Observed Tows
11
92
9
76
75
14
30
74
98
76
45
16
53
89
43
55
19
50
54
60
60
12
57
0
0
46
8
17
61
23
106
39
109
54
82
76
79
Dredge Frame #
1
1
1
2
2
4
4
2
2
2
3
3
3a
3
3
3
3
3
3
3
3
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
Table 2. A summary of flume tank model testing.
Test #
Cutting Bar
Pressure Plate Flume
1
Forward 45°
22 cm
0.85
yes
2
Forward 90°
22 cm
0.85/2.0
yes
3
Forward 90°
22 cm
2.0
None
4
Forward 45°
22 cm
0.85/2.0
None
5
Forward Round
22 cm
0.85/2.0
None
6
Forward 45°
22 cm
2.0
7
“Hat” design
22 cm
1.0/2.0
8
Forward 45°
30 cm
2.0
yes
9
Forward 45°
10 cm
2.0
yes
10
Standard 90°
22 cm
2.0
None
20
Turtle Chain
yes
None
Table 3. Individual dredge results organized by species are presented in this table. It is difficult
to compare results between experimental dredge designs due to a high degree of seasonal and
spatial variability that impact catch results. Pw is the probability using the Wilcoxon test statistic
while Pt is the probability using the Paired T-test.
Scallop (bu)
N
Total Count
Pw
Pt
% Difference from control
Little Skate
N
Total Count
Pw
Pt
% Difference from control
Monkfish
N
Total Count
Pw
Pt
% Difference from control
Fluke
N
Total Count
Pw
Pt
% Difference from control
Yellowtail Fld.
N
Total Count
Pw
Pt
% Difference from control
Fourspot Fld.
N
Total Count
Pw
Pt
% Difference from control
Sand Dab
Dredge 1 Dredge 2 Dredge 3 Dredge 4 Dredge 5
114
404
385
1330 3945.25
3544
0.000
0.007
0.000
0.000
0.001
0.000
10.65% -4.09% 11.18%
43
872
778.5 8972.41
0.170
0.002
0.087
0.004
-6.71% 2.03%
N
Total Count
Pw
Pt
% Difference from control
Winter Fld.
Dredge 1 Dredge 2 Dredge 3 Dredge 4 Dredge 5
114
404
385
43
5577
20177
23515
1861
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
-11.77% -13.28% 10.45% -25.77%
872
16662
0.000
0.000
9.20%
N
Total Count
Pw
Pt
% Difference from control
Barndoor Skate
Dredge 1 Dredge 2 Dredge 3 Dredge 4 Dredge 5
114
2026
0.724
0.292
-2.13%
404
2966
0.945
0.414
-0.80%
385
1562
0.165
0.110
4.69%
43
872
504
724
0.911
0.063
0.500
0.026
0.00% 11.38%
N
Total Count
Pw
Pt
% Difference from control
Winter Skate
Dredge 1 Dredge 2 Dredge 3 Dredge 4 Dredge 5
114
404
53
74
0.260
0.156
0.216
0.063
-15.87% -22.11%
385
43
872
516
33
567
0.974
0.023
0.006
0.433
0.007
0.001
-1.15% -42.11% -16.12%
N
Total Count
Pw
Pt
% Difference from control
Grey Sole
Dredge 1 Dredge 2 Dredge 3 Dredge 4 Dredge 5
114
404
385
43
231
3107
189
965
0.499
0.004
0.000
0.003
0.050
0.002
0.000
0.046
-28.48% -48.19% -15.25% -15.35%
N
Total Count
Pw
Pt
% Difference from control
Dredge 1 Dredge 2 Dredge 3 Dredge 4 Dredge 5
114
404
403
596
0.088
0.000
0.054
0.000
-13.33% -43.35%
385
557
0.501
0.458
-0.71%
872
390
0.143
0.1937
7.14%
21
Dredge 1 Dredge 2 Dredge 3 Dredge 4 Dredge 5
114
404
385
91
464
90
0.014
0.008
0.000
0.006
0.000
0.005
-38.10% -43.76% -31.82%
Dredge 1 Dredge 2 Dredge 3 Dredge 4 Dredge 5
114
404
8
13
0.016
0.000
0.005
0.000
-63.64% -71.74%
Dredge 1 Dredge 2 Dredge 3 Dredge 4 Dredge 5
114
404
507
272
0.916
0.000
0.373
0.000
-2.12% -37.33%
Dredge 1 Dredge 2 Dredge 3 Dredge 4 Dredge 5
114
404
145
451
0.578
0.000
0.335
0.002
5.84% -26.31%
Dredge 1 Dredge 2 Dredge 3 Dredge 4 Dredge 5
114
404
385
374
15
242
0.376
0.009
0.005
0.192
0.003
0.003
-6.50% -54.55% 39.88%
Table 4: This table provides a catch summary for experimental dredges 1, 2, 3, & 5. These dredges were similar
in that they all had the forward positioned cutting bar which is the most significant change from the standard
dredge that was used as the control. Of particular interest is the significant decrease in key flounder species. Pw
is the probability using the Wilcoxon test statistic while Pt is the probability using the Paired T-test
Dredges 1,2,3,5
Scallop
(bu.)
Skate
Summer
Monkfish Flounder
Grey
Sole
Yellowtail Winter Barndoor
Flounder Flounder
Skate
Winter
Skate
Sand
Dab
American
Plaice
Fourspot
Flounder
Number of paired tows
1632
1632
1632
1632
1632
1632
1632
1632
1632
1632
1632
1632
Experimental catch
16786
65864
7230
1205
631
3513
21
777
595
555
208
1946
Control catch
16345
66074
7148
1347
606
6521
67
948
744
1062
97
2436
% Difference from Control
Pw
Pt
2.7%
0.000
-0.3%
na
1.1%
0.057
-10.5%
0.007
4.1%
0.416
-46.1%
0.000
-68.7%
0.000
-18.0%
0.001
-20.0%
0.002
-47.7%
0.000
114.4%
0.000
-20.1%
0.000
0.000
0.401
0.295
0.004
0.265
0.000
0.000
0.000
0.006
0.000
0.000
0.000
22
LIST OF FIGURES
Figure 1. Figure adapted from Milliken et al (2007), illustrating a turtle carcass being run over by a scallop
dredge. The dredge shown is an earlier prototype of the turtle excluder dredge. The experiment both
highlighted the advantages to reducing the bale bars and pointed out the error in this design that lead to the outer
bale extension and removal of the turtle guards.
Figure 2. An illustration of the standard New Bedford Style Scallop dredge (bottom) and the progression of
dredge designs tested during this study.
Figure 3. This picture is of the dredge test frame design used in the flume tank studies. The major dredge
components are highlighted for reference.
Figure 4. A map of the scallop harvesting areas where comparative tows took place demonstrates the range of
bottom types encountered. USGS sediment data taken from: http://pubs.usgs.gov/of/2000/of00-358/.
Figure 5: Rubber disks were used to simulate scallops in the flume tank tests. Lift created by the flow patterns
generated by the cutting bar raised the disks off the bottom of the flume tank. This action improves the chance
of capture by the dredge bag that follows behind the cutting bar.
Figure 6: Dye injection tests in flume tank illustrating hydrodynamic flow around the depressor plate. Water
flow hits the plate and is ramped up and over the dredge.
Figure 7: A yellowtail flounder is shown escaping the turtle excluder dredge by swimming up and over the
frame. In a standard dredge the depressor plate would have blocked this escape route.
23
Figure 1. Figure adapted from Milliken et al (2007), illustrating how a turtle may get overcome by a scallop
dredge and run over. The dredge shown is an earlier prototype of the turtle excluder dredge, and the turtle is a
turtle carcass (and therefore unable to escape). The experiment both highlighted the advantages to reducing the
bale bars, and pointed out the error in this design with the extended cutting bar and turtle guards.
24
Figure 2. A standard New Bedford Style Scallop dredge (bottom) and the progression of dredge designs.
25
Figure 3. Experimental dredge frame design used in the flume tank tests. The major components are highlighted for
reference.
Frame
Pressure plate
Skirt bar
Struts (9” on center)
Cutting bar rotated
to 45 degrees
Turtle chain plate
26
Figure 4. Map scallop harvesting regions where comparative tows took place. USGS sediment data taken from:
http://pubs.usgs.gov/of/2000/of00-358/.
27
Figure 5: Rubber disks were used to simulate scallops in the flume tank tests.
28
Figure 6: Dye injection tests in flume tank illustrating hydrodynamic flow around the depressor plate. Water
flow hits the plate and is ramped up and over the dredge.
29
Figure 7: A yellow tail flounder show escaping the turtle excluder dredge.
30