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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: cfarm@capecod.net

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 V-

shaped 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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Table 1: Cruises, dates of tows, sample areas, and dredge frame tested for each cruise. Vessel ID Date Sailed Area Total Tows Observed Tows Dredge Frame #

Celtic 2006‐1 5/19/2006 SNE open 11 11 1

Celtic 2006‐2 5/25/2006 SE part 218 92 1

Westport 2006‐1 7/31/2006 CAII 27 9 1

Celtic 2006‐3 10/6/2006 CAII 114 76 2

Westport 2006‐2 9/14/2006 CAII 162 75 2

Resolution 2006‐1 111712006 NLSA 25 14 4

Resolution 2006‐2 11/13/2006 CAII 91 30 4

Resolution 2006‐3 12/9/2006 Northern Edge 186 74 2

Nordic Pride 2007‐1 1/612007 Northern Edge 252 98 2

Nordic Pride 2007‐2 2/9/2007 Northern Edge 295 76 2

Westport 2007‐1 3/28/2007 ETAA 68 45 3

Celtic 2007‐1 4/10/2007 ETAA 32 16 3

Freindship 2007‐1 5/15/2007 HCAA 100 53 3a

Freindship 2007‐2 6/5/2007 HCAA 184 89 3

Freindship 2007‐3 6/27/2007 HCAA 161 43 3

Freindship 2007‐4 6/5/2007 HCAA 116 55 3

Freindship 2007‐5 8/22/2007 ETAA 42 19 3

Diligence 2007‐1 9/20/2007 CAl 88 50 3

Diligence 2007‐2 8/20/2007 CAl 93 54 3

Celtic 2007‐6 11/5/2007 ETAA 109 60 3

Westport 2007‐2 11/20/2007 ETAA 100 60 3

Kathy Ann 2008‐2 8/6/2008 ETAA 107 12 5

Tradition 2008‐1 8/6/2008 ETAA 92 57 5

Grand Larson 2008‐1 8/19/2008 ETAA 63 0 5

Elizabeth 2008‐1 10/31/2008 ETAA 60 0 5

Araho 2009‐1 6/4/2009 ETAA 111 46 5

Celtic 2009‐1 6/11/2009 ETAA 106 8 5

Generation 2009‐1 6/17/2009 ETAA 38 17 5

Kathy Ann 2009‐2 6/22/2009 ETAA 118 61 5

Generation 2009‐2 7/8/2009 ETAA 41 23 5

Kathy 2009‐4 7/17/2009 ETAA 203 106 5

Westport 2009‐1 8/25/2009 ETAA 130 39 5

Kathy Ann 2009‐7 9/19/2009 ETAA 239 109 5

Diligence 2009‐3 9/30/2009 ETAA 127 54 5

Tradition 2009‐2 10/9/2009 Delmarva 159 82 5

Celtic 2009‐4 10/13/2009 Delmarva 118 76 5

Diligence 2009‐4 10/13/2009 Delmarva 152 79 5

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Table 2. A summary of flume tank model testing. Test # Cutting Bar Pressure Plate Flume Turtle Chain 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 yes 7 “Hat” design 22 cm 1.0/2.0 None 8 Forward 45° 30 cm 2.0 yes 9 Forward 45° 10 cm 2.0 yes 10 Standard 90° 22 cm 2.0 None

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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) Dredge 1 Dredge 2 Dredge 3 Dredge 4 Dredge 5 Sand Dab Dredge 1 Dredge 2 Dredge 3 Dredge 4 Dredge 5

N 114 404 385 43 872 N 114 404 385Total Count 1330 3945.25 3544 778.5 8972.41 Total Count 91 464 90Pw 0.000 0.007 0.000 0.170 0.002 Pw 0.008 0.000 0.014Pt 0.000 0.001 0.000 0.087 0.004 Pt 0.006 0.000 0.005% Difference from control 10.65% -4.09% 11.18% -6.71% 2.03% % Difference from control -38.10% -43.76% -31.82%

Little Skate Dredge 1 Dredge 2 Dredge 3 Dredge 4 Dredge 5 Winter Fld. Dredge 1 Dredge 2 Dredge 3 Dredge 4 Dredge 5

N 114 404 385 43 872 N 114 404Total Count 5577 20177 23515 1861 16662 Total Count 8 13Pw 0.000 0.000 0.000 0.000 0.000 Pw 0.016 0.000Pt 0.000 0.000 0.000 0.000 0.000 Pt 0.005 0.000% Difference from control -11.77% -13.28% 10.45% -25.77% 9.20% % Difference from control -63.64% -71.74%

Monkfish Dredge 1 Dredge 2 Dredge 3 Dredge 4 Dredge 5 Barndoor Skate Dredge 1 Dredge 2 Dredge 3 Dredge 4 Dredge 5

N 114 404 385 43 872 N 114 404Total Count 2026 2966 1562 504 724 Total Count 507 272Pw 0.724 0.945 0.165 0.911 0.063 Pw 0.916 0.000Pt 0.292 0.414 0.110 0.500 0.026 Pt 0.373 0.000% Difference from control -2.13% -0.80% 4.69% 0.00% 11.38% % Difference from control -2.12% -37.33%

Fluke Dredge 1 Dredge 2 Dredge 3 Dredge 4 Dredge 5 Winter Skate Dredge 1 Dredge 2 Dredge 3 Dredge 4 Dredge 5

N 114 404 385 43 872 N 114 404Total Count 53 74 516 33 567 Total Count 145 451Pw 0.260 0.156 0.974 0.023 0.006 Pw 0.578 0.000Pt 0.216 0.063 0.433 0.007 0.001 Pt 0.335 0.002% Difference from control -15.87% -22.11% -1.15% -42.11% -16.12% % Difference from control 5.84% -26.31%

Yellowtail Fld. Dredge 1 Dredge 2 Dredge 3 Dredge 4 Dredge 5 Grey Sole Dredge 1 Dredge 2 Dredge 3 Dredge 4 Dredge 5

N 114 404 385 43 N 114 404 385Total Count 231 3107 189 965 Total Count 374 15 242Pw 0.004 0.000 0.499 0.003 Pw 0.376 0.009 0.005Pt 0.002 0.000 0.050 0.046 Pt 0.192 0.003 0.003% Difference from control -28.48% -48.19% -15.25% -15.35% % Difference from control -6.50% -54.55% 39.88%

Fourspot Fld. Dredge 1 Dredge 2 Dredge 3 Dredge 4 Dredge 5

N 114 404 385 872Total Count 403 596 557 390Pw 0.088 0.000 0.501 0.143Pt 0.054 0.000 0.458 0.1937% Difference from control -13.33% -43.35% -0.71% 7.14%

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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 1,2,3,5Scallop

(bu.) Skate MonkfishSummer Flounder

Grey Sole

Yellowtail Flounder

Winter Flounder

Barndoor 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 2.7% -0.3% 1.1% -10.5% 4.1% -46.1% -68.7% -18.0% -20.0% -47.7% 114.4% -20.1%

P w 0.000 na 0.057 0.007 0.416 0.000 0.000 0.001 0.002 0.000 0.000 0.000

P t 0.000 0.401 0.295 0.004 0.265 0.000 0.000 0.000 0.006 0.000 0.000 0.000

23

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.

24

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.

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Figure 2. A standard New Bedford Style Scallop dredge (bottom) and the progression of dredge designs.

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Figure 3. Experimental dredge frame design used in the flume tank tests. The major components are highlighted for reference.

Struts (9” on center)

Skirt bar

Pressure plate Frame

Turtle chain plate Cutting bar rotated to 45 degrees

27

Figure 4. Map scallop harvesting regions where comparative tows took place. USGS sediment data taken from:

http://pubs.usgs.gov/of/2000/of00-358/.

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Figure 5: Rubber disks were used to simulate scallops in the flume tank tests.

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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.

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Figure 7: A yellow tail flounder show escaping the turtle excluder dredge.