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Appendix 1.
Daily activity summaries and observations (PDF)
Appendix 2.
Pre- and post-test documentation (PDF)

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

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
2 National Marine Fisheries Serv, Southeast Fisheries Science Cent, 75 Virginia Beach Dr, Miami FL 33149-1003
3 Virginia Institute of Marine Science, Gloucester Point VA 23062
4 National Marine Fisheries Serv, Southeast Fisheries Science Center, 705 Convent Ave, Pascagoula MS 39567
5 Coonamessett Farm, 277 Hatchville Rd, East Falmouth, MA 02536

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.

Information Quality Act Compliance: In accordance with section 515 of Public Law 106-554, the Northeast Fisheries Science Center completed both technical and policy reviews for this report. These predissemination reviews are on file at the NEFSC Editorial Office.

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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.


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.


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.


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).


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:

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.


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.
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