Murray: Magnitude and distribution of sea turtle bycatch in the sea scallop dredge fishery 



679 



closures (Morreale, 1996). The results of this analy- 

 sis indicate that bycatch rates are affected by season, 

 depth, and sea surface temperature. Within certain 

 months and depth zones, therefore, the time when sea 

 surface temperature reaches a threshold level may be 

 the time to trigger an area closure. For example, this 

 type of management approach has been taken in the 

 southeastern United States to regulate turtle bycatch 

 in the large-mesh gill-net fishery. 3 The timing of sea- 

 sonally adjusted area closures is based upon analyzing 

 sea surface temperatures in relation to the presence 

 or absence of sea turtles throughout the area (Epperly 

 et al., 1995; Epperly and Braun-McNeill 4 ). In addition, 

 temperature thresholds currently trigger area closures 

 in the southern California driftnet fishery during El 

 Nino conditions to prevent the incidental capture of log- 

 gerhead sea turtles. 5 



Results from the present study can be used to help 

 evaluate potential bycatch reduction under different 

 management scenarios, given certain assumptions. For 

 example, had the portion of the Hudson Canyon con- 

 trolled access area between depths of 49 and 57 m been 

 closed after surface waters reached 19°C in the summer 

 (the stratum with highest bycatch), the closure would 

 have reduced bycatch by 39%. For this estimate, it is 

 assumed that surface temperatures remain above 19°C 

 throughout the summer and drop below 19°C thereafter. 

 Further, this bycatch reduction scenario also assumes 

 that fishing effort shifts proportionately to the fall and 

 spring season within the same depth zone and that 

 bycatch rates remain the same as those that are cal- 

 culated. Alternatively, fishing effort could shift within 

 a season to shallow and deep depth zones if scallop 

 catch-per-unit-of-effort were not affected. Under this 

 assumption, bycatch would be reduced by 60'"< under 

 the same time and area closure. However, unless there 

 are concurrent reductions in fishing effort, bycatch 

 reductions achieved by these measures could well be 

 offset by increases in bycatch in other depth strata 

 and seasons. 



Gear or fishing modifications Management actions to 

 modify gear or fishing practices can be evaluated in a 

 similar manner. For instance, this analysis indicates 

 that bycatch rates are influenced by the time-of-day 

 when dredges are in the water. Time-of-day was not used 

 to stratify bycatch rates or to extrapolate total bycatch 

 estimates because of limitations in the fishing effort 

 data ( VTR records). If time-of-day had been incorporated 

 into the bycatch model, the model would have predicted 

 higher bycatch rates when dredges were set between 4 

 am and 4 pm (day tows). If the stratum with the highest 

 bycatch rate (summer, high surface temperatures, and 



depths between 49 and 57 m), had been further strati- 

 fied by time-of-day, the model would have predicted a 

 bycatch rate of 0.008 sea turtles/dredge hauls during the 

 day, and 0.002 turtles/dredge hauls during the night. If 

 all the commercial vessels had been fishing during the 

 day in this stratum (;? = 6352 dredges in 2001), the esti- 

 mated bycatch would have been 51 turtles. If the vessels 

 had been fishing during the night, the total estimated 

 bycatch would have been 13 turtles. According to these 

 rates and effort, restricting vessels to night-time tows 

 between the hours of 4 pm and 4 am has the potential to 

 reduce bycatch by 75% in this particular stratum. 



Although specific gear characteristics did not show 

 a strong relationship to sea turtle bycatch in this 

 analysis, further work should be conducted to evaluate 

 whether specific gear characteristics could be modified 

 to decrease bycatch. For example, the near significance 

 with the model incorporating number of tickler chains 

 (P=0.07) warrants further testing of this gear charac- 

 teristic. Tickler chains cover the mouth of the dredge in 

 a grid-like configuration with the vertical up and down 

 chains. The number of chains on the bag and distance 

 between the chains may help to prevent sea turtles from 

 entering the dredge bag. This dredge configuration is 

 currently being tested for sea turtle bycatch reduction 

 in the Hudson Canyon area (DuPaul and Smolowitz 6 ). 

 Further research should also examine the behavior 

 of sea turtles in relation to dredge gear for a more 

 complete understanding of how and when turtles are 

 entrapped. 



Sea turtles and scallop dredge interactions cannot be 

 viewed in isolation from other gear types and conserva- 

 tion measures. Some fisheries that co-occur with sea 

 turtles may have an equal, if not greater, impact on 

 turtles than do scallop dredges (e.g., the shrimp trawl 

 fishery in the Gulf of Mexico (Henwood and Stuntz, 

 1987]). Changes in sea turtle abundance, or shifts in 

 fishing effort, may increase the likelihood of encounters 

 in both net and dredge fisheries. If environmental condi- 

 tions associated with high bycatch rates in the Hudson 

 Canyon and Virginia Beach areas are consistent across 

 years, it may be possible to anticipate and deter future 

 interactions from occurring. 



Acknowledgments 



I would like to thank Debra Palka and Marjorie Ross- 

 man for help with analytical and statistical approaches 

 to bycatch estimation. Andy Solow and Andy Beet at the 

 Marine Policy Center, Woods Hole Oceanographic Insti- 

 tute, also provided guidance in the statistical analysis. 

 David Mountain provided invaluable help in acquiring 



3 Final Rule. FR 67: 71895-71900. 3 December 2002. 



4 Epperly, S. P. and J. Braun-McNeill. 2002. Unpubl. data. 

 The use of AVHRR Imagery and the management of sea turtle 

 interactions in the Mid-Atlantic Bight. NMFS Southeast 

 Fisheries Science Center, Miami, Florida, 33149. 



5 Final Rule, FR 68: 69962-69967, 16 December 2003. 



6 DuPaul. W. P., and R. Smolowitz. 2003. Unpubl. data. 

 Industry trials of a modified sea scallop dredge to minimize 

 the catch of sea turtles. Virginia Institute of Marine Sci- 

 ence, Gloucester Point, Virginia, 23062, and Coonamessett 

 Farm, East Falmouth, Massachusetts, 02536. 



