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



675 



Annual bycatch was the sum of the stratified bycatch 

 estimates. The finite population correction factor (Co- 

 chran, 1977) was applied to bycatch estimates in stratas 

 where the observer coverage was greater than 10%. 



Number of dredge hauls in the VTR database without 

 coordinate positions (32%) were prorated between the 

 stratified areas according to the percentage of dredge 

 hauls with known coordinates from the same year, 

 state, and stratified areas. 



Results 



Observed bycatch 



Nine and 16 turtle bycatch were observed in 2001 and 

 2002, respectively, in the Hudson Canyon controlled 

 access area. Of the 25 turtles taken in the Hudson 

 Canyon area across both years, 21 (84%) were taken 

 during summer months. Two turtle bycatch were 

 observed in the Virginia Beach access area during fall 

 2001 — the only time when there was observer coverage 

 in this area across both years. 



GAM smoothers 



Plots of the smoothed functions in the GAM revealed 

 whether the continuous variable in the model explained 

 any error in the bycatch rate estimates. For example, 

 a plot of the smooth function for depth as a covariate 

 revealed that bycatch rates may be higher between 49 

 m (27 fm) and 57 m (31 fm) and lower around this zone 

 (Fig. 2). Likewise, a plot of the smooth function for tem- 

 perature as a covariate revealed that bycatch rates may 



be higher above 19°C. These plots helped bin the continu- 

 ous variables into categories (Appendix 1) which could 

 then be tested in the GLM. All continuous variables in 

 the GAM were categorized in a similar manner. 



GLM bycatch model 



Significant factors affecting sea turtle bycatch were 

 season, sea surface temperature, depth zone, and time- 

 of-day (Table 1). These variables were significant despite 

 the order in which they were tested in the model. The 

 model with the lowest AIC value was considered the 

 "best" model, although time-of-day could not be included 

 in the final model to predict bycatch rates. This level of 

 information is not recorded in commercial fisheries log- 

 books; therefore bycatch rates based on time-of-day could 

 not be extrapolated to total bycatch. Width of the scallop 

 dredge frame, number of tickler chains, and number of 

 up and down chains were not significant variables. 



Model fit 



The number of predicted sea turtle bycatch closely 

 matched the observed bycatch in both years in all 

 bycatch strata (Table 2). Strata were defined according 

 to variables identified in the GLM as having a significant 

 effect on bycatch rates. The relationship between actual 

 and observed takes was strong (r 2 =0.93), indicating that 

 the predictions from the model fitted the data well. 



Bycatch rate estimates 



Bycatch rates were stratified by season, temperature inter- 

 val, and depth zone (Table 3). Because year was not a 



