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Fishery Bulletin 103(4) 



tended to be lower over the Grand Banks than off the 

 U.S. east coast and the mean ratio of immature blue 

 shark tended to be higher. 



Discussion 



Our results indicate that blue shark tolerance to the 

 stresses associated with longline capture decreases with 

 animal size at levels that vary with set duration. These 

 results are consistent with the findings of Neilson et al. 

 (1989) and Milliken et al. (1999) who observed greater 

 discard mortality among the smallest sizes classes of 



Figure 2 



(A) Observed proportions of blue shark discarded alive 

 (ra = 37) for each fork-length set duration combination; 

 and (B) predicted response surface. 



Iongline-caught Atlantic halibut (Hippoglossus hippo- 

 glossus) and cod (Gadus morhua), respectively. In our 

 study, set duration represented the maximum possible 

 time a given fish was "on-hook," and thus was only the 

 coarsest of measures of the magnitude and duration of 

 stress experienced by hooked fishes. Nevertheless, this 

 crude measure appears to have captured enough of the 

 cumulative stress effects on fish survival to emerge as 

 a significant factor. In contrast, water temperature did 

 not emerge as important in our analysis. However, we 

 suspect this resulted because surface water tempera- 

 tures (the only temperature measurements available) are 

 poor indicators of the levels and changes in temperature 

 actually experienced by captured sharks. Presumably, 

 better predictions of condition at boat-side (and thus 

 live discard quantities) could be made with knowledge 

 of time-on-hook, depth, and temperature of capture, 

 rate of gear retrieval, sea conditions, etc. Unfortunately, 

 many of the measurements that are likely most relevant 

 to recording shark condition at boat-side can only be 

 made by distributing and retrieving large quantities of 

 electronic instruments (i.e., temperature-depth recorders 

 and hook-timers, see Boggs, 1992) near the hooks, and 

 for each set. Such an approach is not only costly, but also 

 difficult to implement without a research team dedicated 

 for this purpose. Similarly, only by directed research 

 can questions of postrelease mortality be addressed. 

 Clearly, the proportions of living blue shark considered 

 in our study are minimum estimates of fishing impacts 

 because they do not account for delayed mortality of 

 individuals released injured or otherwise impaired. For 

 gauging postrelease mortality of Iongline-caught blue 

 shark, tagging studies are warranted (Neilson et al., 

 1989; Kohler et al., 2002). 



Evident in the maps is that the proportion of blue 

 sharks available for live release was not homogeneous 

 throughout the spatial range examined. Overall the 

 proportion of blue shark released alive was higher (0.78) 

 along the U.S. Atlantic east coast and decreased over the 

 Grand Banks (0.67) (Fig. 3A). The maps also indicated 

 that overall the proportion of immature blue sharks was 

 highest over the Grand Banks (0.93) compared to the 

 U.S. Atlantic east coast (0.63) (Fig. 3B). In their exami- 

 nation of U.S. Atlantic east coast longline catches south 

 of the present study (i.e., between 35° and 22°N latitude), 

 Beerkircher et al. (2004) found that 0.87 of blue shark 

 caught were alive at boat-side. It seems likely, therefore, 

 that contributing to the relatively higher survival ob- 

 served by Beerkircher et al. (2004) was that only about 

 half of the blue shark in their analysis were immature 

 (as inferred from size). Blue shark interactions over the 

 Grand Banks deserve special attention because most in- 

 dividuals discarded by the U.S. pelagic longline fleet are 

 captured in that area. In 2002, for example, two thirds 

 of the estimated 4335 blue shark mortalities attributed 

 to U.S. Atlantic pelagic longline fleet were captured in 

 this area (Diaz, unpubl. data 3 ). 



Diaz, G. A. 2005. NMFS Pelagic longline logbook pro- 

 gram. NMFS/SEFSC Miami, FL 33149. 



