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Fishery Bulletin 103(1) 



1987). These mortalities appear to have occurred too 

 soon to have been caused by infection (Bourke et al., 

 1987) and too late to have been caused by lactic aci- 

 dosis. Postexertion recovery in istiophorid billfishes is 

 poorly studied, but Skomal and Chase (2002) reported 

 significant perturbations in blood chemistry, includ- 

 ing elevation in blood Cortisol levels in bluefin tuna 

 iThunnus thynnus), yellowfin tuna (Thunnus albacares), 

 and white marlin exposed to prolonged angling bouts 

 (mean=46 min). Acoustic tracks of these animals re- 

 vealed recovery periods characterized by limited diving 

 behavior for two hours or less after release. The death 

 of white marlin DR02-04 after apparent recovery (Fig. 

 3C) may be the result of natural mortality, another 

 capture event, or delayed mortality associated with 

 release from recreational fishing gear. Mortality associ- 

 ated with the trauma induced by retained fishing hooks 

 need not be immediate. Blue sharks with fishing hooks 

 embedded in the esophagus or perforating the gastric 

 wall have been found to experience systemic debilitat- 

 ing disease that may affect survival over longer time 

 intervals (Borucinska et al., 2001, 2002). 



We also cannot discount predation as a possible cause 

 of mortality for any of the white marlin that died in our 

 study. Acoustic tagging studies have described preda- 

 tion on tagged and released sailfish (Jolley and Irby, 

 1979), blue marlin (Block et al., 1992) and black marlin 

 (Pepperell and Davis, 1999) by sharks. Recently, Ker- 

 stetter et al. (2004) observed results consistent with 

 scavenging and predation on PSAT-tagged white marlin 

 and opah (Lampris guttatus) by sharks. Both Block et 

 al. (1992) and Kerstetter et al. (2004) documented at- 

 tacks on tagged marlin that exhibited prolonged surface 

 associations — the same pattern shown by DR02-04 im- 

 mediately following its release and prior to mortality. 



One tag (MA04) in our study failed to transmit data 

 and was eliminated from all analyses. In previous PSAT 

 studies demonstrating billfish survival, mortalities of 

 tagged istiophorids were not directly observed (Graves 

 et al., 2002; Kerstetter et al., 2003), and the authors 

 conservatively regarded nonreporting tags to be evidence 

 of mortality. The early tag models used in these stud- 

 ies may have failed to transmit data because moribund 

 animals were located at depths that exceeded the toler- 

 ance limit (650 m) of the tags or because of other factors, 

 including tag malfunction, mechanical damage (Graves 

 et al., 2002; Kerstetter et al., 2003) or tag ingestion 

 (Kerstetter et al., 2004), or a combination of these fac- 

 tors. Other authors, using newer models of PSATs rated 

 to withstand pressure equivalent to a depth of 3000 m, 

 have clearly documented several mortalities and have 

 chosen to eliminate nonreporting tags from their analy- 

 ses (Domeier et al, 2003, present study). Treating nonre- 

 porting tags as mortalities will bias mortality estimates 

 upwards if tags fail to report for reasons other than 

 catch-and-release-induced mortality (Goodyear, 2002). 



Relatively small sample sizes and fairly limited spa- 

 tial coverage in the present study precluded the use of 

 these data to infer Atlantic-wide estimates of postrelease 

 mortality rates for white marlin. Given the need to ac- 



count for geographical differences in body sizes of white 

 marlin, fishing gears, drop-back durations, angler skill 

 level, habitat variables, predator densities, and loca- 

 tions, the sample size needed to generate an accurate 

 estimate of postrelease mortality for the entire Atlantic 

 recreational sportfishery could easily require more than 

 a thousand tags (Goodyear, 2002). Results of simulated 

 experiments suggest that if the true underlying J-hook 

 mortality rate is 35%, more than 200 PSATs would have 

 to be deployed on white marlin caught on this terminal 

 tackle to reduce the 95% confidence intervals to ±5% of 

 the true value. The cost of such an experiment (~$1 mil- 

 lion for tags alone) is presently prohibitive, particularly 

 considering that these estimates are derived under the 

 assumption of ideal conditions (no premature releases, 

 no tag-induced mortality, and no natural mortality) 

 (Goodyear, 2002). The presence of any confounding fac- 

 tors would increase the necessary sample size and the 

 total cost of such an experiment (Goodyear, 2002). 



Despite a relatively small sample size, the present 

 study clearly demonstrates the importance of hook type 

 for the postrelease survival of white marlin. Our results 

 indicate that a highly significant proportion of released 

 white marlin caught on straight-shank ("J") hooks per- 

 ish and that these hooks are significantly more likely 

 to hook fish deeply and cause internal damage. In con- 

 trast, the survival rate of all white marlin caught on 

 circle hooks indicates that a simple change in terminal 

 tackle can significantly reduce postrelease fishing mor- 

 tality in the recreational fishery. 



Acknowledgments 



The authors would like to thank Captains Mike Adkins 

 (South Jersey Champion), O. B. O'Bryan (Sea-D), Jimmy 

 Grant (Vintage), Gene Hawn (Ocean Fifty Seven), Ryan 

 Higgins (Caliente), Ken Neill (Healthy Grin), Steve Rich- 

 ardson (Backlash), Rod Ryan (White Witch), and Rom 

 Whittaker (Release), as well as their crews, for their skill 

 in finding white marlin and for their patience with us as 

 we deployed PSATs. We thank Phil Goodyear for kindly 

 providing the bootstrapping software for simulations, Eric 

 Prince (NMPS) for suggestions regarding bait rigging 

 techniques, Paul Howey and Lissa Werbos (Microwave 

 Telemetry, Inc) for technical assistance with the tags, 

 Lorraine Brasseur (VIMS) for assistance with MATLAB 

 programming, Robert Diaz (VIMS) for advice with sta- 

 tistical methods, and David Kerstetter (VIMS) for helpful 

 comments on this manuscript. We gratefully acknowledge 

 the logistical support of Guy Harvey, Dick Weber, and 

 John Wendkos. This project was funded by the National 

 Marine Fisheries Service and Marine Ventures, Inc. 



Literature cited 



Agresti, A. 



1990. Categorical data analysis, 558 p. John Wiley and 

 Sons, Inc., New York, NY. 



