134 



An evaluation of pop-up satellite tags for 

 estimating postrelease survival of blue marlin 

 {Makaira nigricans) from a recreational fishery* 



John E. Graves 



Virginia Institute of Marine Science 

 College of William and Mary 

 Rt. 1208 Greate Road 

 Gloucester Point, Virglna 23062 

 E-mail address gravesfaJvims edu 



Brian E. Luckhurst 



Division of Fisheries 



Department of Agnculture and Fistienes 



PO Box CR 52 



Crawl CRBX, Bermuda 



Eric D. Prince 



National Marine Fistieries Service 

 75 Virginia Beach Dnve 

 Miami, FL 33146 



Blue marlin [Makaira nigricans) rep- 

 resent an important commercial and 

 recreational resource throughout trop- 

 ical and subtropical oceanic waters. 

 In the Atlantic Ocean, blue marlin 

 are managed as a single, oceanwide 

 stock. In the most recent assessment of 

 Atlantic blue marim ( ICCAT, 2001), the 

 Standing Committee for Research and 

 Statistics (SCRS) of the International 

 Commission for the Conservation of 

 Atlantic Tunas (ICCAT) estimated the 

 current biomass of blue marlin to be 

 about 40% of that required for maxi- 

 mum sustainable yield (MSY). Further- 

 more, the assessment indicated that 

 the current level of fishing mortality 

 (F) was about four times higher than 

 F,y,gY and that catch levels in recent 

 years were more than twice the equi- 

 librium yield, contributing to a fur- 

 ther decline of the overexploited stock. 

 Based on the most recent stock assess- 

 ment, fishing-induced mortality must 

 be reduced by about 60% to halt the 

 decline of the stock (Goodyear, 2000). 



The greatest source of billfish (Istio- 

 phoridae) mortality occurs as a result 

 of incidental catches by longline gear 

 deployed for tunas and swordfish (IC- 

 CAT, 1997, 2001). These highly migra- 

 tory species co-occur in the subtropical 



and tropical epipelagic environment, 

 and all are vulnerable to the pelagic 

 longline. Not all billfish are dead at the 

 time of capture (haulback) on longline 

 gear; data from observers on vessels 

 in the Venezuelan industrial longline 

 fishery indicate that about 49% of blue 

 inarlin caught on pelagic longline gear 

 are alive at the time of capture (Jack- 

 son and Farber, 1998). 



To reduce billfish mortality ICCAT in 

 1997 required nations to reduce their 

 landings of Atlantic blue marlin by 25% 

 from 1996 levels. Furthermore, the IC- 

 CAT SCRS has recommended that the 

 Commission consider requiring the re- 

 lease of all live billfish taken on long- 

 line gear (ICCAT 1997, 2001). It is 

 believed that such a management mea- 

 sure would be more acceptable to mem- 

 ber nations than an overall reduction 

 in longline effort that would also re- 

 duce catches of target species. How- 

 ever, representatives from several na- 

 tions have pointed out that there are 

 not sufficient data to estimate postre- 

 lease survival of billfish; therefore the 

 conservation impact of a recommenda- 

 tion requiring live released fish cannot 

 be evaluated. In fact, low recovery rates 

 of billfish tagged and released with 

 conventional tags by recreational and 



commercial fishermen ( <2%; Jones and 

 Prince, 1998; Ortiz et al, 1998) are con- 

 sistent with high postrelease mortality. 

 However, factors such as tag shedding 

 and failure to report tag recaptures 

 could also account for low rates of tag 

 returns ( Bayley and Prince, 1994; Jones 

 and Prince, 1998). Clearly, data are 

 needed to support or refute the hypothe- 

 sis that the release of live billfish would 

 significantly eliminate fishing mortali- 

 ty of blue marlin (Graves et al.'). 



Acoustic tracking studies designed 

 to investigate billfish physiology and 

 behavior have provided insights into 

 the postrelease survival of billfish tak- 

 en on recreational gear Specifically, ob- 

 sei-ved and inferred mortalities during 

 the course of the acoustic tracks indi- 

 cate that not all released billfish sur- 

 vive (reviewed in Pepperell and Davis, 

 1999). Unfortunately, it is not possible 

 to estimate levels of postrelease mor- 

 tality of billfish from previous acoustic 

 tracking studies for several reasons. 

 First, owing to the high cost of ship 

 and personnel tiine, relatively few ani- 

 mals have been investigated in acous- 

 tic tracking studies. Second, because 

 ocean conditions can deteriorate quick- 

 ly, many of the acoustic tracks were 

 for less than 12 hours duration, pro- 

 viding a limited opportunity to ob- 

 ser\'e mortality after 12 hours. Third, 

 billfish were caught and subsequently 

 tracked under a variety of conditions, 

 making cross-study comparisons diffi- 

 cult. Finally, an estimate of postrelease 

 mortality rates resulting from acoustic 

 studies may be biased because in soine 

 cases only healthy fish were selected to 

 carry acoustic transmitters. 



The development of pop-up satellite 

 tag technology may present a possible 

 means to estimate postrelease mortali- 

 ty of billfish. Although relatively expen- 

 sive pop-up satellite tags reduce the 

 need to use a tracking vessel to follow 



* Contribution 2416 of the Virginia Insti- 

 tute of Marine Science, College of William 

 and Mary, Gloucester Point, VA 2.3062. 



' Graves, J. E., G. Skomal. and E. D. Prince. 

 1995. Report of the billfish mortality 

 workshop. Contribution rep. MIA-94/ 

 95-49, 7 p. Southeast Fisheries Science 

 Center, Miami, FL. 



Manuscript accepted 30 July 2001. 

 Fish, Bull. 100434-142 (2002). 



