Baremore et al.: Gillnet selectivity for |uvenile Carcharhmus limbatus 
239 
be done with a proper context and with supporting 
length-frequency data when possible. 
Based on data from fisheries observers, the average 
mesh size used from 2005 through 2010 in the commer- 
cial anchored gillnet fishery in the U.S. Atlantic Ocean 
was 11.1 cm, with a range of 8.5-16.0 cm (Passerotti 4 ). 
The modal length calculated by using the equation 
in Table 3 for normal models indicates that blacktip 
sharks approximately 77 cm FL should be most vulner- 
able to the average mesh size in the commercial gillnet 
fishery. When calculated by using the full range of mesh 
sizes, the predicted modes range from 59 to 111 cm FL. 
Average lengths of blacktip sharks measured by ob- 
servers captured by commercial anchored gillnets from 
2005-2010 ranged from 79 to 107 cm FL (Baremore et 
al., 2007; Passerotti and Carlson, 2009, 2010; Passerotti 
et al., 2010, 2011). The observed lengths are consis- 
tent with the selectivity model estimated for blacktip 
sharks. Blacktip sharks are born at approximately 40 
cm FL and mature between 120 and 130 cm FL (5-7 
yr) in the U.S. Atlantic Ocean (Carlson et al., 2006), 
suggesting that the blacktip sharks most vulnerable to 
commercial gear are juveniles. Juvenile blacktip sharks 
use inshore nursery areas during spring and summer 
months, but migrate into deeper waters in the fall and 
winter (Castro, 1993; Heupel et al., 2007). Commercial 
gillnet fishermen operating in states with gillnet bans 
are required to fish at least 4.8 km from shore (federal 
waters) in the U.S. Atlantic Ocean; therefore the small- 
est juvenile blacktip sharks may not be as vulnerable 
to bycatch in these areas, especially during summer 
months. However, in states such as North Carolina, 
which allow commercial gillnet fishing in state waters, 
the potential for gear interaction with juvenile blacktip 
sharks year-round is higher. Observer data show that 
blacktip sharks <120 cm FL are captured in commercial 
gillnet fisheries and therefore juvenile blacktip sharks 
are likely affected by both offshore and inshore gillnet 
fisheries. 
Thorpe and Frierson (2009) reported a mortality rate 
of 90.5% for blacktip sharks captured in experimental 
gillnets. Although soak time was not reported, the gill- 
nets and sampling protocol in their study were designed 
to mimic those commonly used by commercial gillnet 
fishermen in North Carolina; therefore it is probable 
that juvenile blacktip sharks interacting with commer- 
cial gillnets may also experience high bycatch mortal- 
ity. Demographic evidence suggests that population 
growth rates are more sensitive to survival of juvenile 
life stages of sharks than adults (Cortes, 2002). There- 
fore, modeling of the gear selectivity of gillnet fisheries, 
and particularly modeling bycatch from fisheries that 
have the potential to impact juveniles, is especially 
important. 
4 Passerotti, M. 2011. Personal commun. Panama City 
Laboratory, Southeast Fisheries Science Center, National 
Marine Fisheries Service, National Oceanic and Atmos- 
pheric Administration, 3500 Delwood Beach Rd., Panama 
City, Florida 32408. 
Blacktip sharks are a commercially exploited species 
in U.S. waters, and the stock status in the Atlantic 
Ocean and Gulf of Mexico is assessed by the National 
Marine Fisheries Service on a regular basis (NMFS, 
2002; SEDAR, 2006). Bycatch estimates for blacktip 
sharks are available from observer data (Passerotti 
et al., 2010), and fishing intensity of the Spanish and 
king mackerel gillnet fisheries has been previously es- 
timated (SEDAR 2008; 2009). These fishery-dependent 
data, along with selectivity curves provided by this 
study, can be used by assessment scientists to estimate 
the selectivity of blacktip sharks caught as bycatch by 
commercial gillnet fisheries in the U.S. Atlantic Ocean. 
Bycatch data are equally as important as primary catch 
data for stock assessment models (NMFS 5 ; SEDAR, 
2006), though often more difficult to attain because 
bycatch is generally discarded at sea. This study pro- 
vides valuable information for assessment scientists and 
managers tasked with estimating the size structure of 
blacktip sharks caught by commercial gillnet fisheries. 
Conclusions 
Juvenile blacktip sharks are caught as bycatch in com- 
mercial gillnet fisheries in the U.S. Atlantic Ocean, 
although the impact on the population has not been 
assessed. The results from this study showed that gill- 
net selectivity for juvenile blacktip sharks caught in 
the fishery-independent survey was best described by a 
normal selectivity curve with fixed spread and with fish- 
ing intensity proportional to mesh size. Because many 
commercial gillnet fisheries use mesh sizes similar to 
those used to produce these results, it may be possible 
to estimate the length frequencies of juvenile blacktip 
sharks influenced by these coastal fisheries. Selectiv- 
ity estimates may also be applicable to other sharks 
of similar size for which species-specific information 
is unavailable. Future studies should focus on fishery- 
dependent gillnet selectivity estimates to determine if 
selectivity changes with gear, location, or target species. 
Acknowledgments 
We thank everyone at the NOAA Fisheries Panama 
City Laboratory who aided in field work and gear 
maintenance. We would especially like to thank the 
numerous volunteers and unpaid interns who gave 
countless hours collecting data over the years. J. Carl- 
son reviewed a version of the manuscript and gave 
useful comments. E. Cortes imparted expertise on the 
subject of gillnet selectivity and M. Passerotti provided 
5 NMFS (National Marine Fisheries Service). 2002. Stock 
assessment of large coastal sharks in the U.S. Atlantic Ocean 
and Gulf of Mexico: final meeting report of the 2002 shark 
evaluation workshop. Contribution report 02-03-177, 64 
p. Sustainable Fisheries Div., National Marine Fisheries 
Service, NOAA, Silver Spring, MD. 
