Baremore et at: Gillnet selectivity for |uvenile Carcharhinus limbatus 
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setts throughout the Gulf of Mexico in U.S. coastal wa- 
ters (McEachran and Fechhelm, 1998). Juvenile blacktip 
sharks use nursery areas such as bays and nearshore 
habitats during spring and summer months (Castro, 
1993; Heupel and Hueter, 2002). Because of their range 
and life history characteristics, juvenile blacktip sharks 
are likely to encounter commercial gillnets. 
Gillnet selection curves are a useful way to represent 
the retention probabilities of different mesh sizes for a 
given species of fish. Retention probability by gillnets 
is usually considered to be dome-shaped and can be 
described by the equation 
where r-(Z-) is the retention probability that a fish of 
length l in size class i is caught by mesh size j, and n and 
a represent the mean and spread of the curve (Millar 
and Fryer, 1999). However, the selection curve may be 
skewed because of snagging, rolling, or entangling of 
animals, and can result in a gamma or lognormal curve 
(Millar and Fryer, 1999). 
Generally, selectivity can be measured in two ways: 
directly and indirectly (Millar and Fryer, 1999). Direct 
experiments are performed on a population for which 
the size distribution is known, and size selection is 
calculated by comparison of the population with the 
catch distributions. Indirect, or comparative, experi- 
ments are more common and usually involve simulta- 
neously fishing gillnets of differing mesh sizes with 
equal effort. 
Commercial fishing gear selectivity curves are in- 
corporated into modern stock assessment models, 
and changes in the parameters have the potential 
to impact the assessed status of the stock (Maunder, 
2002). Size selectivity is used in the estimation of the 
length-frequency of a stock, estimation of fishing-in- 
duced mortality, and in age-based assessment models 
(Millar and Fryer, 1999). Although important for the 
stock assessment models, fishery-independent selec- 
tivity models are rare for many large shark species 
(McAuley et al., 2007). Selectivity for bycatch species 
is also becoming an important issue in stock assess- 
ment, but direct estimates are likewise rare for most 
fisheries. The goal of this study is to determine the 
relationship between gillnet mesh size and selectivity 
for juvenile blacktip sharks using fishery-independent 
data. 
Materials and methods 
Sampling 
Data necessary for indirect calculation of gillnet mesh 
selectivities were obtained from the Gulf of Mexico 
Shark Pupping and Nursery (GULFSPAN) survey, 
which is a fishery-independent gillnet survey of coastal 
shark populations in the northeastern Gulf of Mexico 
(Carlson and Brusher, 1999). Catch data for C. lim- 
batus were collated over 17 years (1994-2010) from 
five bay systems in northwest Florida: St. Andrew Bay, 
Crooked Island Sound, St. Joseph Bay, the gulf side 
of St. Vincent Island, and Apalachicola Bay (Fig. 1). 
Six gillnet panels of differing stretched mesh sizes 
were strung together in increasing mesh size, anchored, 
and fished concurrently as a single gillnet. Each panel 
was 30.1 m long and 3.4 m deep (Table 1). From 1994 
through 2005, stretched mesh sizes ranged from 8.9 cm 
to 14.0 cm, increasing by 1.3-cm (0.5-in) intervals, with 
an additional panel of 20.3 cm. In 2006, the 20.3-cm 
panel was removed and a 7.6-cm panel was added ad 
hoc. The largest mesh panel was removed because of 
its historically low catch of juvenile small coastal shark 
species, and the 7.6-cm panel was added to increase 
catch of small neonatal sharks. Unless otherwise indi- 
cated, all mesh sizes reported in the present study are 
stretched mesh sizes. 
Sampling occurred each year from late March 
through October. Net set locations within bay sys- 
tems were randomly chosen over a variety of habitat 
and depth combinations. The majority of sets were 
short (<1 hr) as a means of reducing mortality, es- 
pecially when water temperatures were above 25°C. 
However, some nets were soaked for longer periods 
of time, depending on the research priorities at the 
time. Captured sharks were removed from the net, 
their sex was determined, and they were measured 
for fork length (FL) on a rigid measuring board in a 
straight line from the tip of the nose to the fork in 
the tail. Sharks in poor condition were sacrificed for 
research projects and those in good condition were 
tagged and released. Maturity state was determined 
by clasper calcification for males, internal examina- 
tion for sacrificed female sharks, and released females 
were considered to be mature when greater than 115 
cm FL (Carlson et al., 2006). Sexes were combined 
for data analyses. 
Data analysis 
Catch data were pooled by mesh size into 5-cm-FL size 
bins, and the midpoint of each size class (/,) was used to 
calculate a selectivity curve for each mesh size (Millar 
and Holst, 1997). Four gillnet selectivity models were 
fitted to the / ( for each mesh size (m) (Millar and Holst, 
1997), by using the SELECT (share each length’s catch 
total) method (Millar and Holst, 1997; Millar and Fryer, 
1999; Millar, 2003, 2010). The selection curves were 
fitted to the data by using the “gillnetfunctions” package 
in R statistical software (Millar, 2003, 2010; R Develop- 
ment Core Team, 2009). The SELECT method applies 
the method of maximum likelihood, which estimates 
selectivity parameters from a general log-linear model. 
The expected catch of sharks of length class i in gillnet 
j is described by 
v u = PA r j’ (1) 
