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for sharks do not include year-specific selectivity func- 
tions (SEDAR, 2006). 
Gillnet selectivity is more highly influenced by mor- 
phological features such as girth and the presence or 
absence of hard structures than it is by the length of a 
fish (Reis and Pawson, 1999; Carol and Garcia Berthou, 
2007). Nevertheless, straight-line measured length can 
often be used as a proxy for girth in selectivity studies 
because of the close direct relationship between the two 
parameters (Reis and Pawson, 1999), with exceptions 
for cases of unusual morphological features (e.g., in 
hammerheads [Thorpe and Frierson, 2009]) or behav- 
ioral response to entanglement (e.g., finetooth sharks 
[Carlson and Cortes, 2003]). Many sharks in the family 
Carcharhinidae share similar body shape and struc- 
ture (Compagno and Niem, 1998), with girth and the 
rigidity of fins acting as a limiting factor for capture 
by gillnets (Carlson and Cortes, 2003; McAuley et ah, 
2007). Girth-to-length relationships have been found to 
be similar among related species of sharks (McLoughlin 
and Stevens, 1994). It is therefore possible that selectiv- 
ity curves could be family-specific rather than species- 
specific for sharks. The most recent data have shown 
that the normal selectivity curves may provide the best 
fit for sharks in the family Carcharhinidae (McAuley 
et ah, 2007; Thorpe and Frierson, 2009), indicating 
that the results for blacktip sharks could be useful for 
other carcharhinids of similar size. Selectivity param- 
eters estimated for the blacktip shark could be used 
as a proxy for other species in the same family when 
species-specific selectivity estimates are unavailable. 
This theory could be tested by applying this method 
to other similar-size shark species for which a gillnet 
selectivity curve has been estimated, and should be 
pursued further as more data become available. 
Thorpe and Frierson (2009) found length modes of 97 
and 88 cm FL for blacktip sharks caught in mesh sizes 
7.6 and 10.2 cm, respectively, whereas we estimated 
modes of 46 and 62 cm FL for the same mesh sizes 
(Table 6). However, Thorpe and Frierson (2009) failed 
to fit a selectivity curve to the individual mesh sizes 
because of the wide spread of the sparse length data. 
Their study was based on a small number of samples 
(n = 76) and the modes for only two mesh sizes were 
estimated. The low sample size reported by Thorpe 
and Frierson (2009) was likely due to the relatively 
short duration of sampling, which was conducted over 
a period of eight months. Additionally, Thorpe and 
Frierson (2009) conducted their survey more than 1 
km from shore, where the likelihood of small juveniles 
coming in contact with the gear was low. Total effort 
was not reported; however, catch rates were low in 
all gillnet panels (<0.15 blacktip sharks caught per 
hour of fishing). It is also possible that the size classes 
sampled in both studies were not reflective of the true 
size structure of the population because localized con- 
centrations of sharks in each area that were available 
to the gear probably differed. The true availability of 
blacktip sharks to gillnets in different regions cannot be 
known; therefore applying selectivity functions should 
