Baremore et at. Gillnet selectivity for |uvenile Carcharhmus limbatus 
233 
Table 2 
Selectivity curves for normal, gamma, and lognormal models used to estimate gillnet selectivity for blacktip sharks (Carcharhi- 
nus limbatus ): m / is the mesh size for panel j ( j= 1-7 panels) and / ( is the midpoint of length class i (i= 1-22 length classes). Rela- 
tive fishing intensity is modeled separately. Equations in the right hand column are the last two terms in the log-linear model 
Model 
Selection curve 
A /i("vO + A 
Normal: 
fixed spread 
exp 
( / \2 
(, l,-k m J 
2cx 
2 
Normal: 
proportional 
spread 
exp 
{ l , ~ a i m j)~ 
2 a 2 
2 
Gamma: 
proportional 
spread 
/, 
(a-l)-k-m 
a - 1 - 
[«-!]■ 
/, 
m 
j 
Lognormal: 
proportional 
spread 
-exp 
^ +log l H y 
log(/,)-p, - log 
2d" 1 
^ | log (Z,.)- log 
— 1 --log' 
m, J 2 
Table 3 
Equations used to estimate the modal length of blacktip 
sharks (Carcharhinus limbatus) caught with each gillnet 
mesh size {mf for all four gillnet selectivity models. 
Model 
Mode 
Normal (fixed and 
proportional spread) 
Mode (m ] )=k-m J 
Gamma 
Mode (m )=(a- 1) • k-m } 
Lognormal 
f m \ 
Mode (m,)=exp(ii-a 2 ) • — 
J ^ r l m l ) 
where factor (l J indicates that length class is fitted as 
a factor of the model, and f x (m t , j) and f 2 (m r j ) are the 
selectivity functions of rrij and j (right hand column of 
Table 2). When p is assumed to be proportional to mesh 
size (logp-=logm-j, the form of the log-linear model is as 
follows: 
l°g ( v u ) = log (m y ) + factor (/,) + 
Pif,(m J ,j) + l3 2 f 2 (m J j). 
The parameters /3 ; and /3, are related to the form of the 
selectivity curve and are defined in Table 2. The follow- 
ing assumptions were made for all models: 1) catches 
were independent; and 2) gillnet panels were fished 
with equal effort. The mode, or maximum selected size 
for each panel, was calculated according to equations 
listed in Table 3. All models were fitted to the data twice, 
once under the assumption of equal fishing intensity 
and again under the assumption of fishing intensity 
proportional to mesh size. Overdispersion, or lack of 
fit, was tested by calculating the dispersion parameter, 
which is the model deviance divided by the degrees of 
freedom. When the dispersion parameter is >1, data are 
considered to be overdispersed. 
Results 
A total of 1720 blacktip sharks were measured from 1994 
through 2010 (Table 4). Blacktip sharks were collected 
during 14 of the 17 years of the survey. Average net soak 
time was 2.67 hr (range: 0.17-23.83 hr) over 1573 sets. 
Some outliers were excluded when sampling protocol 
was considered to be out of the ordinary procedure. The 
majority (97%) of blacktip sharks caught in all panels 
were immature and less than 110 cm FL (mode = 65 cm 
FL, Fig. 2). There was a general increase in the mean 
size of blacktip sharks with increasing mesh size. For 
the panels that were fished concurrently for all years 
(8.9-14.0 cm mesh), the total sample sizes of measured 
sharks were similar (Table 4). 
