234 
Fishery Bulletin 1 10(2) 
Table 4 
Length distribution for all blacktip sharks ( Carcharhinus 
limbatus ) caught in each gillnet mesh panel in the Gulf 
of Mexico Shark Pupping and Nursery (GULFSPAN) 
Survey in northwest Florida, 1994-2010. 
Fork Mesh sizes (cm) 
lengin 
(cm) 
7.6 
8.9 
10.2 
11.4 
12.7 
14.0 
20. 
42.5 
4 
4 
4 
0 
1 
0 
0 
47.5 
2 
10 
14 
18 
6 
4 
0 
52.5 
13 
25 
29 
57 
21 
22 
2 
57.5 
13 
20 
32 
53 
30 
43 
3 
62.5 
12 
24 
21 
41 
63 
41 
0 
67.5 
7 
30 
31 
29 
47 
31 
4 
72.5 
6 
26 
30 
34 
20 
32 
6 
77.5 
13 
36 
34 
21 
30 
20 
5 
82.5 
8 
15 
20 
22 
17 
28 
7 
87.5 
4 
29 
24 
34 
25 
16 
9 
92.5 
7 
15 
17 
20 
14 
15 
9 
97.5 
0 
14 
12 
15 
13 
14 
18 
102.5 
2 
4 
16 
15 
9 
8 
17 
107.5 
0 
10 
5 
1 
5 
4 
12 
112.5 
0 
1 
0 
2 
4 
0 
6 
117.5 
0 
2 
0 
4 
3 
0 
8 
122.5 
0 
2 
1 
1 
0 
0 
4 
127.5 
0 
0 
0 
1 
0 
0 
2 
132.5 
0 
0 
0 
0 
0 
0 
0 
137.5 
0 
0 
0 
0 
0 
0 
2 
142.5 
0 
0 
0 
1 
0 
0 
1 
147.5 
1 
0 
1 
0 
0 
0 
0 
Totals 
92 
267 
291 
369 
308 
278 
115 
The normal, fixed-spread models had the lowest model 
deviance overall, with the model incorporating fishing 
intensity proportional to mesh size having the low- 
est total model deviance (Fig. 3, Table 5). The ratio of 
model deviance to degrees of freedom was higher than 1 
(2.9), indicating overdispersion of the data. This result 
indicates that blacktip sharks may not have behaved 
independently (e.g., with schooling behavior), violating 
the first assumption of independent catches. Residual 
plots showed a similar degree of bias for all models 
(Fig. 3), with none demonstrating markedly different 
fits to the data. The biggest difference among models 
was for the largest mesh (20.3 cm) for which the normal 
(proportional spread), lognormal, and gamma curves 
under-represented some of the smaller length classes. 
The highest number of positive residuals was seen for 
the smaller length classes (50-70 cm FL) in mesh sizes 
11.4 cm and 12.7 cm and, to a lesser degree, the 14.0 cm 
panel for all models (Fig. 3). The plots indicated that 
more of the smaller individuals were caught in these 
panels than predicted by the models. The largest and 
smallest mesh sizes (20.3 and 7.6 cm) caught fewer of 
the smallest sharks than predicted by the models. The 
residuals did not indicate systematic bias in any of the 
models aside from the lack of fit to the smallest size 
classes (Fig. 3). Predicted selectivity curves for the 
normal, fixed-spread model assuming proportional fish- 
ing intensity plotted with observed length-frequencies 
for each mesh size (Fig. 4) showed that the model fitted 
the observed data well. 
Discussion 
In previous gillnet selectivity studies on 
sharks, a gamma-shaped distribution has been 
assumed (Carlson and Cortes, 2003; Kirkwood 
and Walker, 1986; McLoughlin and Stevens, 
1994; Simpfendorfer and Unsworth, 1998), 
based on the specialized SELECT method 
described by Kirkwood and Walker (1986). 
However, a more recent study on the gillnet 
selectivity for sandbar sharks C. plumbeus 
(McAuley et al., 2007) found that all four 
models estimated by the Millar and Holst 
(1997) method provided better fits than the 
Kirkwood and Walker (1986) gamma model. 
Our study on blacktip sharks indicated that 
the normal, fixed spread models provided the 
best fit. A more limited study in North Caro- 
lina (Thorpe and Frierson, 2009) found that 
the normal model with spread proportional to 
mesh size generally provided the best fit for 
blacknose (C. acronotus), bonnethead ( Sphyrna 
tiburo), and blacktip sharks. Although the 
method of Kirkwood and Walker (1986) was 
not employed in this study, the gamma curve 
estimated by the Millar and Holst (1997) 
SELECT method provided a poorer fit than 
the normal and lognormal models. Therefore, it 
