Santos et al.: Effects of leader material on catches of longline fisheries in the Indian Ocean 
229 
Size (cm) 
Figure 5 
Size distributions for the 2 major species caught during 2013-2014 in the southwest 
Indian Ocean. (A) blue shark (Prionace glauca) caught with 2.5-mm monofilament nylon: 
(B) swordfish (Xiphias gladius) caught with 2.5-mm monofilament nylon; (C) blue shark 
{Prionace glauca) caught with 1.2-mm multifilament stainless wire; (D) swordfish (Xi¬ 
phias gladius) caught with 1.2-mm multifilament stainless wire. Size measurements are 
given in centimeters: lower-jaw fork length for swordfish and fork length for blue shark. 
offs because these bite-offs were more frequent on sets 
with the highest shark catch rates. Moreover, several 
shark bite-offs were observed during the fishing experi¬ 
ments by the fishery observers. However, other species 
that also have sharp teeth, such as the snake mackerel 
or the dolphinfish, could also escape the longline by 
severing the leaders. Other important aspects related 
to bite-off rates are the hook styles used and hooking 
location. Circle hooks primarily embed in the corner of 
the jaw (Prince et al., 2002; Skomal et al., 2002) and 
J hooks are more likely to be swallowed, causing deep 
hooking in the throat or gut. Deep hooking, which was 
the most common retention mode recorded in our study 
with the J hook, can result in leaders becoming more 
exposed to abrasion against teeth. 
Very few authors have discussed the effects of gear 
material on mortality at haulback. As in our study, 
Afonso et al. (2012) also did not find significant dif¬ 
ferences when comparing shark MPUE between leader 
types, although the mortalities were slightly higher 
when using wire leaders. On the other hand, we re¬ 
corded higher swordfish MPUE on wire. We cannot ex¬ 
plain such a result, particularly as the mean CPUEn 
and CPUEb were similar for both leader materials. In 
addition, the size selectivity of gear material has been 
poorly studied. Although it would appear that leader 
material has no effect on size selectivity for most spe¬ 
cies, in the case of blue shark, a wider size range and 
a larger mean size were observed with the use of wire 
leaders, which reinforces the previously reported obser¬ 
vation (Afonso et al., 2012) that wire leaders retain the 
more resilient and larger blue shark specimens. 
Although a relatively small number of specimens 
of other shark species were caught during our experi¬ 
ment, the dead versus alive ratios (mortality at haul- 
back) observed were consistent with those reported by 
Coelho et al. (2012b) for a similar fishery in the Atlan¬ 
tic Ocean. The exception was the shortfin mako, which 
in this case showed a much lower mortality ratio at 
haulback (7-15%). Coelho et al (2012b) reported a rela¬ 
tionship between hooking mortality and specimen size 
for the shortfin mako in the Atlantic Ocean, with larger 
specimens having a lower probability of being dead at 
haulback. Because the size distribution in this region 
of the Indian Ocean tends to be composed of larger 
specimens than those in the Atlantic Ocean, as report- 
