219 
National Marine 
Fisheries Service 
NOAA 
Fishery Bulletin 
ft- established in 1881 ■<?. 
Spencer F. Baird 
First U.S. Commissioner 
of Fisheries and founder 
of Fishery Bulletin 
Effects of leader material on catches of 
shallow fielagic longline fisheries in the 
southwest Indian Ocean 
Migue! N. Santos 
Pedro G. Lino 
Rui Coelho (contact author) 
Email address for contact author: rpcoelho@ipma.pt 
Instituto Portugues do Mar e da Atmosfera (IPMA) 
Avenida 5 de Outubro s/n 
8700-305 Olhao, Portugal 
Abstract—-Catches from a commer¬ 
cial longline fishery targeting sword¬ 
fish (Xiphias gladius) on monofila¬ 
ment nylon leaders were compared 
with catches on wire leaders in 
the Indian Ocean. More taxa were 
caught on wire leaders, which also 
showed higher catch rates (13% 
and 56%, in number and weight, re¬ 
spectively) of blue shark (Prionace 
glauca). In contrast, catch rates of 
swordfish were not significantly af¬ 
fected by leader material. Nylon 
leaders showed lower at-haulback 
mortality for most bony fishes, ex¬ 
cept swordfish. Higher bite-off rates 
were observed on nylon monofila¬ 
ment, likely owing to the escape of 
species with sharp teeth, such as 
sharks. Both leader types caught 
most species within similar size 
ranges, but larger mean sizes of blue 
shark were recorded on wire leaders. 
The value per unit of effort (VPUE) 
of the retained catch did not differ 
between leader materials; however, 
VPUEs are highly dependent on 
market fluctuations. Banning wire 
leaders could be an effective way of 
reducing shark catches, particularly 
blue shark catches, in the southwest 
Indian Ocean. 
Manuscript submitted 6 May 2016. 
Manuscript accepted 17 February 2017. 
Fish. Bull.:219-232 (2017). 
Online publication date: 10 March 2017. 
doi: 10.7755/FB115.2.9 
The views and opinions expressed or 
implied in this article are those of the 
author (or authors) and do not necessarily 
reflect the position of the National 
Marine Fisheries Service, NOAA. 
Pelagic longlines have historically 
been used by distant water fleets 
to catch large tunas {Thunnus spp.) 
and swordfish (Xiphias gladius) on 
the high seas. However, there has 
been considerable concern over the 
ecological effects of these fisheries 
because longline gears also catch 
other species, particularly billfish 
(Istiophoridae) and pelagic sharks 
(Elasmobranchii), and, to a less ex¬ 
tent, marine-turtles, sea-birds and 
marine-mammals (Lewison et al., 
2004; Watson and Kerstetter, 2006; 
Huang, 2011). This range of species 
is due to the fact that these groups 
of animals occupy broad geographic 
ranges spanning geopolitical bound¬ 
aries and oceanographic regions that 
support different fisheries (Wallace 
et al., 2010). Fisheries bycatch—the 
unintended capture of nontarget 
organisms during fisheries opera¬ 
tions—is therefore a major problem 
worldwide because it occurs with vir¬ 
tually all Ashing fleets and is a glob¬ 
al issue for management of marine 
resources (Hall et al., 2000; Soykan 
et al., 2008). Despite the existence of 
extensive differences in bycatch spe¬ 
cies and the magnitude of these dif¬ 
ferences from one fishery to another, 
bycatch can be a driver of declines 
in marine megafauna populations 
(Lewison et al., 2004; Read, 2007; 
Wallace et al., 2010). Such declines 
can be particularly important in the 
case of bycatch species that have 
long life cycles and low productivity, 
and therefore low potential for popu¬ 
lation recovery, as is the case for sev¬ 
eral pelagic sharks. 
The large pelagic longline fisher¬ 
ies in the Indian Ocean date back 
to the 1950s, when the Asian fleets 
targeted mainly tropical tunas (Lee 
et al., 2005) and caught billfishes 
and sharks as bycatch. During the 
early 1990s, the shallow-setting pe¬ 
lagic longliners (mostly European) 
expanded the range of their sword¬ 
fish fishery from the Atlantic Ocean 
to the Indian Ocean. A few changes 
were incorporated into the fishing 
gear in the early 2000s, specifically 
a shift from traditional to modern 
gear (Watson and Kerstetter, 2006), 
making use of mainlines and branch 
lines of monofilament leader and us¬ 
ing lightsticks or flashlights. More¬ 
over, in the same period, the land¬ 
ings of pelagic sharks increased as 
a result of the increasing interest in 
the international markets for shark 
products. Between the mid-2000s 
and early 2010s, owing to increasing 
costs (mostly related to the oil pric¬ 
es) and taking advantage of the high 
