220 
Fishery Bulletin 115(2) 
abundance of sharks in particular areas or seasons (or 
both), the fisheries targeting swordfish started replac¬ 
ing the traditional monofilament leaders with multi¬ 
filament steel leaders, and baiting the hooks with fish, 
mostly mackerel (Scomber spp.), instead of squid (Illex 
spp.). These gear changes raised concerns about their 
effects, particularly on shark stocks, because those gear 
changes were occurring on an ocean-wide scale. 
A number of research initiatives have focused on 
the mitigation of longline bycatch by making several 
technological and methodological changes, all aiming 
at increasing the selectivity of fishing gear and reduc¬ 
ing mortality. With regard to shark bycatch, particular 
attention has been given to the use of different hook 
styles (Watson et al., 2005; Yokota et al., 2006; Coel- 
ho et al., 2012a; Amorim et al., 2015) and bait types 
(Watson et al., 2005; Coelho et al., 2012a; Amorim et 
al., 2015). However, there are few published studies on 
the effects of wire leaders on longline catches, despite 
the general concern regarding increasing catches of 
sharks. Moreover, a number of these previous studies 
have reported some contradictory results: Branstetter 
and Musick (1993) conducted 71 longline sets (using 
50 branchlines with nylon leaders and 100 branchlines 
with wire leaders) and reported higher catch rates of 
sharks on nylon leaders in inshore waters, but found 
the opposite in offshore waters; Berkeley and Campos 
(1988), on the basis of 13 longline sets (about 25% us¬ 
ing wire leaders), reported lower catches of sharks on 
wire leaders than on nylon, but the difference was not 
statistically significant; Stone and Dixon (2001) com¬ 
pleted 10 sets with alternately spaced monofilament 
and tarred multifilament nylon leaders and reported 
lower catches for the tarred multifilament nylon lead¬ 
ers; Afonso et al. (2012) completed 17 longline shallow 
sets, using nylon and wire leaders, and reported higher 
catches of blue shark (Prionace glauca), and all sharks 
combined on wire leaders; Vega and Licandeo (2009) 
conducted 37 fishing sets using polyamide monofila¬ 
ment (hooks baited with squid or mackerel) and steel 
multifilament 3-strand wire (hooks baited with mack¬ 
erel) and showed marginal differences in shark catch 
rates, but no differences for the blue shark; and finally, 
Ward et al. (2008) conducted the largest experiment 
involving 5 commercial vessels (177 longline sets) tar¬ 
geting tuna (with deep setting), using wire (6 strands) 
and nylon leaders and reported a higher shark catch 
rate on wire leaders but did not report any data for the 
blue shark (the most common species caught on pelagic 
longlines). Most of these studies were conducted in the 
northwestern Atlantic Ocean (Branstetter and Musick, 
1993; Stone and Dixon, 2001) and the southwestern 
equatorial regions (Afonso et al., 2012); whereas the 
remaining studies were conducted in the southwest 
Pacific Ocean off northeastern Australia (Ward et al., 
2008) , and in the southeast Pacific Ocean off Easter 
Island and Salas y Gomez Island (Vega and Licandeo, 
2009) . 
There is a lack of information specific to the Indi¬ 
an Ocean on longlining despite the global landings of 
sharks on the order of 90,000 metric tons (t) (Indian 
Ocean Tuna Commission [lOTC] Online Data Query¬ 
ing Service, website). Interestingly, a recent European 
Union funded research project estimated potential 
shark catches for the Indian Ocean alone to be on the 
order of 160,000 t (Murua et al.^). In 2015, the Scien¬ 
tific Committee of the lOTC completed its first stock 
assessment for the blue shark, concluding that the un¬ 
certainties in the data and model structure were high, 
and as such the stock status was uncertain; however, 
the possibility of the stock being currently overfished 
was not ruled out (lOTC^). 
Our study was designed to test the effect of differ¬ 
ent combinations of the material used on the termi¬ 
nal tackle of the branch lines (monofilament nylon and 
multifilament wire) on the catches of the shallow pe¬ 
lagic longline fishery targeting swordfish in the south¬ 
west Indian Ocean, an area of significant pelagic shark 
catches. We provide, for the first time for the Indian 
Ocean and on the basis of a large experimental study, 
a comparison between the catch composition, catch and 
yield rates, mortality rates, bite-off rates, and catch-at- 
size for both target and bycatch species caught by the 
2 types of leader materials currently used on pelagic 
longline fisheries traditionally targeting swordfish. 
Materials and methods 
Experimental design and data collection 
A total of 82 longline sets were carried out during 2 
trips in the southwest Indian Ocean, over wide lati¬ 
tudinal (23-32°S) and longitudinal (56-71°E) ranges 
(Fig. 1) between November 2013 and March 2014. The 
experiments were conducted by a commercial fishing 
vessel from the Portuguese pelagic longline fleet. The 
fishing gear consisted of a standard monofilament poly¬ 
amide mainline (3.6 mm in diameter), with 6 branch 
lines between floats (82 m from each other). The 
branch lines were approximately 18.6 m in length, and 
were attached to the main line by a 12.5-cm snap. Each 
branch line had 4 sections: 1) the first section consisted 
of 2.5-mm nylon monofilament (11.85 m long) connected 
by a swivel (4.5 cm) to the next section; 2) the second 
section consisted of a 0.7-m weighed rope (weighing 50 
g), connected by a loop to the following section; 3) the 
third section consisted of 2.2-mm nylon monofilament 
(5.4 m in length), connected by a loop to the following 
1 Murua, H., F. J. Abascal, J. Amande, J. Ariz, P. Bach, P. 
Chavance, R. Coelho, M. Korta, F. Poisson, M. N. Santos, et 
al. 2013. Provision of scientific advice for the purpose of 
the implementation of the EUPOA sharks. Final report. Eu¬ 
ropean Commission Studies for Carrying out the Common 
Fisheries Policy (MARE/2010/11-LOT 2), 336 p. [Available 
from website.] 
2 lOTC (Indian Ocean Tuna Commission). 2015. Report of 
the 18th Session of the lOTC Scientific Committee. Indian 
Ocean Tuna Commission. Bali, Indonesia, 23-27 November. 
IOTC-2015-SC18-R[E], 175 p. [Available from website.] 
