Santos et al.: Effects of leader material on catches of longline fisheries in the Indian Ocean 
221 
Figure 1 
Map of the locations, indicated with black lines, of the experimental 
longline sets conducted in the southwest Indian Ocean between Novem¬ 
ber 2013 and March 2014 to determine possible effects of leader mate¬ 
rial on catches of pelagic longline fisheries. 
section; in the case of the control; 4) the fourth sec¬ 
tion consisted of a 2.5-mm nylon monofilament leader 
(0.65 m in length) with a hook in the terminal tackle, 
whereas in the case of the treatment (the wire leader), 
the fourth section consisted of a 1.2-mm multifilament 
stainless steel leader (3 strands, 0.65 m long) with a 
hook in the terminal tackle. A battery flashlight (green 
color) was attached to the loop connecting the second 
and third sections of the branch lines. Only one hook 
type was used, specifically a stainless steel 10° offset 
J hook (model EC-9/O-R^, Won Yang Fishing Tackle Co. 
Ltd., Pusan, Korea) that corresponds with the tradi¬ 
tional J hook used by the fishery, whose characteristics 
are summarized in Figure 2. Only one bait type, squid 
(Illex spp.), was used throughout the experiments. Stan¬ 
dardized bait was used in all longline sets (squid 24.5 
cm [standard deviation (SD) 1.64]). All characteristics 
of the fishing gear and fishing practices (e.g., gear sec¬ 
tion placement, setting time, light color, bait size, and 
hook) were standardized along the 2 trips. The total 
number of hooks was constant in each set and was the 
same for each leader type in every set (504 hooks for 
each leader type material in each set) and fishing oc¬ 
curred at depths of approximately 20-50 m. Gear de¬ 
ployment began traditionally at 1730 h, and haulback 
started the next day at about 0600 h. Leader type was 
alternated section by section along the longline, and 
each section had 84 hooks that were stored in individ- 
^ Mention of trade names or commercial companies is for iden¬ 
tification purposes only and does not imply endorsement by 
the National Marine Fisheries Service, NOAA. 
ual “baskets.” This alternating of leader 
types minimized potential confounding 
effects specific within each set: for ex¬ 
ample, location, water temperature, fish 
density, or other factors. Moreover, the 
branch line type of the first section was 
changed every set, according to a fixed 
scheme (i.e., mono:wire:mono:wire, and 
so on). 
Following Watson et al. (2005), we 
carried out power tests in order to es¬ 
timate the experimental fishing effort 
required to detect a fishing method that 
has different degrees of effectiveness 
in catching swordfish and blue shark 
in comparison with the control fish¬ 
ing method. The control fishing method 
was assumed to be the combination of 
gear and bait most commonly used in 
the fishery, specifically J hooks baited 
with squid, and the power calculations 
were based on the necessary number of 
hooks required to detect a 25% and 50% 
change in the number of swordfish and 
blue shark caught. A trained observer 
from the Institute Portugues do Mar e 
da Atmosfera monitored the experimen¬ 
tal trials and collected the data on the 
vessel. Whenever a specimen was caught in the long- 
line, the observer identified the species, recorded the 
leader line material, the fate (retained or discarded) 
of a specimen, condition at haulback (alive or dead) 
and if discarded (alive or dead) and the type of inter¬ 
action (i.e., hooking location: mouth or jaw, when the 
hook was visible; and hooking mode: deeply ingested, 
hook ingested and located in the throat or gut; exter¬ 
nally hooked when the hook was located externally). 
The condition of the leaders (bitten-off or not) was re¬ 
corded. In the case of marine turtles, when possible, 
they were netted with a large dip net and, whenever 
possible, the observer and crew attempted to remove 
fishing gear with long-handled de-hookers and line cut¬ 
ters. The sex of the specimens, both turtles and fish, 
was determined and size was measured to the nearest 
lower 1 cm (lower-jaw fork length for billfishes; fork 
length [FL] for other fishes; carapace curved length for 
turtles). However, because of the size or weight of some 
species (i.e., manta rays [Manta spp.]) and to increase 
their survivorship, some specimens were immediately 
released by cutting off the line. 
In the study area, the fleet is currently using main¬ 
ly monofilament leaders and hooks baited with squid, 
therefore we considered that the main target species 
was swordfish. The catch was assigned to 1 of 5 group¬ 
ings: billfish, which included swordfish and marlins, 
the latter of which were considered a bycatch; tuna 
(Thunnus spp.), considered bycatch; sharks, which in¬ 
cluded all elasmobranchs, considered bycatch; other 
bony fishes, which were assigned exclusively to bycatch 
species; and turtles. Finally, all the species that were 
