Driggers et al.: Influence of bait type on catch rates of predatory fish on longline gear 
51 
does not decrease catch rates of target species yet re- 
duces rates of shark capture could be the most easily 
implemented and likely to be readily accepted within 
a fishery. 
Past studies have shown that catch rates of certain 
shark species are affected by the use of specific bait 
types. For example, Gilman et al. (2007) analyzed data 
collected in the pelagic longline fishery in Hawaii and 
determined that the catch rate of blue sharks (Priona- 
ce glauca) was reduced by 36% when fish, rather than 
squid, were used as bait. Similarly, Watson et al. (2005) 
conducted experimental longline sets in the western 
North Atlantic Ocean and found catch rates of blue 
sharks were 31-40% (depending on hook type) lower 
on hooks baited with fish than on hooks baited with 
squid. Although results of these studies are potentially 
biased by use of multiple hook types and sizes, their 
results strongly indicate that use of a specific bait type 
could be a means to reduce bycatch of sharks. 
Hook-based fisheries in the northern Gulf of Mexico 
that target grouper (Serranidae), snapper (Lutjani- 
dae), and tilefish (Malacanthidae) species frequently 
capture sharks (Gulak et al., 2013). For example, the 
Atlantic sharpnose shark (Rhizoprionodon terraenouae) 
was the sixth-most captured fish species among the ap- 
proximately 180 fish species reported in observer data 
collected from the bottom longline fishery for reef fish 
in the Gulf of Mexico (Scott-Denton et al., 2011). Fur- 
thermore, smoothhound (Mustelus spp.) and blacknose 
(Carcharhinus acronotus) sharks were among the 20 
most frequently captured fish taxa within the same 
fishery (Scott-Denton et al., 2011). The size and type 
of hooks vary in the bottom longline fishery for reef 
fish in the Gulf of Mexico (Gulak et al., 2013), and bait 
type is inconsistent, depending on personal preference, 
availability, and price, among other factors (Prytherch, 
1983; Scott-Denton et al., 2011). Because there is high 
variability in the gear and bait used within this fish- 
ery, use of observer data to examine potential effects of 
bait type on the catch rates of specific species is prob- 
lematic. Our goal was to conduct a controlled experi- 
ment to test the effects of 2 readily available bait types 
commonly used in the bottom longline fishery for reef 
fish on catch rates of sharks and economically impor- 
tant teleosts in this region. Additionally, we examined 
density-dependent effects on preferences for the 2 bait 
types for both groups of fish species. 
Materials and methods 
Bottom longline gear was deployed from the NOAA 
Ship Oregon II at sampling sites in the northern Gulf 
of Mexico from 11 March through 13 April 2015. Sam- 
pling sites were selected on the basis of 18.5-km grids 
within predefined geographic areas (from Pascagoula, 
Mississippi, to Cape San Bias, Florida) and depth 
(9-1000 m) constraints. However, obstructions (e.g., 
other vessels, reefs, petroleum platforms, and safety 
fairways) caused the locations of some sampling sites 
to be different from the positions that were originally 
planned. The bounds of the sampling universe were se- 
lected to maximize sampling opportunities within tem- 
poral limitations. 
Bottom longline gear consisted of 1842 m of 4.0-mm 
monofilament mainline and 100 gangions. Each gan- 
gion was 3.7 m in length and constructed of an AK snap 
(size 150), 3.2 m of 3.0-mm-diameter monofilament, 0.5 
m of 2.4-mm-diameter fishing wire, and a 15/0 circle 
hook (Mustad #39960Di, O. Mustad & Son A.S, Gjorvik, 
Norway). Each gangion was baited with Atlantic mack- 
erel {Scomber scombrus) or northern shortfin squid {II- 
lex illecebrosus). Both bait types were cut so that they 
were of the same approximate dimensions and appro- 
priately sized for the hook. Gangions were deployed so 
that each bait type alternated along the length of the 
mainline (i.e., northern shortfin squid, Atlantic mack- 
erel, northern shortfin squid, Atlantic mackerel), and 
the starting bait type was selected randomly for each 
longline set. 
Gear soak time, defined as time elapsed between de- 
ployment of the terminal high flyer during gear deploy- 
ment and retrieval of the first high flyer during haul- 
back, was approximately 1 h at sampling sites with 
depths less than 400 m and 2 h at sampling sites with 
depths greater than 400 m. However, the actual time 
each hook spends in the water can vary due to a num- 
ber of factors, such as hook position along the mainline, 
differences in gear setting and retrieving speeds, and 
delays related to handling times that were associated 
with the number of organisms captured. Therefore, the 
time each hook entered the water at deployment and 
exited the water during retrieval was electronically 
monitored. The elapsed time between deployment and 
retrieval of each hook was considered hook soak time. 
The status of each retrieved hook was monitored 
and recorded as whole bait present, partial bait pres- 
ent, no bait present, missing hook, or organism cap- 
tured. To determine whether baits of Atlantic mackerel 
and northern shortfin squid were retained equally on 
hooks, data from gangions classified with a status of 
whole bait present were compared by using chi-square 
tests with Yates correction for continuity. Using the 
same test, we investigated potential differences in 
bait retention between Atlantic mackerel and northern 
shortfin squid when there were hook interactions with 
feeding organisms other than those retained on a hook. 
For this investigation, the categories partial bait pres- 
ent and no bait present were combined. We combined 
them because of the subjective nature of the partial 
bait present category (i.e., the status of a hook was 
classified the same whether, for example, a small piece 
of bait tissue had been removed or most of the bait 
had been removed). The category missing hook was not 
included in analyses because, in the limited number 
of cases for which this status was recorded, the hook 
1 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. 
