Driggers and Hannan: Efficacy of 2 common bait types in reducing bycatch of coastal sharks 
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3 t Blacktip shark 
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Figure 2 
Length-frequency distributions of sharks captured on longline sets baited with Atlantic mackerel 
(Scomber scombrus) (gray bars) and northern shortfin squid (Ilex illecebrosus ) (white bars) in 
the northern Gulf of Mexico during Mareh-April 2017. There were no significant species-specific 
differences in the median fork length (FL) of sharks caught on the 2 bait types, with the exception 
of Atlantic sharpnose sharks (Rhizoprionodon terraenovae ), which had a larger median FL on 
mackerel-baited hooks. Note that the scales of x- and y-axes differ among species. 
Gulf smoothhound; however, Cortes (1999) summarized 
the diet of 13 species within the morphologically conser¬ 
vative genus Mustelus and demonstrated that these 
fishes have a diverse diet. For example, dietary studies 
of the smooth dogfish, which is sympatric with and has 
the same dental morphology as the Gulf smoothhound, 
indicate that its diet consists of decapod crustaceans, 
teleosts, and non-decapod invertebrates in decreasing 
order (Cortes, 1999). 
Pitcher et al. (1982) empirically tested the effect of 
shoal size on the amount of time it took goldfish ( Caras- 
sius auratus ) and minnows (Phoxinus phoxinus ) to locate 
a food source, and they found that foraging efficiency 
increased with shoal size for both species. Among coastal 
shark species captured during 3500 fishery-independent 
bottom longline sets conducted throughout the northern 
Gulf of Mexico as part of a survey by the National Marine 
Fisheries Service, Southeast Fisheries Science Center, 
Mississippi Laboratories, the Atlantic sharpnose shark 
is not only the most commonly caught species (re=15,494 
individuals) but also the species most commonly caught 
in schools of 10 individuals or larger (number of sets=513, 
school size range of 10-72 sharks). If there is a link 
between shoal size and preferred bait detection for Atlan¬ 
tic sharpnose sharks, as our results indicate, it is possi¬ 
ble that other species have a preference for a specific bait 
when shoaling. Because individuals of some shark species, 
such as the blacktip shark, spinner shark (Carcharhinus 
brevipinna), and scalloped hammerhead, form large shoals 
within the Gulf of Mexico during certain times of the year 
(senior author, unpubl. data), it is possible that bait prefer¬ 
ence could be exhibited during those periods but absent at 
other times when individuals are more dispersed. Future 
research should examine this hypothesis and test how the 
use of bait type as a bycatch mitigation measure can vary 
seasonally. 
The combined results of this study and Driggers et al. 
(2017) provide the first comparison, as far as we are aware 
of, between the effect of bait type on catch rates of sharks 
caught on standardized longline gear with single-bait-type 
