192 
Fishery Bulletin 117(3) 
Table 2 
Number of individuals (n), size range (millimeters fork length [FL]), mean size (mm FL), median size (mm FL), and test statistics 
for species caught on longline sets baited with Atlantic mackerel (Scomber scombrus) or northern shortfin squid (Ilex illecebrosus ) 
in the northern Gulf of Mexico during March-April 2017. Statistics from t-tests, Mann-Whitney W tests, and chi-square tests are 
presented with associated P-values in parentheses. An asterisk (*) indicates data that did not meet the assumptions of normality 
or homoscedasticity. 
Species 
Bait type 
n 
Size range 
Mean (SE) 
Median 
t-value ( P) 
W test (P) 
Chi-square test ( P) 
Gulf smoothhound 
Mackerel 
Squid 
51 
39 
573-1207 
402-1143 
902.4 (19.8) 
867.7 (21.3) 
900.0 
819.0 
1.70 (0.08) 
769.0 (0.07) 
1.34 (0.25) 
Atlantic sharpnose 
shark 
Mackerel 
Squid 
488 
302 
323-970 
487-961 
793.8 (3.1) 
759.5 (5.0) 
806.0 
781.0 
6.22 (<0.Q1)* 
49,994.4 (<0.01) 
43.32 (<0.01) 
Blacknose shark 
Mackerel 
Squid 
33 
21 
584-1057 
589-1015 
861.1 (21.8) 
855.6 (33.8) 
884.0 
922.0 
0.14(0.89) 
363.0 (0.78) 
2.24 (0.13) 
Blacktip shark 
Mackerel 
Squid 
18 
13 
990-1214 
1015-1250 
1100.3(62.1) 
1125.9(23.7) 
1115.0 
1096.0 
-0.98 (0.33) 
148.5 (0.58) 
0.52 (0.47) 
Scalloped 
hammerhead 
Mackerel 
Squid 
46 
53 
643-1880 
646-1820 
1254.5 (45.8) 
1243.5 (43.5) 
1237.0 
1210.0 
0.17(0.86) 
1177.0(0.77) 
0.36 (0.55) 
Red snapper 
Mackerel 
Squid 
198 
202 
530-880 
605-881 
708.1 (4.4) 
705.3 (3.4) 
706.0 
699.5 
0.49 (0.62)* 
17,554.0 (0.33) 
0.02 (0.88) 
northern shortfin squid bait was evident for Atlantic 
sharpnose sharks both when not accounting for and when 
accounting for sets with unusually high catches. This 
result, in agreement with the findings of Driggers et al. 
(2017), demonstrates that bait type is a viable option for 
shark bycatch reduction albeit on a species-specific basis. 
Further, the catch of red snapper was not affected by bait 
type in this study or in Driggers et al. (2017), indicating 
that shark catch can be reduced without affecting the 
catch of target species when bait type is used as a bycatch 
mitigation measure. 
For blacknose sharks, there was no difference in catch 
rates between bait types unless unusually high catch rates 
identified as outliers were removed from the data set. 
Driggers et al. (2017) reported that, when both baits were 
alternated on a single set, the catch of blacknose sharks 
was over 5 times greater when Atlantic mackerel was used 
for bait compared with catch when northern shortfin squid 
was used. This finding is consistent with past reports that 
the diet of blacknose sharks is primarily piscivorous (e.g., 
Castro, 2011). For example, Dodrill (1977) reported that 
stomach contents of all blacknose sharks he examined 
contained teleost prey. Similarly, Cortes (1999) reported 
that the diet of blacknose sharks consisted of 98.2% tele- 
osts, with less than 2% consisting of crustaceans and non- 
cephalopod mollusks. Ford (2012), on the basis of a sample 
size larger than the combined sample size reported in the 
Dodrill (1977) and Cortes (1999) studies, found that fish 
compose 63.4% of the diet of blacknose sharks off the East 
Coast of the United States, with the remainder composed 
of invertebrates, including squid and octopods (11.4%). 
Results of our study and those of Ford (2012) indi¬ 
cate that blacknose sharks are more opportunistic than 
previously thought. Additionally, when considered in 
conjunction with the findings of Driggers et al. (2017), 
although the species shows a clear preference for tele- 
osts, blacknose sharks will opportunistically prey on 
other organisms when their preferred prey is not present. 
Although there were fewer blacknose sharks captured on 
sets baited with northern shortfin squid during our study, 
the difference in capture rates between the 2 baits was not 
significantly different. Therefore, on the basis of the data 
from our study, bait choice does not appear to be an effi¬ 
cient bycatch mitigation measure for the blacknose shark. 
However, it is important to note that only 54 blacknose 
sharks were captured during this study, and if a real effect 
was present, it could have been obfuscated by the rela¬ 
tively low sample size. 
Like blacknose sharks, blacktip sharks are reported to 
feed primarily on teleosts (e.g., Castro, 1996; Bethea 
et al., 2004; Barry et al., 2008). For example, Hoffmayer 
and Parsons (2003) examined the diet of blacktip sharks 
in the Mississippi Sound and determined that 95% of the 
diet of this species was composed of teleosts. Results of 
this study demonstrate that blacktip sharks are oppor¬ 
tunistic and, although they specialize in feeding on tele¬ 
osts, they will feed on both squid and teleost baits, 
negating the value of bait type as a bycatch mitigation 
measure for this species. A similar conclusion was 
reached for the effect of bait type on catch rates of the 
Gulf smoothhound, scalloped hammerhead, and red 
snapper. This result was not unexpected because scal¬ 
loped hammerheads and red snapper are known to have 
relatively wide dietary breadths, including teleosts and 
cephalopods (Stevens and Lyle, 1989; Szedlmayer and 
Lee, 2004; Wells et al., 2008; Vaske et al., 2009; 
Torres-Rojas et ah, 2010). Further, to our knowledge, no 
species-specific diet studies have been conducted for the 
