52 
Fishery Bulletin 1 15(1) 
Table 1 
Catch composition and length ranges, by bait type, of fish species caught on bottom longline gear in the northern Gulf of 
Mexico in March and April 2015. The bait types are Atlantic mackerel {Scomber scombrus) and northern shortfin squid {Illex 
illecebrosus). All measurements are reported in fork length, with the exceptions of values for tilefish {Lopholatilus chamae- 
leonticeps) and red drum (Sciaenops ocellatus) that are reported in total length, as well as values for clearnose skates {Raja 
eglanteria) that are reported in disc width. Results of Kolmogorov-Smirnov (K-S), Mann-¥/hitney (M-W), and chi-square 
(with Yates correction) tests, used to test for differences in species-specific length distributions, median lengths, and catch 
rates between bait types, respectively, are provided with associated P-values. 
Species 
Atlantic mackerel 
n size range (mm) 
Northern shortfin squid 
n size range (mm) 
K-S 
{P) 
M-W 
(P) 
Chi- 
square {P) 
Carcharhinus acronotus 
61 
563-1053 
12 
587-1061 
1.19 (0.12) 
238.0 (0.06) 
31.56 (<0.01) 
Carcharhinus plumbeus 
45 
1025-1800 
7 
1183-1546 
0.95 (0.33) 
98.0 (0.13) 
26.33 (<0.01) 
Centrophorous uyato 
43 
610-951 
21 
478-948 
0.77 (0.59) 
419.5 (0.65) 
6.89 (<0.01) 
Galeocerdo cuvier 
20 
910-2620 
10 
785-1812 
1.12 (0.16) 
52.5 (0.05) 
2.70 (0.10) 
Hyporthodus flavolimbatus 
14 
593-865 
6 
560-795 
0.62 (0.83) 
31.5 (0.83) 
2.45 (0.12) 
Lopholatilus chamaeleonticeps 
12 
497-895 
14 
465-905 
0.70 (0.72) 
97.0 (0.52) 
0.04 (0.85) 
Lutjanus campechanus 
56 
370-876 
45 
536-860 
0.55 (0.92) 
1208.0 (0.89) 
0.99 (0.32) 
Mustelus canis 
18 
754-1187 
6 
810-1081 
0.86 (0.45) 
39.0 (0.42) 
5.04 (0.02) 
Mustelus sinusmexicanus 
40 
686-1167 
24 
713-1120 
0.76 (0.62) 
379.0 (0.34) 
3.52 (0.06) 
Raja eglanteria 
7 
384-410 
16 
349-446 
0.78 (0.57) 
47.0 (0.97) 
2.78 (0.09) 
Rhizoprionodon terraenovae 
344 
574-895 
139 
486-860 
0.69 (0.73) 
21,969.0 (0.80) 
86.16 (<0.01) 
Sciaenops ocellatus 
5 
825-910 
21 
838-1005 
1.32 (0.06) 
82.0 (0.06) 
8.65 (<0.01) 
Sphyrna lewini 
12 
860-1890 
9 
750-1860 
1.13 (0.14) 
78.0 (0.09) 
0.19 (0.66) 
Squalus complex 
21 
410-680 
10 
445-655 
0.64 (0.80) 
89.5 (0.66) 
3.22 (0.07) 
could have been hung on the bottom and, as a result, 
gear failure could have occurred during retrieval. Ad- 
ditionally, to determine whether retention of the 2 bait 
types differed with increasing soak time, the distribu- 
tion of hook soak times for hooks retrieved with whole 
bait present of Atlantic mackerel was compared to the 
distribution of soak times for hooks retrieved with 
whole bait present of northern shortfin squid by using 
a Kolmogorov-Smirnov test. 
All captured individuals were identified to the low- 
est possible taxonomic level and measured to the near- 
est millimeter. Fork length (FL) and total length (TL) 
were measured from the tip of the rostrum to the cau- 
dal notch and to the tip of the upper lobe of the cau- 
dal fin while the fin was fully extended, respectively. 
For batoids, disc width was measured between apices 
of the pectoral fins. Species-specific identifications were 
not possible in a limited number of instances as a re- 
sult of a fish escaping a hook before being landed (i.e., 
catch was confirmed but identification was not possi- 
ble). In those cases, all individuals could be identified 
to at least the family level. A Kolmogorov-Smirnov test 
was used to test for differences in the species-specific 
length distributions of individuals caught on each bait 
type. When length distribution data of a species were 
normally distributed (as indicated by values of kurtosis 
and skewness being between -2 and 2) and homosce- 
dastic (as assessed with an F-test), a ^-test was used 
to determine whether differences existed in species-spe- 
cific mean length at capture for each bait type. When 
length distribution data failed to meet the assumptions 
of parametric statistics, a Mann-Whitney W test was 
used to compare median length of each species caught 
by bait type. j 
Chi-square tests with Yates correction for continuity i 
v/ere used to determine whether there was a bait-relat- 
ed effect on species-specific catch rates. Species includ- 
ed in analyses were limited to those that had a mini- 
mum of 20 individuals captured regardless of bait type 
(Table 1). Additionally, because of low species-specific 
capture rates and the close morphological similarities i 
of the Cuban dogfish (Squalus cubensis) and the short- 
spine spurdog {S. mitsukurii) (both species with fewer 
than 20 individuals captured), these squalid shark spe- 
cies were treated as a complex. For those species that 
showed a significant bait preference and were captured 
on a minimum of 10 longline sets, we examined if there 
was a change in the degree of bait preference with in- 
creasing total catch. For this examination, an index of 
bait preference (IBP) was calculated with the following 
equation: 
IBP = {number of individuals captured on mackerel 
- number of individuals captured on squid) 
/ total number of individuals captured. 
The IBP values ranged from 1 (all individuals cap- 
tured with Atlantic mackerel bait) to -1 (all individuals 
captured with northern shortfin squid bait). The value 
of 0 indicated that an equal number of individuals were 
caught on both bait types. To obtain a nonparametric 
estimation of the relationship between species-specif- 
ic IBP and total catch, a locally weighted scatterplot 
