62 
Fishery Bulletin 1 1 1 (1) 
On seafloor 
o 
CD 
CD 
.2 
c: 
CD 
o 
CD 
CL 
Near seafloor 
■ Submersible (410 fishes, 8%) □ ROV (4083 fishes, 59%) 
50 - 
45 - 
40 - 
Toward Away Left Right Down 
CD 
CD 
03 
CD 
CL 
Midwater 
Submersible (724 fishes, 22%) □ ROV (461 1 fishes, 73%) 
50 - 
Toward Away Left Right Down 
Figure 4 
Percentage of fishes that reacted in a particular direction 
to the manned submersible or the remotely operated vehicle 
(ROV) that were used in 2007 off central California in our 
study of the reactions of fishes to underwater vehicles rela- 
tive to the initial position of those fishes: on the seafloor, near 
(<1 m above) the seafloor, or in the midwater (>1 m above the 
seafloor). Total number of fishes that reacted to each vehicle, 
and the percentage of the total number of fishes in the survey 
that reacted, are shown in parentheses for each initial position. 
es in natural behavior” (Lorance and Trenkel, 
2006) and as “a marked change in activity level 
and/or locomotion behavior” in the other article 
(Uiblein et al., 2003). General categories of re- 
actions (such as a fish avoided or was attracted 
to a vehicle, or a fish had no reaction) were 
used in 6 studies (Adams et al., 1995; Trenkel 
et al., 2004a; Trenkel et al., 2004b; Costello et 
al., 2005; Trenkel and Lorance, 2011, Baker et 
al., 2012), but specific definitions of the reac- 
tions (in contrast to natural movements) were 
not reported for these studies. 
In our surveys, reaction of a nonmoving fish 
was defined as a distinct movement greater 
than one body length. We used this proportional 
measure instead of a specific distance because 
the total length of observed fishes varied from 
5 to 100 cm. The use of our definition of a reac- 
tion as at least one body length could be prob- 
lematic, especially for quantification of relative- 
ly small movements. However, in our study, the 
minimum distance that any fish traveled was 
0.5 m in reaction to the submersible (with 96% 
of these fishes moving 1 m or greater) and 1.0 
m in reaction to the ROV. Therefore, the reac- 
tions of even the smallest fishes could be read- 
ily discerned. 
It can be argued that a fish in motion when 
first seen in a video footage was already mov- 
ing in reaction to the survey vehicles (Uiblein 
et al., 2003; Lorance and Trenkel, 2006). In our 
study, we surveyed numerous benthopelagic 
species that were slowly moving when first 
observed in the video footage. Such movement 
was not considered a reaction unless a fish ob- 
viously changed course or speed. Because a fish 
could not be seen before it came into view on a 
video footage, it could not be determined if that 
fish was initially motionless and then reacted 
as the vehicle approached. This type of behav- 
ior could be indicated by signs like a dust cloud 
where a fish had contact with the seafloor, a 
fish quickly darting into the video footage, or 
loose aggregations of fishes moving in many dif- 
ferent directions. In our study, these types of 
behavior were rarely, if ever, observed. 
Few quantitative studies have been con- 
ducted on fish reactions to a submersible or an 
ROV, and no direct comparisons between the 
reactions of specific fish species to a submers- 
ible and ROV have been found in the literature. 
General reactions to an ROV (fishes moving 
into and out of a video frame) were quantified 
during surveys on mud habitats off central Cal- 
ifornia (Adams et al., 1995). In that study, most 
fishes that occurred on the seafloor did not re- 
act to a relatively large working-class ROV, al- 
though 2 species typically observed off the sea- 
floor exhibited avoidance behavior: 44% of all 
