Laidig et al Reactions of fishes to a manned submersible and a remotely operated vehicle 
61 
20 cm or less in length (68% of all fish- 
es in the submersible surveys and 89% 
in the ROV surveys). The fishes that 
were <20 cm in total length accounted 
for 75% of all reactions observed from 
the submersible and 94% of all reac- 
tions observed in video footage from 
the ROV surveys. 
80 
70 
60 
= 50 
40 
30 - 
20 
10 - 
I Submersible (n = 10,550) □ ROV (n = 1 6, 1 58) 
(n= 6976) 
(n= 6287) 
(n= 2895) 
(n=6009) 
(n=3280) 
1 
On seafloor 
Near seafloor 
Midwater 
Figure 3 
Percentages of fishes that reacted to the manned submersible or the re- 
motely operated vehicle (ROV) that were used in 2007 off central Cali- 
fornia 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). The 
total number of reactions is indicated for each vehicle. Numbers above 
bars indicate the total number of fishes (i.e., sample size) in each cat- 
egory for each vehicle. 
Discussion 
Although fishes reacted to both sur- 
vey vehicles, there were proportionally 
greater numbers of reactions to the 
ROV than to the submersible. The ROV 
and submersible traveled at similar 
speeds and maintained similar heights 
off the seafloor, yet substantial differ- 
ences were observed in fish reactions 
to the 2 vehicles. Possible reasons for 
these differences in reactions include 
the presence of a tether that attach- 
es the ROV to the support ship (the 
manned submersible is autonomous 
and untethered), forward lighting on 
the ROV compared with lighting large- 
ly on the starboard side of the sub- 
mersible, differences in vehicle noise, 
and disparity in vehicle dimensions. 
Both vehicles were much larger than 
common predators (e.g., large fishes and pinnipeds) of 
most of these species, and we, therefore, surmise that 
size alone was not the factor that caused fishes to re- 
act. It is possible that the smaller ROV, which was 
about one-half the height and length of the submers- 
ible, appeared to be more like a large predator to the 
fishes than did the submersible, but this idea is dif- 
ficult to establish. 
The magnitude of pressure waves generated in front 
of each vehicle could have differed because the submers- 
ible was of solid construction and the ROV comprised a 
frame with attached instruments and a trailing tether. 
Indeed, pressure waves generated from a deepwater 
drop-camera system that operated about 130 m above a 
midwater aggregation of Orange Roughy ( Hoplostethus 
atlanticus) off Tasmania caused those fishes to disperse 
rapidly up to 40 m (Koslow et al. 1995). 
Fish reactions to vehicles can also depend on envi- 
ronmental conditions (e.g., type of seafloor sediments, 
relief, ambient light levels, and water currents) and 
some attributes of the survey itself (e.g., vehicle speed 
and height off the seafloor). To reduce the effects of 
some of these conditions, we surveyed only during day- 
light hours, in similar habitats, during the same time 
of the same year, at similar speeds, and at similar 
heights off the seafloor. 
Whatever the reasons that fishes react to survey ve- 
hicles, the reaction of the target species must be con- 
sidered in selection of underwater vehicles to conduct 
surveys on fish abundance. Population abundance can 
be either over-or under-estimated if fish reactions to 
the survey vehicles are not quantified. Once the reac- 
tion rates are determined, correction factors can be 
developed to account for species-specific differences in 
reaction to the survey vehicles and to adjust resultant 
abundance estimates. Knowledge of fish reactions as- 
sociated with each survey tool can help ascertain the 
most appropriate survey method for target species. 
Clear description and quantification of fish reactions 
to underwater survey vehicles are not common in the 
literature. From a review of the literature, fish reac- 
tions were defined in only 2 of 37 published papers that 
reported on the reactions of fishes to underwater vehi- 
cles (see review in Stoner et al. 2008; Davis et a!., 1997; 
Krieger and Ito, 1999; Else et al., 2002; Moore et al., 
2002; Uiblein et al., 2003; Costello et al., 2005; Gartner 
et al., 2008; Luck and Pietsch, 2008; Benefield et al., 
2009; Trenkel and Lorance, 2011; Baker et al., 2012; 
O’Connell et al. 2 ). A fish reaction was defined in one of 
these 2 articles as a “disturbed” behavior or “differenc- 
2 O’Connell, V., D. Carlile, and C. Brylinsky. 2001. Demersal 
shelf rockfish stock assessment and fishery evaluation 
report for 2002. Regional Information Report 1J01-35, 
42 p. Alaska Dept. Fish Game, Division of Commercial 
Fisheries, Juneau, AK. 
