Hart et al.: Abundance, distribution, and activity patterns of demersal fishes on Hecate Bank, Oregon 
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(Moulton, 1977), on rocky kelp forests off Santa Bar- 
bara, California (Ebeling and Bray, 1976), and on Santa 
Catalina Island, California (Hobson and Chess, 1976; 
Hobson et al., 1981), the small-size adult and juvenile 
rockfish that remain exposed from refuge during the 
night stay close to the seafloor. 
Only two taxa showed distinct day and night activity 
patterns (diurnal: rosethorn rockfish and nocturnal: 
hagfish), indicating that marked differences in day 
and night activity are not as prevalent as those found 
in shallow tropical coral reef and temperate fish com- 
munities (Table 2) (Hobson, 1972; Helfman, 1978). The 
diurnal activity pattern exhibited by rosethorn rockfish 
was independent of habitat, indicating that habitat type 
may not be a significant factor in determining differ- 
ences between day and night activities for this species 
(Tables 1 and 2). The nocturnal activity pattern that 
hagfishes exhibited was similar to that observed by 
Ooka-Souda et al. (1985) of another hagfish species 
(Eptatretus burgeri ), in the laboratory. During the day 
on Heceta Bank, most hagfish were observed coiled up 
over mud or sand, or around cobble, whereas during 
the night fish of this taxon was observed swimming 
above bottom or moving in contact with the bottom 
in a twisted manner. Like silver hake ( Merluccius bi- 
linearis) which use sand waves (transverse ridges of 
sand) for refuge during the day (Auster et al., 2003) 
and that forage during the night (Bowman and Bow- 
man, 1980; Auster et al., 1995), hagfishes on Heceta 
Bank may use mid-depth cobble and mud habitats for 
resting during the day and may forage at night. Not 
all rosethorn rockfish and hagfishes strictly followed a 
diurnal or nocturnal activity pattern; however, the fish 
activity measure we used was not sensitive enough to 
detect subtle differences in behavior, such as resting 
individuals found hovering close to but not in contact 
with the seafloor. 
Potential bias exists in observing and attempting 
to quantify activity in fishes when using video survey 
methods (Uzmann et al., 1977; Sale and Douglas, 1981; 
Wakefield and Smith, 1990). Our inability to identify 
many of the rockfishes to species is potentially problem- 
atic, and illustrates a limitation of this survey method. 
The majority of historical studies show that very few 
fishes exhibit changes in activity with the presence of 
an ROV or submersible, although a handful of taxa do 
show behavioral responses (High, 1980; Carlson and 
Straty, 1981; Pearcy et al., 1989). SCUBA-based video 
surveys of fish abundance and behavior indicate that 
although the majority of fishes show no noticeable reac- 
tion, some species may avoid or be attracted to divers 
outside the camera’s field of visibility and may even 
follow divers (Moulton, 1977). It has been argued that 
SCUBA surveys do not significantly affect counts be- 
cause most fishes that follow divers remain behind the 
field of view of the camera (Powles and Barans, 1980). 
In our study, anecdotal evidence indicated that the 
ROV had limited effect on fish behavior, except in cases 
where the ROV came in contact with the substrata. 
Further investigation is needed, however, to fully grasp 
the impacts of observational vehicles on fish responses 
(Stoner et al., 2008). 
Implications for groundfish surveys 
Day and night activity patterns in demersal fishes have 
been shown to dramatically change the catchability of 
some species on the West Coast (Hannah et al., 2005), 
in the Northwest Atlantic (Bowman and Bowman, 1980), 
in Newfoundland (Casey and Myers, 1998), and in the 
North Sea (Petrakis et al., 2001). In this study, we 
found that daytime surveys could underestimate the 
abundance of certain species that are more abundant 
or active at night, such as sharpchin rockfish. Highly 
significant differences in day and night abundance 
of schooling rockfishes found in this study indicate 
that daytime trawl surveys over small- to medium-size 
habitat features may be biased for some fish species. 
Migration of fishes into the overlying water column, hori- 
zontally off the bank, or into hiding among medium- to 
large-size features is likely the most common day and 
night behaviors that would decrease the availability of 
fishes to the ROV. Specifically, Puget Sound rockfish may 
be more available over deeper, smaller-size rock struc- 
tures, whereas other dominant day taxa (pygmy rockfish, 
pygmy-Puget Sound rockfish complex, and unidentified 
juvenile rockfish) and sharpchin rockfish are likely less 
available to trawl surveys over large-size features in 
shallower portions of the bank. We speculate that the 
reduction in abundance of the four most dominant day 
taxa is due to fish seeking refuge around medium- to 
larger-size structures because of the potential presence 
of large piscivores (Wilkins, 1986; Adams, 1987), rather 
than to schooling in the water column. 
Acknowledgments 
We would especially like to thank the following col- 
leagues who contributed to this study: B. Barss, B. 
Embley, G. Hendler, M. Hixon, D. Markle, B. McCune, S. 
Merle, B. Tissot, M. Yoklavich, and K. York. A. Whitmire 
and T. Cowles generously contributed information on 
ambient light levels. L. Britt, C. Whitmire, and L. Cian- 
nelli provided constructive reviews of the manuscript. 
This portion of the Heceta Bank project was funded by 
the West Coast and Polar Regions Undersea Research 
Center of the National Oceanographic and Atmospheric 
Administration’s (NOAA) National Undersea Research 
Program, the Northwest and Southwest Fisheries Sci- 
ence Centers, NOAA’s Pacific Marine Environmental 
Laboratory, and the Cooperative Institute for Marine 
Resources Studies at Oregon State University. We would 
like to thank the professional personnel who operated 
the ROV ROPOS and the NOAA RV Ronald Brown. T. 
Hart was supported through the Oregon Agricultural 
Experiment Station project ORE00102, the H. Richard 
Carlson Scholarship, the Collaborative Marine Fisher- 
ies Fellowship and the Bill Wick Award through Oregon 
State University. 
