Comparison of Fish survey methods
Surveys were conducted at the ExxonMobil Platform Harmony located in
the Santa Barbara Channel (34°22'N, 120°10'W), southern California
(Figure 1). Harmony was installed in 1989, is 10.3 km from shore,
and sits in a bottom depth of about 363 m. To reduce possible
variation, the two comparison surveys were conducted by the same
ROV, at a single platform, over the same depths, and on the same
days. The research was conducted along crossbeams during daylight
hours on 25–26 August 2018 at water column depths of 17 m, 38 m, 61
m, and 182 m, and at the bottom-most crossbeam at 363 m. At each
depth, we surveyed the north, west, and south sides of the platform
(i.e., the east side was not surveyed). We used a typical industry
work-class Comanche-type ROV for these surveys.
Using two survey methods, that we termed “biological” (UCSB survey
method in the data table) and “industry,” we compared the densities
and diversities of fishes associated with these crossbeams. The
biological surveys were based on methodologies
we have utilized when surveying fishes around California platforms
using both manned submersibles and an ROV. In the biological method,
the ROV traveled parallel to a platform crossbeam, the camera was
aimed at a 90° angle to that crossbeam, and the ROV remained about 2
m from the structure. Importantly, when surveying the bottom-most
crossbeam (which sat on or just off the sea floor), both the sea
floor bottom-crossbeam interface and any crossbeam undercut were
kept in the field of view. This was because research has shown that
off California, fishes dwelling around platform bases tend to
associate with those portions of the bottom crossbeam that are
undercut (creating a crevice) rather than those sections where there
is no gap. We note that in the biological method, the upper part of
the bottom crossbeam was sometimes not visible.
The other, industry, technique was designed to
replicate industry platform inspection surveys. The goal of industry
inspections is to examine the jacket, including the crossbeams, for
structural integrity issues. When using the industry method, we
requested that the ROV pilot, who had conducted many industry
platform surveys with this type of device, conduct a survey of the
crossbeams as if he were conducting an industry survey. The major
differences between the two methods were 1) during the industry
survey the pilot tended to angle the ROV such that, as the ROV moved
forward, the pilot could see what was ahead as the ROV traversed a
crossbeam. In practice, this meant that the ROV often, although not
always, maintained an angle of less than 90° to the crossbeam
(compared to a camera angle of 90° during the biological surveys).
2) The distance from the ROV to the jacket was variable and was
often closer to the crossbeam than in the biological inspection. 3)
Importantly when surveying the bottommost
crossbeam (immediately adjacent to the sea floor), the pilot tended
to remain somewhat above the sea floor, higher than for our
biological surveys. Thus, compared to the biological method, in the
industry survey the bottommost crossbeam was viewed from a higher
vantage and the sea floor-crossbeam interface was sometimes not
visible and the crevice under that crossbeam (if present) was never
visible.
