A comparison of two remotely operated vehicle (ROV) survey methods used to estimate fish assemblages and densities around a California oil platform.

Offshore oil and gas platforms have a finite life of production operations. Once production ceases, decommissioning options for the platform are assessed. The role that a platform's jacket plays as fish habitat can inform the decommissioning decision. In this study, conducted along the crossbeams of a California platform jacket and using an ROV, we compared estimates of fish diversity and densities determined from a targeted "biological" survey with those from a replicated "structural" survey. We found that the water column fish species assemblages characterized by the two methods were similar. By contrast, the two survey methods yielded different species assemblages inhabiting the crossbeam at the platform jacket base. This difference occurred because, at least off California, the platform jacket base species diversity tends to be highest where the bottom crossbeam is undercut, creating sheltering sites for many species. Because the structural method inadequately imaged the seafloor-crossbeam interface, particularly where a gap occurred between crossbeam and seafloor, substantial numbers of fishes were not visible. While we cannot extrapolate from this study to all platforms' worldwide, it is clear that routine platform structural integrity surveys may be a valuable source for opportunistic marine community surveys. Intentional planning of the structural survey to incorporate relatively minor variations (e.g., maintaining fixed ROV distance from the infrastructure and consistent 90° camera angle) coupled with a deliberate consideration of the platform ecology (e.g., positioning the ROV to capture the seafloor-crossbeam interface) can substantially improve the effects on fish assemblage assessments from routine structural surveys without compromising the integrity assessment. We suggest that these biases should be both acknowledged and, understood when using routine structural surveys to inform platform ecology assessment. Additional consideration may be given to structural surveys that incorporate incremental adjustments to provide better data applicability to biological assessments.

Introduction

Each of the thousands of offshore oil and gas platforms worldwide has a finite production life. Once a decision is made to cease production, governmental agencies undertake a decommissioning process to decide on the disposition of that platform jacket (most often either partial or total removal, [1]). The role that a platform’s jacket plays as fish and invertebrate habitat can be part of that decision-making process [1], and the increasing number of platforms to be decommissioned increases the need for this information. Historically, these characterizations have come from targeted biological surveys (e.g., conducted by scuba divers, manned submersibles, remotely operated vehicles (ROVs), bioacoustics, and nets) of a limited number of offshore oil and gas structures [2-7]. However, there also exists oil and gas ROV engineering surveys of platforms and associated infrastructure (i.e., pipelines, subsea equipment and wellheads) conducted as part of routine physical integrity inspections. In the past few years, researchers off western Australia [8-12] and in the North Sea [13, 14] have begun to use this archival footage to characterize the biological communities associated with the offshore oil and gas structures.

This relatively recent development has led to discussions on the value of using industry structural surveys for biological assessments [12, 15, 16]. While this research makes clear that structural surveys can be useful in surveying marine life around platform jackets, there remain questions regarding the potential biases of this methodology [15, 16]. Thus, there is a need for studies that compare biological data from targeted biological surveys with those taken to assess structural integrity. In this study, using an ROV, we conducted a pilot project that compared estimates of fish diversity and densities determined from a targeted biological survey with those from a replicated structural survey at a platform jacket.

Materials and methods

Surveys were conducted at the ExxonMobil Platform Harmony located in the Santa Barbara Channel (34°22'N, 120°10'W), southern California (Fig 1). Harmony was installed in 1989, is 10.3 km from shore, and sits at a bottom depth of about 363 m [17]. 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. We used a work-class Comanche-type ROV for these surveys. All surveys were conducted at the same speed (about 0.5 knots) using a SubC 1CamMk5 HDf video camera and lights at all depths. 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 (crossbeam lengths are shown in Table 1). At each depth, we surveyed the north, west, and south sides of the platform jacket (i.e., the east side was not surveyed). All surveys were conducted during daylight hours, and decisions regarding which side was surveyed first, which method was first used on a specific side, and the length of time between surveys on a specific side were made haphazardly (Table 2).

Fig 1

Location of Platform Harmony, Santa Barbara Channel, southern California.

Table 1

Crossbeams included in 2018 ROV surveys
Depth (m)	North and South (m)	West (m)
17	46.0	28.9
38	50.3	32.6
61	54.9	36.6
182	79.2	58.1
363	117.3	91.6

Lengths of Platform Harmony crossbeams surveyed in this study. The east crossbeam was not surveyed.

Table 2

Depth (m)	Side of platform	1st pass	2nd pass	1st pass ends	2nd pass begins	Elapsed time between passes	Notes
17	North	1	4	15:35:53	16:08:44	0:32:51	"Pass" refers to transect number
17	West	2	5	15:42:03	16:30:00	0:47:57	Color coding:
17	South	3	6	15:50:00	16:44:29	0:54:29	Structural survey
38	North	30	35	17:12:02	17:55:35	0:43:33	Biological survey
38	West	31	34	17:20:05	17:49:05	0:29:00
38	South	32	33	17:29:58	17:35:26	0:05:28
61	North	24	25	15:27:02	15:30:25	0:03:23
61	West	26	27	15:50:50	15:53:46	0:02:56
61	South	28	29	16:17:25	16:19:45	0:02:20
183	North	18	19	12:51:30	12:55:45	0:04:15
183	West	20	21	13:24:16	13:27:36	0:03:20
183	South	22	23	14:03:59	14:06:01	0:02:02
372	West	8	12	19:01:45	22:07:00	3:05:15
372	North	9	11	19:26:57	20:25:30	0:58:33
372	South	10	13	20:01:05	22:32:37	2:31:32

Sequence of crossbeam surveys, “structural” and “biological,” conducted at Platform Harmony. Included are 1) crossbeam depth, 2) crossbeam designation, 3) order of the two surveys (structural or biological) at a given crossbeam, 4) time of survey, and 5) time between surveys at a specific crossbeam.

Using two survey methods, that we termed “biological” and “structural,” 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 [2] and an ROV [18, 19]. 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 (Fig 2). Importantly, when surveying the bottom-most crossbeam (which sat on or just off the seafloor), both the seafloor 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 [19]. We note that in the biological method, the upper part of the bottom crossbeam was sometimes not visible.

Fig 2

A downward looking view of the orientation of the ROV along horizontal crossbeams in “biological” and “structural” surveys.

In the biological surveys, the camera was oriented at a 90° angle to the crossbeam and the ROV was kept at about 2 m from the jacket. In a simulation of structural surveys, the camera angle to the crossbeam varied but was centered around 45° and the ROV was usually less than 1 m from the crossbeam.

The structural technique was designed to replicate an industry engineering platform inspection survey. The goal of structural inspections is to examine the oil and gas infrastructure including the jacket crossbeams, for structural integrity issues. When using the structural method, we requested that the ROV pilot, who had conducted many ROV structural platform surveys, conduct a survey of the crossbeams as if he were conducting a typical structural survey. The major differences between the two methods were 1) during the structural 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 (Fig 2). In practice, this meant that the camera angle to the crossbeam varied but was centered around 45° (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 (primarily 1 m or less) than in the biological inspection. 3) Importantly when surveying the bottom-most crossbeam (immediately adjacent to the seafloor), the pilot tended to remain somewhat above the seafloor, higher than for the biological surveys. Thus, compared to the biological method, in the structural survey the bottommost crossbeam was viewed from a higher vantage and the seafloor-crossbeam interface was sometimes not visible and the crevice under that crossbeam (if present) was never visible. All footage, along all crossbeams, was included in the analyses.

Statistical analysis

We identified fish taxa to the lowest taxonomic level possible. Due to the challenges on distinguishing some fish species at their young-of-year stage, these unidentified young-of-year fish were grouped as one species in the following analysis. Therefore, herein, our use of the term “species” refers not only to single species but also to species aggregates. Species density was calculated as the number of individuals for a given species on a given transect divided by the length of the transect. To better visualize the density values in figures, we multiplied the density by 100 to obtain density in the unit of number per 100 m2.

We examined the effect of the survey methods (biological and structural methods) on fish density at each of the habitats (midwater and base). Mixed-model ANOVA was used to statistically test whether the difference in fish density was driven by the survey methods, and the depth of crossbeam was a random factor to account for fish density variance in the water column.

To visualize the relationships between species assemblages over habitats and survey methods, we created a two-dimensional, non-metric multidimensional scaling (nMDS) plot using the “metaMDS” function in the “vegan” package in R [20]. The sample matrix in the NMDS analysis was was comprised of the densities of the top ten most abundant species that consisted of 83% of the total fish count.

The resulting stress value of 0.08 indicated that the reduced dimensions well represented the original community assemblage. To statistically test whether the species assemblages between survey methods and habitats were different, we conducted the Analysis of Similarities (ANOSIM), the anosim() function in “vegan” package. Both nMDS and ANOSIM were performed on the Bray-Curtis dissimilarity indices from the sample matrix using vegdist() function.

Results and discussion

Platform jacket midwaters

The midwater species assemblages characterized by the two methods were similar (Fig 3). We observed a minimum of 19 fish species using the biological method and 18 species with the structural technique amid crossbeams between 17 m and 182 m (Table 3). Two “species”, Sebastes flavidus/serranoides and Rhinogobiops nicholsii (along with a handful of unidentified fishes), were only observed using the biological method, and two species, Sebastes carnatus and Sebastes serriceps were unique to the structural surveys. All were observed in very small numbers. Among the commercially important taxa observed were the squarespot rockfish, Sebastes hopkinsi, unidentified young-of-the-year (YOY) rockfishes, painted greenling, Oxylebius pictus, unidentified rockfishes, Sebastes spp., blue rockfish, Sebastes mystinus, and widow rockfish, Sebastes entomelas. Regardless of the protocol, the majority of taxa observed were rockfishes (15 of 19 species for the biological method and 14 of 18 species for the structural method), and rockfishes dominated by both numbers (266/299 and 181/223, respectively) and densities (90.8% and 79%, respectively) (Table 2).

Fig 3

NMDS plot of the 10 most abundant species (83% of all fishes) observed at Platform Harmony from the ROV surveys.

Table 3

Method
	Biological		Structural
Species	Number	Density	Number	Density
Sebastes hopkinsi	93	11.5	29	3.9
YOY Sebastes	87	10.9	44	5.5
Oxylebius pictus	22	2.8	36	4.6
Sebastes spp.	20	2.0	8	0.7
Sebastes mystinus	14	1.4	3	0.3
Sebastes entomelas	14	1.1	47	6.2
Sebastes dallii	8	0.8	5	0.5
Sebastes caurinus	6	0.7	11	1.3
Sebastes paucispinis	5	0.4	13	0.9
Sebastes rubrivinctus	5	0.5	4	0.4
Unidentified fishes	5	0.6	0	0.0
Sebastes saxicola	4	0.5	5	0.6
Sebastes miniatus	4	0.4	1	0.2
Chromis punctipinnis	2	0.3	2	0.2
Scorpaenichthys marmoratus	2	0.2	2	0.3
Sebastes semicinctus	2	0.3	1	0.2
Sebastes flavidus/serranoides	2	0.3	0	0.0
Rhinogobiops nicholsii	1	0.2	0	0.0
Sebastes atrovirens	1	0.1	6	0.7
“KGB” YOY1	1	0.1	2	0.2
Sebastomus sp.	1	0.1	2	0.2
Sebastes carnatus	0	0.0	1	0.1
Sebastes serriceps	0	0.0	1	0.1
Total	299	35.2	223	26.9
Total number of species	19		18
Percent Sebastes by number	90.0		81.2

1Sebastes atrovirens, Sebastes carnatus, Sebastes caurinus, and/or Sebastes chrysomelas.

Number and densities (number per 100 m2) of fishes observed in the midwaters of Platform Harmony by two ROV survey methods, “biological” and “structural” (these are defined under Methods). YOY–young-of-the-year.

Although mean densities derived from the biological method were higher than from the structural method (35.2 m-2 and 26.9 m-2, respectively), they were not statistically different (p >0.05) (Table 3, Fig 4). Particularly high densities occurred at three crossbeams. When we more closely examined the video footage, we found that these extremely high densities were due to loose and moving aggregations of young rockfishes. These aggregations were present during one of two passes along the crossbeams (Points A, B, and C in Fig 4), each pass being a different survey method, and densities along the same crossbeams were substantially lower on the other. Importantly, Point A represents a second pass along a crossbeam using the structural method following a first pass using the biological method when lower density was observed, and Points B and C, first passes using the biological method when densities were higher than the second passes. Therefore, to the extent that we can determine, in these instances differences in densities were not due to the difference in ROV survey method nor to the order by which the two methods were conducted along a crossbeam.

Fig 4

Median densities and 25% and 75% quantiles of all fishes combined in the midwaters and at the base of Platform Harmony using two protocols, “biological”: And “structural”.

Each dot represents the density of fishes along a single crossbeam between corners of the platform (n = 12, two sets of two points overlap and appear as one). Fish density values were multiplied by 100 to obtain density units of fish per 100 m2. The dots labeled A-D identify outliers.

Platform jacket base

The nMDS plot shows dissimilarity between the two assemblages at the base (Fig 3), although the few samples precluded testing the statistical significance of this difference. Compared to the midwaters, the platform jacket base harbored fewer species (10 for each protocol), although rockfishes were again the dominant group. Important species in the biological survey of the base of the platform jacket included unidentified Sebastes (presumably Sebastes aurora and/or Sebastes melanostomus), Sebastes melanostomus, Sebastes aurora, and Sebastes babcocki, as well as Sebastolobus spp. (Fig 4, Table 4). The important species in the structural survey at the base were unidentified Sebastes, Cataetyx rubrirostris, and S. melanostomus. Two “species”, a single Eptatretus stouti and an unidentified Nettastomatidae, were unique to the biological survey and three “species”, an unidentified flatfish, a Sebastomus sp., and three Leuroglossus stilbius individuals, were only see in the structural survey. These were all observed in very small numbers. As in the midwaters, overall mean densities were higher using the biological survey technique than the structural technique (32.1 m-2 versus 17.7 m-2) although the difference was not statistically significant. A single extreme value accounted for the difference (Point D, Fig 4). This highest fish density was observed on the north side of the platform jacket, on the second pass, using the biological method, and was driven primarily by two species groups: unidentified Sebastes with a density of 23 m-2 and Sebastes melanostomus, with a density of 13 m-2. By comparison, the lowest density estimate was observed using the structural method on the north side, and the two survey techniques yielded similar density estimates on the south and west sides of the platform jacket (Fig 5).

Table 4

	Biological		Structural
Species	Number	Density	Number	Density
Unidentified Sebastes1	88	14.7	58	9.7
Sebastes melanostomus	46	7.5	14	2.3
Sebastes aurora	17	2.8	3	0.5
Sebastolobus spp.2	17	2.5	5	0.7
Sebastes babcocki	10	1.6	2	0.3
Cataetyx rubrirostris	9	1.3	15	2.3
Unidentified fishes	4	0.6	4	0.7
Microstomus pacificus	3	0.4	2	0.3
Parmaturus xaniurus	2	0.3	1	0.2
Eptatretus stouti	1	0.2	0	0.0
Unidentified Nettastomatidae	1	0.2	0	0.0
Unidentified flatfish	0	0.0	1	0.1
Leuroglossus stilbius	0	0.0	3	0.5
Sebastomus sp.	0	0.0	1	0.2
Total	198	32.1	109	17.7
Total number of species	10		10
Percent Species by number	81.3		71.6

1Likely Sebastes aurora and/or Sebastes melanostomus.

2Likely Sebastolobus alascanus.

Fig 5

Densities of all fishes combined in the midwaters and at the base of Platform Harmony, by method and crossbeam side (N = north, S = south, W = west).

Number and densities (number per 100 m2) of fishes observed at the base of Platform Harmony by two ROV survey methods, “biological” and “structural” (these are defined under Methods).

Why would the biological and structural survey methods yield different species assemblages along the crossbeam base? And, why would the density of fishes be substantially higher along one side of the platform jacket using the biological survey method? To address these questions, we compared:

the structural characteristics of the three crossbeams at the base of the platform jacket (specifically the amount they were undercut);

the position of fishes associated with these crossbeams (whether they were associated with the seafloor-crossbeam interface or associated with the sides or tops of the crossbeams);

how much of the seafloor-crossbeam interface was visible using the two methods.

The most apparent structural difference among the crossbeams along the three sides of the platform jacket was that the north side was completely undercut as observed in the biological survey which was designed to view the interface between the seafloor and the crossbeam (Fig 6). A continuous crevice was visible beneath the north crossbeam. In comparison, 55% and 13.4% of the crossbeam was undercut on the west side and south side of the platform jacket, respectively (Fig 6).

Fig 6

The percentage of the lengths of the north, west, and south crossbeams that were undercut, and the percentage of the seafloor-crossbeam interface that was visible at each crossbeam using the biological and structural survey method.

The entire length (100%) of the seafloor-crossbeam interface along each crossbeam was visible in the biological survey.

Both types of survey techniques revealed that fishes were more abundant on the seafloor than above the seafloor along the side and upper surface of crossbeams (Table 5). On all three sides of the platform jacket, a greater proportion of fishes were seen on the seafloor in the biological survey than in the structural survey.

Table 5

Biological
North	West	South
Total number of fishes: 96	Total number of fishes: 47	Total number of fishes: 57
Number of fishes on bottom: 84	Number of fishes on bottom: 36	Number of fishes on bottom: 49
Number of fishes off bottom: 12	Number of fishes off bottom: 11	Number of fishes off bottom: 8
Percent of fishes on bottom: 87.5	Percent of fishes on bottom: 76.6	Percent of fishes on bottom: 80.6
Structural
Total number of fishes: 21	Total number of fishes: 43	Total number of fishes: 46
Number of fishes on bottom: 12	Number of fishes on bottom: 24	Number of fishes on bottom: 31
Number of fishes off bottom: 9	Number of fishes off bottom: 19	Number of fishes off bottom: 15
Percent of fishes on bottom: 57.1	Percent of fishes on bottom: 55.8	Percent of fishes on bottom: 67.4

A comparison of the numbers of fishes at each crossbeam (north, west, and south) and the number and percentages of fishes associated either on the “bottom” and near or under the sea floor-crossbeam interface or “off-bottom” along the crossbeam sides and upper surfaces.

Importantly, between the two methods, there was a substantial difference in the amount of the seafloor-crossbeam interface that was visible. The interface along all three crossbeams was completely visible using the biological method because the ROV was near the seafloor and its camera was aimed directly at the interface. In contrast, the structural survey, where the ROV was positioned higher above the seafloor with the field of view aimed to include the top and side of the crossbeam, the amount of the seafloor-crossbeam interface visible varied between 8.8% of the length of the north beam and 83.0 and 97.0% of the lengths of the west and south beams, respectively (Fig 6). This difference between the north beam and the other two crossbeams occurred because in the structural method the seafloor was visible only where the crossbeam was sufficiently buried such that both the top of the crossbeam and the seafloor-crossbeam interface were within the field of view. In contrast, from this vantage of the ROV, the undercut area if present beneath the crossbeam was obscured from view by the beam itself, and consequently the fishes in the undercut area below the crossbeam would not be visible (Fig 7).

Fig 7

Two images of the same section of the west crossbeam.

Image (A) was taken during the biological survey and image (B) during the structural survey. Note that in the biological survey, the seafloor-crossbeam interface is visible as is the undercut beneath the crossbeam. In image (B), the seafloor-crossbeam interface is not visible; however, compared to (A) more of the middle and upper part of the crossbeam is visible in (B).

Previous surveys at other California platform jackets have demonstrated that some species occur at greater densities in areas undercut below a crossbeam than adjacent areas where the cross beam is embedded in the sediment [21]. They termed these species members of the “sheltering habitat guild” and Sebastes aurora, one of the species responsible for the differences in assemblages, is a member of that guild. Because the structural method inadequately imaged that seafloor-crossbeam interface, particularly along the north crossbeam, substantial numbers of fishes were obscured from view and not surveyed.

Other studies have raised the potential advantages and limitations of using repurposed structural surveys (e.g., 14–16). As one study noted [16] “Most ROV operations are conducted by industry in a way that fulfills immediate industry requirements but which can confound scientific interpretations of the data. For example, there is variation in video resolution, ROV speed, distance above the substrate [italics ours] and time (e.g. both seasonal and time of day).” Similarly, as other investigators [14] successfully used inspection ROV footage to assess invertebrates and fishes along North Sea pipelines, they too highlighted several aspects that can bias the biological assessments. In particular, and similar to our study, differing camera angles “led to inconsistencies in the section of pipeline assessed for each sample according to each camera view” and “The angle of lateral cameras also influenced the amount of seabed visible either side of the pipeline.”

Our study did not address other issues inherent in using ROVs as fish survey tools [10, 16, 22]. As an example, a survey off central California [22] compared the reactions of fishes between a human-operated vehicle (HOV) and an ROV on natural habitats. It found that significantly more fishes reacted (by swimming away) from an ROV than an HOV. The authors speculated that these differences could be due to the presence of a tether on the ROV (absent from the HOV), differences between the ROV and HOV in the positioning of lighting (forward versus starboard, respectively), difference in sounds emitted by the vehicles, and the difference in the size of the vehicles. Following up on this work [23], these researchers compared fish diversities, densities, and sizes obtained during this same study. They found that 1) density estimates of taxa associated with the seafloor were higher in the HOV surveys, 2) a greater percentage of HOV-observed fishes could be sized, and 3) a higher percentage of fishes in the HOV surveys could be identified to species.

Due to the small sample size (n = 12) for each of the survey methods and the short survey time (2 days), only simple statistics were computed in this study to explore the overall difference between these two survey methods. We note that we cannot extrapolate from this study limited to one platform jacket to surveys of all of California platform jackets, or to platform jackets worldwide, as platform jacket structure, fish behaviors, and ROV operator behaviors are likely to be quite variable. In addition to variances introduced by distance to infrastructure and camera angle, biological assessments from structural surveys can be biased if organisms tend to associate with specific, localized features of a platform jacket and if these features are poorly sampled by the structural surveys. In this study, in the midwaters of Harmony, there was no evidence that fishes disproportionately associated with particular parts of the crossbeams and, as importantly, during both biological and structural surveys the span of the diameter of the crossbeam remained in view. Thus, species assemblages and fish densities were similar using the two methods. In contrast, it is clear that, at the base of the platform, fishes were more likely to 1) associate with the seafloor-crossbeam interface and, more particularly, they tended to associate with those portions of the crossbeam that were undercut. During the structural survey, the ROV was not routinely positioned to image the seafloor-crossbeam interface, and thus a substantial number of the fishes were obscured from view that were observed in the biological survey.

This study suggests that structural inspection surveys can be a valuable tool in assessing fish assemblages associated with platform jackets. Integrating a more robust biological methodology for structural surveys may be considered to provide guidance to ROV operators. For instance, a recent study [24] highlighted changes to ROV survey methodology, related to lighting and camera operations, that provided a more complete picture of the biological community around a platform in the Gulf of Mexico. To limit costs, incremental adjustments to regular structural surveys should be considered. Guidance should include the use of HD cameras, maintaining a fixed distance from the platform, speed of the ROV (when ROV is in transit between different structural survey points of interest) and camera angle. In our study, a camera angle positioning the ROV at the seafloor-crossbeam interface to capture the fish assemblages would have proved valuable. These minor adjustments would be expected to increase the data applicability of the already valuable structural surveys to be more in line with targeted biological surveys.

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29 Jun 2020

PONE-D-20-16316

A Comparison of Two ROV Survey Methods Used to Estimate Fish Assemblages and Densities Around a California Oil Platform

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Reviewer #1: Review of PONE-D-20-16316 A comparison of two ROV survey methods used to estimate fish assemblages and densities around a California Oil Platform

General:

The paper provides a succinct assessment of how dedicated biological surveys of fish communities on a platform jacket compare to data that could be derived from typical industry surveys. The results are valuable as they demonstrate the utility of industry surveys and how slight adjustments to protocol can yield important biological data. It is also rare for such surveys to be conducted, with an effective doubling of sampling effort required by industry to undertake this research. I note the paper is focussed on fish, but examination of epibiota may also yield interesting results.

The paper is generally well written and clear with just a few grammatical errors (see below). I have made some specific suggestions below to improve the manuscript. In particular, further explanation of the differences in methodologies is required and the resulting implications for the study.

Specific:

The addition of a Figure that shows the reader what the jacket structure looks like (surface-seabed with crossbeams) would be a good addition and could be incorporated into Figure 2.

Table 1: Include crossbeam length into the text within the table to make this clear.

Line 88 – Add a sentence explaining why these depths were chosen. Is this the only depths that crossbeams were evident? There is a big gap between 61 and 182 and 182 and the seabed.

Methods in general: I would like to see more information on the differences between the methods. Did the structural survey use lights? How did the speed compare between both surveys? Also, would a structural survey undertake transects in the same manner as that used by the biological survey, i.e. of these same components or did they only survey these same crossbeams for comparison to the biological surveys? This is important because structural surveys may not survey the entire jacket but just sections of interest to industry.

Line 104 - A 2 m distance for a biological survey (focussed on fish) seems very close. McLean et al. (2019) used 5 m to provide an optimal view of the fish assemblage around the jacket. Do you feel that you missed larger species?

Line 138: I would stick with the raw numbers here rather than inflating by x100. Just adjust the scales on Figures so that the pattern is evident and present as per m2. This would mean altering Figures 4, 5 to present the correct values per m2. I really cant see this inflation as being necessary.

Line 145 – missing a word “..we created a two-dimensional..”

Line 147: spelling. Abundance should be abundant.

Table 2: Add “structural” to the industry column for consistency with text. Same for Table 3 and Table 4. I also note that the figures present structural first then biological. You could keep the tables in the same order.

Results – add that three species were uniquely observed by the biological survey and two by the structural survey, albeit each in very low numbers.

Line 180 – should this be 26.9 not 27.1? (to match Table 2).

Line 183: So here you are stating that whichever survey method occurred first was more likely to view these aggregations of young rockfishes. Likely because they were scared away or avoided the ROV subsequently. The following sentence needs further justification then, i.e. for each of the three sides of the jacket or days did you start with a different survey method? (randomised order).

Figure 4 legend: Make it clear in the legend that densities have been inflated (x100)

Line 202 – make sure numbers match the table. They are slightly off.

Line 204 – do you mean that the highest density estimate was obtained on the north side using the biological method? This is unclear here? If referring to Point D, what is the actual species that is driving this? Is it Sebastes melanostomus?

Line 205 – change industrial to “structural”

Table 4 legend is missing a close bracket.

Line 284: missing word. “It showed that..”

Line 293: Great! Bring on HOV surveys into the future.

Line 295: California’s platform jackets – plurals needed.

Line 310: remove invertebrates from the sentence.

Line 312: This sentence does not make sense or is missing a few words.

Reviewer #2: Love and colleagues compared fish assemblage and density data derived from two types of ROV surveys along an ExxonMobil Platform off the coast of southern California. The goal was to determine if video observations from routine approaches used in the industry could be utilized by scientists to monitor fish populations, as is being done by others elsewhere (e.g., Gulf SERPENT project). The first approach was one used by the industry to examine structural integrity (structure) and involved variable camera angles along cross-beams. The second, used more by scientists (biological), involved a 90 degree scan and incorporated the seabed on the bottom cross-beam. Overall, the authors identified community similarities between the two techniques, suggesting a potential benefit of the industry-based methods, although there were significant differences when considering communities at the base of the platform.

The main weakness of the manuscript is that it is based on so few comparative surveys (n=12) between the two approaches, which are preformed at a single platform, spread across different depths of the water column and different "faces" of the platform jacket. It is understood that these surveys are quite costly to implement, but the lack of statistical rigor should be further emphasized. Further, there is a tremendous amount of detail lacking that could affect the interpret-ability of the results. As the authors themselves admit, fish can become deterred by ROV presence. So, a major question becomes: how were these surveys performed (in terms of timing) and could one have impacted the results of the other (e.g., was biological always before industry)? Was randomization possible? All we are provided with are the dates over which the surveys were completed, and that they were done by the same ROV type. It was also clear that the authors were not well-versed in their community analyses as there are terms used incorrectly, and again were lacking details on how they set up their statistical analysis. I have identified these shortcomings in the attached PDF.

Stylistically, the manuscript is well-written and the figures strongly convey the results in a readable manner. However, there were several typos throughout, which have also been highlighted in my review.

In all, I believe Love et al. have some intriguing preliminary evidence that industry-type ROV surveys could provide meaningful data to be used by scientists for monitoring fish densities and assemblages at oil platforms. I am not sure that the findings are particularly "earth-shattering" given the limited number of surveys in a constrained time-period, and feel this manuscript would probably be better suited as a rapid communication.

Specific Comments (by line numbers):

ABSTRACT

Line 13: Abstract - this is way too long. It shouldn't exceed 300 words

INTRODUCTION

Line 67: Consider adding reference:

Ajemian, M. J., Wetz, J. J., Shipley-Lozano, B., & Stunz, G. W. (2015). Rapid assessment of fish communities on submerged oil and gas platform reefs using remotely operated vehicles. Fisheries Research, 167, 143-155.

Line 79: Strange return

MATERIALS AND METHODS

Line 87: Which survey was first?

Line 112: List camera type and specs

Line 131: Need more details on the chronology of the surveys

Line 131: More details on whether all footage was analyzed or if there was a sub-sampling regime

Line 141: Wouldn't this be just a mixed-model ANOVA?

Line 144-147: Need to state data type used in analysis (density, presence/absence)

Line 147: Replace 'abundance' with "abundant"

Line 150: Change 'Dissimilarity' to "similarity:

Line 151: What type of matrix was used?

RESULTS AND DISCUSSION:

Lines 166-168: The NMDS is simply an ordination; it does not give you a significance level. Is this from the ANOSIM?

Line 169: Change 'MNDS' to "NMDS"

Lines 183-186: This is exactly why the chronology of the surveys needs explanation in the methods.

Line 197: For 'primarily,' did you mean "presumably" - it does not make sense otherwise

Line 210 (table 3): I think it's important to discuss the species that showed up in the structure survey that didn't in the biological survey

Lines 218-220: Not sure this format of questioning is customary

Line 240: Recommend making this table a figure

Line 264: Move reference to the end of the sentence

Line 277: Remove commas after 'they' and 'too'

Line 278: Change 'biased' to "bias"

Line 284: 'It that' - there is a missing or wrong word here

Line 310: This is the first time invertebrates are mentioned - I don't think it is appropriate to say this.

**********

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9 Sep 2020

Response to Reviewer’s Commonts of:

A Comparison of Two ROV Survey Methods Used to Estimate Fish Assemblages and Densities Around a California Oil Platform

Authors: Love, Milton S., Mary M. Nishimoto, Scott Clark, Li Kui, Aivy Aziz, David Palandro

Thanks to the reviewers’ comments and suggestions, which greatly improve the quality of the manuscript. Please see the responses below.

Reviewer #1: Review of PONE-D-20-16316 A comparison of two ROV survey methods used to estimate fish assemblages and densities around a California Oil Platform

General:

The paper provides a succinct assessment of how dedicated biological surveys of fish communities on a platform jacket compare to data that could be derived from typical industry surveys. The results are valuable as they demonstrate the utility of industry surveys and how slight adjustments to protocol can yield important biological data. It is also rare for such surveys to be conducted, with an effective doubling of sampling effort required by industry to undertake this research. I note the paper is focussed on fish, but examination of epibiota may also yield interesting results.

The paper is generally well written and clear with just a few grammatical errors (see below). I have made some specific suggestions below to improve the manuscript. In particular, further explanation of the differences in methodologies is required and the resulting implications for the study.

Specific:

The addition of a Figure that shows the reader what the jacket structure looks like (surface-seabed with crossbeams) would be a good addition and could be incorporated into Figure 2.

Done

Table 1: Include crossbeam length into the text within the table to make this clear.

Sorry we don’t understand this request.

Line 88 – Add a sentence explaining why these depths were chosen. Is this the only depths that crossbeams were evident? There is a big gap between 61 and 182 and 182 and the seabed.

We have added the following: “As we did not have sufficient funding to include all of the crossbeams in this survey, we surveyed midwater crossbeams of representative depths along with the crossbeam at the bottom.”

Methods in general: I would like to see more information on the differences between the methods. Did the structural survey use lights? How did the speed compare between both surveys?

We have inserted the following: “All surveys were conducted at the same speed using the SubC 1CamMk5 HDf video camera using lights at all depths..”

Also, would a structural survey undertake transects in the same manner as that used by the biological survey, i.e. of these same components or did they only survey these same crossbeams for comparison to the biological surveys? This is important because structural surveys may not survey the entire jacket but just sections of interest to industry.

Yes, the more complete surveys would cover the same crossbeams.

Line 104 - A 2 m distance for a biological survey (focused on fish) seems very close. McLean et al. (2019) used 5 m to provide an optimal view of the fish assemblage around the jacket. Do you feel that you missed larger species?

An interesting point. The fish community that McLean et al. (2019) surveyed is quite different from that found off California. McLean’s community is a tropical one, with substantial numbers of larger, much more motile jacks and snapper. Our community is composed, in midwaters, primarily of juvenile rockfishes, that form more-or-less slow to respond aggregations and, at the bottom, again of rockfishes and other relatively small, and importantly sedentary taxa. In addition, most of the fishes are either so small or so difficult to distinguish from congenerics that distances greater than about 2 m would mean that identifications would be either extremely problematic or impossible.

Line 138: I would stick with the raw numbers here rather than inflating by x100. Just adjust the scales on Figures so that the pattern is evident and present as per m2. This would mean altering Figures 4, 5 to present the correct values per m2. I really cant see this inflation as being necessary.

We understand your concerns, but we also prefer to keep the inflated density number in this manuscript. Here are some conventions: 1) Other researchers studying California platform fishes, i.e. Martin and Lowe (2009), have also used the same metric. 2) There is a convention in reporting densities of California reef and soft sea floor fishes to use the expanded area (x100). 3) all of our other platform studies are in those units, so we need this consistency in order for us, and for anyone else, to be able to compare the results in this study to those of other platform studies.

Line 145 – missing a word “..we created a two-dimensional..”

Done.

Line 147: spelling. Abundance should be abundant.

Done.

Table 2: Add “structural” to the industry column for consistency with text. Same for Table 3 and

Table 4. I also note that the figures present structural first then biological. You could keep the tables in the same order.

Done.

Results – add that three species were uniquely observed by the biological survey and two by the structural survey, albeit each in very low numbers.

We have the following: Two “species”, Sebastes flavidus/serranoides and Rhinogobiops nicholsii (along with a handful of unidentified fishes), were only observed using the biological method, and two species, Sebastes carnatus and Sebastes serriceps were unique to the structural surveys. All were observed in very small numbers.

Line 180 – should this be 26.9 not 27.1? (to match Table 2).

We changed to 26.9.

Line 183: So here you are stating that whichever survey method occurred first was more likely to view these aggregations of young rockfishes. Likely because they were scared away or avoided the ROV subsequently. The following sentence needs further justification then, i.e. for each of the three sides of the jacket or days did you start with a different survey method? (randomised order).

We have made substantial changes to the text and added a new Table 2 to help explain both the methods (order of surveys at each crossbeam) and the results.

In the Introduction we have added the following: All surveys were conducted during daylight hours, and decisions regarding which side was surveyed first, which method was first used on a specific side, and the length of time between surveys on a specific side were made haphazardly (Table 2).

In addition, in the Results section we clarified the role that multiple passes may have on densities estimates: “Although mean densities derived from the biological method were higher than from the structural method (35.2 m-2 and 26.9 m-2, respectively), they were not statistically different (p >0.05) (Table 3, Fig 4). Particularly, high densities occurred at three crossbeams. When we more closely examined the video footage, we found that these extremely high densities were due to loose and moving aggregations of young rockfishes. These aggregations were present during one of the two survey techniques along the crossbeams (Points A, B, and C in Fig 4), and then densities along the same crossbeams were substantially lower on the other after the aggregations moved away. Importantly, Point A represents a second pass and Points B and C, first passes. Therefore, to the extent that we can determine, in these instances differences in densities were not due to the difference in ROV survey method, but rather to other, unknown, environmental parameters.”

Figure 4 legend: Make it clear in the legend that densities have been inflated (x100)

We add a sentence in the caption to clarify that. “Fish density values were multiplied by 100 to obtain density units of fish per 100 m2.”

Line 202 – make sure numbers match the table. They are slightly off.

Done.

Line 204 – do you mean that the highest density estimate was obtained on the north side using the biological method? This is unclear here? If referring to Point D, what is the actual species that is driving this? Is it Sebastes melanostomus?

We inserted the following text to clarify the issue: “This highest fish density was observed on the north side of the platform jacket using the biological method, and was driven primarily by two species groups: unidentified Sebastes with a density of 23 m-2 and Sebastes melanostomus, with a density of 13 m-2. By comparison, the lowest density estimate was observed using the industrial method on the north side, and the two survey techniques yielded similar density estimates on the south and west sides of the platform jacket (Fig 5).”

Line 205 – change industrial to “structural”

Done.

Table 4 legend is missing a close bracket.

The errant parenthesis was removed.

Line 284: missing word. “It showed that..”

Done. We used “found” rather than “showed.”

Line 293: Great! Bring on HOV surveys into the future.

Ha! If only we could…

Line 295: California’s platform jackets – plurals needed.

Done.

Line 310: remove invertebrates from the sentence.

Done.

Line 312: This sentence does not make sense or is missing a few words.

Done.

Reviewer #2: Love and colleagues compared fish assemblage and density data derived from two types of ROV surveys along an ExxonMobil Platform off the coast of southern California. The goal was to determine if video observations from routine approaches used in the industry could be utilized by scientists to monitor fish populations, as is being done by others elsewhere (e.g., Gulf SERPENT project). The first approach was one used by the industry to examine structural integrity (structure) and involved variable camera angles along cross-beams. The second, used more by scientists (biological), involved a 90 degree scan and incorporated the seabed on the bottom cross-beam. Overall, the authors identified community similarities between the two techniques, suggesting a potential benefit of the industry-based methods, although there were significant differences when considering communities at the base of the platform.

The main weakness of the manuscript is that it is based on so few comparative surveys (n=12) between the two approaches, which are preformed at a single platform, spread across different depths of the water column and different "faces" of the platform jacket. It is understood that these surveys are quite costly to implement, but the lack of statistical rigor should be further emphasized. Further, there is a tremendous amount of detail lacking that could affect the interpret-ability of the results. As the authors themselves admit, fish can become deterred by ROV presence. So, a major question becomes: how were these surveys performed (in terms of timing) and could one have impacted the results of the other (e.g., was biological always before industry)? Was randomization possible? All we are provided with are the dates over which the surveys were completed, and that they were done by the same ROV type. It was also clear that the authors were not well-versed in their community analyses as there are terms used incorrectly, and again were lacking details on how they set up their statistical analysis. I have identified these shortcomings in the attached PDF.

We certainly recognize that the sample size was relatively small, and the survey time was short, which were due to the expense of this kind of survey. We have added several sentences in the discussion to emphasize the limitation of this study. For instance, we begin the penultimate paragraph “Due to the small sample size (n=12) for each of the survey methods and short survey time (2 days), only the simple statistics were computed in this study to explore the overall difference between these two survey methods.” The relevant statistical method descriptions have also been revised to clarify the analysis.

Stylistically, the manuscript is well-written and the figures strongly convey the results in a readable manner. However, there were several typos throughout, which have also been highlighted in my review.

In all, I believe Love et al. have some intriguing preliminary evidence that industry-type ROV surveys could provide meaningful data to be used by scientists for monitoring fish densities and assemblages at oil platforms. I am not sure that the findings are particularly "earth-shattering" given the limited number of surveys in a constrained time-period, and feel this manuscript would probably be better suited as a rapid communication.

Specific Comments (by line numbers):

ABSTRACT

Line 13: Abstract - this is way too long. It shouldn't exceed 300 words

Done. Now 294 words.

INTRODUCTION

Line 67: Consider adding reference:

Ajemian, M. J., Wetz, J. J., Shipley-Lozano, B., & Stunz, G. W. (2015). Rapid assessment of fish communities on submerged oil and gas platform reefs using remotely operated vehicles. Fisheries Research, 167, 143-155.

Done.

Line 79: Strange return

MATERIALS AND METHODS

Line 87: Which survey was first?

See the responses given for Reviewer 1 re line 183.

Line 112: List camera type and specs

We included this in a sentence around lines 129–131.

Line 131: Need more details on the chronology of the surveys

See the responses given for Reviewer 1 re line 183.

Line 131: More details on whether all footage was analyzed or if there was a sub-sampling regime

We added “All footage, along all crossbeams, was included in the analyses.”

Line 141: Wouldn't this be just a mixed-model ANOVA?

Yes. We changed this in the statistical analysis section.

Line 144-147: Need to state data type used in analysis (density, presence/absence)

It is the density value and we added it into the manuscript. The sentence now reads “The response variable was a matrix of fish density of the top ten most abundant species that consisted of 83% of the total fish count.”

Line 147: Replace 'abundance' with "abundant"

Thanks! We corrected the spelling.

Line 150: Change 'Dissimilarity' to "similarity”

Done

Line 151: What type of matrix was used?

Species density matrix. We added it into the manuscript. The sentence now reads “To statistically test whether the species assemblages between survey methods and habitats were different, we used the Analysis of Dissimilarity (ANOSIM), the anosim() function in “vegan” package, following by calculating the matrix of species dissimilarity using vegdist() function.”

RESULTS AND DISCUSSION:

Lines 166-168: The NMDS is simply an ordination; it does not give you a significance level. Is this from the ANOSIM?

Yes, it is from the ANOSIM. We added in more detail: “However, the ANOSIM analysis for the survey method was not significant (p = 0.69), suggesting that survey method was not likely to contribute to the species assemblage difference.”

Line 169: Change 'MNDS' to "NMDS"

Done.

Lines 183-186: This is exactly why the chronology of the surveys needs explanation in the methods.

See the responses given to Reviewer 1, line 183.

Line 197: For 'primarily,' did you mean "presumably" - it does not make sense otherwise

Done.

Line 210 (table 3): I think it's important to discuss the species that showed up in the structure survey that didn't in the biological survey

The fishes that were not held in common were observed in very small, and likely no meaningful, numbers. We have added the following to the results: “Two “species”, a single Eptatretus stouti and an unidentified Nettastomatidae, were unique to the biological survey and three “species”, an unidentified flatfish, a Sebastomus sp., and three Leuroglossus stilbius individuals, were only see in the structural survey. These were all observed in very small numbers.”

Lines 218-220: Not sure this format of questioning is customary

We would prefer to keep as is, as we consider that the wording assists the reader.

Line 240: Recommend making this table a figure

While the numbers in this table are presented in Figure 5, we believe the reader is best served through this table.

Line 264: Move reference to the end of the sentence

Done.

Line 277: Remove commas after 'they' and 'too'

Done.

Line 278: Change 'biased' to "bias"

Done.

Line 284: 'It that' - there is a missing or wrong word here

We added “found” to make “It found that”

Line 310: This is the first time invertebrates are mentioned - I don't think it is appropriate to say this.

Removed.

Submitted filename: Response to Reviewers.docx

Click here for additional data file.

22 Sep 2020

PONE-D-20-16316R1

A comparison of two remotely operated vehicle (ROV) survey methods used to estimate fish assemblages and densities around a California oil platform

PLOS ONE

Dear Dr. Love,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

Academic Editor

This paper will be acceptable for publication when you have addressed the two minor points raised by the reviewer.

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Additional Editor Comments (if provided):

Academic Editor

This paper will be acceptable for publication when you have addressed the two minor points raised by the reviewer.

[Note: HTML markup is below. Please do not edit.]

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #2: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #2: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #2: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #2: Yes

**********

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Reviewer #2: Yes

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Reviewer #2: Thank you for making the various revisions to the manuscript. It seems that the majority of the recommended changes have been incorporated and the manuscript is vastly improved.

A few final (minor) comments:

ABSTRACT: Thanks for reducing this, although there is a return in there (not sure if that was accidental) so it seems like two paragraphs still.

Line 178: Remove comma after 'particularly'

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Reviewer #2: No

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13 Oct 2020

This is the final version of:

A Comparison of Two Remotely Operated Vehicle (ROV) Survey Methods Used to Estimate Fish Assemblages and Densities Around a California Oil Platform

Love, M. S., M. M. Nishimoto, S. Clark, L. Kui, A. Aziz, D. Palandro

In response to the comments of reviewer #2:

1) I have changed the abstract, so that it is only a single paragraph.

2) I have removed the comma after “particularly” in line 177.

Milton Love

Research Biologist

Marine Science Institute

University of California

Santa Barbara, California

Submitted filename: Response to Reviewers.22 September 2020.docx

Click here for additional data file.

26 Oct 2020

A comparison of two remotely operated vehicle (ROV) survey methods used to estimate fish assemblages and densities around a California oil platform

PONE-D-20-16316R2

Dear Dr. Love,

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Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Reviewers' comments:

29 Oct 2020

PONE-D-20-16316R2

A comparison of two remotely operated vehicle (ROV) survey methods used to estimate fish assemblages and densities around a California oil platform

Dear Dr. Love:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please let them know about your upcoming paper now to help maximize its impact. If they'll be preparing press materials, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

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on behalf of

Professor Maura (Gee) Geraldine Chapman

Academic Editor

PLOS ONE