OCS Study 

BOEM 2013-217 

Coastal Marine Institute 

U.S. Department of the Interior 

Bureau of Ocean Energy Management 

Gulf of Mexico OCS Region

Cooperative Agreement 

Coastal Marine Institute 

Louisiana State University

Deepwater Coral Distribution and

Abundance on Active Offshore Oil and

Gas Platforms and Decommissioned 

Rigs-to-Reefs Platforms
OCS Study 

BOEM 2013-217 

Published by 

U.S. Department of the Interior 

Bureau of Ocean Energy Management 

Gulf of Mexico OCS Region

Cooperative Agreement 

Coastal Marine Institute 

Louisiana State University

Coastal Marine Institute 

Deepwater Coral Distribution and

Abundance on Active Offshore Oil and

Gas Platforms and Decommissioned 

Rigs-to-Reefs Platforms

Author 

Paul W. Sammarco 

December 2013 

Prepared under BOEM Contract 

M08AC12865 

by 

Louisiana Universities Marine Consortium (LUMCON) 

8124 Hwy. 56 

Chauvin, LA 70344 

iii

DISCLAIMER

This report was prepared under contract between the Bureau of Ocean Energy Management 

(BOEM) and Louisiana State University. This report has been technically reviewed by BOEM, 

and it has been approved for publication. Approval does not necessarily signify that the contents 

reflect the views and policies of BOEM, nor does mention of trade names or commercial 

products constitute endorsement or recommendation for use.

REPORT AVAILABILITY

To download a PDF file of this Gulf of Mexico OCS Region report, go to the U.S. Department of 

the Interior, Bureau of Ocean Energy Management, Environmental Studies Program Information 

System website and search on OCS Study BOEM 2013-217.

This report can be viewed at select Federal Depository Libraries. It can also be obtained from 

the National Technical Information Service; the contact information is below.

U.S. Department of Commerce

National Technical Information Service

5301 Shawnee Rd.

Springfield, Virginia 22312

Phone: (703) 605-6000, 1(800)553-6847

Fax: (703) 605-6900

Website: http://www.ntis.gov/

CITATION

Sammarco, Paul W. 2013. Deepwater coral distribution and abundance on active offshore oil 

and gas platforms and decommissioned Rigs-to-Reefs platforms. U.S. Dept. of the 

Interior, Bureau of Ocean Energy Management, Gulf of Mexico OCS Region, New 

Orleans, LA. OCS Study BOEM 2013-217. 45 pp.

ABOUT THE COVER

Cover photo by Scott A. Porter. © 2013 by S.A. Porter. All rights reserved. Used with 

permission. 

v

CONTRIBUTING AUTHORS

• Gregory S. Boland : Environmental Section, Bureau of Ocean Energy Management, U.S.

Department of the Interior, Herndon, Virginia

• Melissa Genazzio : Louisiana Universities Marine Consortium (LUMCON), Chauvin, 

Louisiana

• Angela Lirette : Louisiana Universities Marine Consortium (LUMCON), Chauvin, Louisiana

• Paul W. Sammarco : Louisiana Universities Marine Consortium (LUMCON), Chauvin, 

Louisiana

• James Sinclair: Environmental Section, Bureau of Ocean Energy Management, U.S.

Department of the Interior, New Orleans, Louisiana

• Yuasung Fiona Tung : Louisiana Universities Marine Consortium (LUMCON), Chauvin, 

Louisiana

ACKNOWLEDGEMENTS

Many thanks to all of those who assisted us with diving, sample collection, topside, 

laboratory, and other support: Volunteer divers and topside support: D. Ball, M. Boatwright, H. 

Butler, C. Chauffe, C. Chauvin, K. Collins, J. Culbertson, L. Dauterive, J. Dunn, K. Fiand, Craig 

Gentry, C. Horrell, Greg Kozlowski, A. Miller, Lt. Col. D. Perrenod, S.A. Porter, A. Schultze, 

K.B. Strychar, P. Sjordal, Amanda Thronson. ARACAR ROV operations: J. Blitch, A. 

Campbell, J. Vice. LUMCON ROV operations: M. Wike. M/V Fling: N. Baltz, K. Bearden, K. 

Bush, J. Dibble, P. Dagenais, B. Ellen, G. Harding, S. Hauer, K. Kimmey, J. Koobles, C. Kozio, 

J. Prentice, M. McReynolds, G. Phillips, B. Shurtleff. LUMCON accounting: H. Boudreaux, C. 

Luke, and S. Parrott. Texas Parks and Wildlife Service for permission to access the Rigs-toReefs platforms. The oil, gas, and other companies and their staff who provided us access to their 

offshore platforms and other assistance: Apache Oil and Chevron Texaco: W. Cormier, K. 

McMinn, C. Powell; Chevron Texaco: B.E. Hocutt, K.A. Mire, F. Sanders, D. Sennett. We also 

thank L. Rouse of the Louisiana State University Coastal Marine Institute and the U.S.

Department of Interior Bureau of Ocean Energy Management for their support and for funding 

this study under Cooperative Agreement #1435-01-04CA-32806/0106TO39929-19145to PWS.

vii

TABLE OF CONTENTS

LIST OF FIGURES .....................................................................................................................................ix

LIST OF TABLES.......................................................................................................................................xi

EXECUTIVE SUMMARY .......................................................................................................................xiii

1.0 INTRODUCTION ................................................................................................................................. 1

1.1 General Background: Platforms in the Northern Gulf of Mexico and the

Flower Garden Banks.................................................................................................................. 1

1.2 Corals in the Northern Gulf of Mexico....................................................................................... 1

1.3 The Rigs-to-Reefs Program ........................................................................................................ 2

1.4 Objectives ................................................................................................................................... 3

1.5 Duration ...................................................................................................................................... 3

2.0 MATERIALS AND METHODS........................................................................................................... 5

2.1 Study Sites .................................................................................................................................. 5

2.2 ROV Surveys.............................................................................................................................. 5

2.3 Statistical Analyses..................................................................................................................... 7

2.4 Technical Considerations and Alteration of Original Plan ......................................................... 7

3.0 RESULTS .............................................................................................................................................. 9

3.1 Coral Species Composition......................................................................................................... 9

3.2 Coral Density .............................................................................................................................. 9

3.3 Coral Depth Distribution........................................................................................................... 13

4.0 DISCUSSION...................................................................................................................................... 19

4.1 Overall Density Comparisons................................................................................................... 19

4.2 Madracis decactis..................................................................................................................... 19

4.3 Tubastraea coccinea ................................................................................................................. 19

4.5 Phyllangia americana............................................................................................................... 20

4.6 No Evidence for Enhancement of Coral Community Structure and Resultant Fish Habitat .... 20

5.0 CONCLUSIONS ................................................................................................................................. 21

REFERENCES ........................................................................................................................................... 23

ix

LIST OF FIGURES

Figure 1. Map of platforms surveyed in this study for scleractinian corals............................... 6

Figure 2. Graph showing the total coral density in no./10m2 on standing and toppled 

Rigs-to-Reefs platforms on the northern GOM outer continental shelf. .................. 10

Figure 3. Graph showing coral density in no./10m2 in two scleractinian species: 

Tubastraea coccinea (upper graph), an azooxanthellate coral, and Madracis 

decactis (lower), a zooxanthellate one...................................................................... 11

Figure 4. Graph depicting coral density in no./10m2 in two azooxanthellate scleractinian 

coral species: Phyllangia americana (upper graph) and Oculina diffusa (lower).... 12

Figure 5. Depth distribution of all scleractinian corals. Corals distributed significantly 

deeper on the standing platforms than on the toppled Rigs-to-Reefs structures (p 

< 0.001, RxC frequency G-test)................................................................................ 14

Figure 6. Depth distribution of Madracis decactis. No significant difference between 

depth distributions of this species between the two sets of structures (p > 0.05, 

RxC frequency G-test).............................................................................................. 15

Figure 7. Depth distribution of Tubastraea coccinea. Highly significant difference 

between depth distributions of this species between the two sets of structures (p 

< 0.001, RxC frequency G-test)................................................................................ 16

Figure 8. Depth distribution of Oculina diffusa. Corals distributed significantly deeper on 

the standing platforms than on the toppled Rigs-to-Reefs structures (p < 0.001, 

RxC frequency G-test).............................................................................................. 17

Figure 9. Depth distribution of Phyllangia americana. Corals distributed significantly 

deeper on the standing platforms than on the toppled Rigs-to-Reefs structures (p 

< 0.001, RxC frequency G-test)................................................................................ 18

xi

LIST OF TABLES

Table 1. List of oil and gas platforms on the continental shelf of the northern GOM 

surveyed during this study .......................................................................................... 7

xiii

EXECUTIVE SUMMARY

Thousands of oil platforms in the northern Gulf of Mexico (GOM) provide hard substratum 

which has not been present in GOM shallow water for thousands of years. These platforms, 

acting as numerous artificial islands throughout the region, have facilitated the geographic 

expansion of Caribbean reef coral populations. The U.S. DOI Bureau of Ocean Energy 

Management (BOEM) has provided for the “reefing” of -platforms as artificial reefs to create 

additional hard substratum habitat in this region, with the ultimate objective being to promote 

fisheries development. We assessed the effectiveness of these Rigs-to-Reef structures in 

promoting coral community development on the toppled platforms in deep water using ROV 

reconnaissance (max. depth ~110 m). We compared coral community development on five 

toppled platforms with two active, standing production platforms. 

The corals found on the platforms were the hermatypic/zooxanthellate Madracis decactis and 

several ahermatypic/azooxanthellate: Tubastraea coccinea, Oculina diffusa, and Phyllangia 

americana. 

Comparisons of standing and toppled platforms show no significant difference in total coral 

density. This was due primarily to varying, non-correlated, opposing species-specific changes in 

abundance between the two types of structures. That is, M. decactis and T. coccinea densities 

were significantly higher on toppled Rigs-to-Reefs structures. In the latter case, the corals were 

found on the “bottom” portion of the legs that had been up-ended during the toppling process, 

presumably having settled and grown there after toppling. On the other hand, P. americana was 

more abundant on standing platforms than on the toppled ones. The densities of O. diffusa were 

found to be equivalent on both types of structures. 

With respect to depth distribution, when all corals were considered together, they were 

distributed more deeply on standing platforms than on toppled Rigs-to-Reefs structures. This 

was particularly true in O. diffusa, P. americana, and T. coccinea. There was no significant 

difference between depth distributions between the two types of structures in M. decactis. All 

corals of this species were found to occur in depths of < 50 m. This was most likely due to this 

species being zooxanthellate and requiring light for survival and growth. It is believed that the 

density reduction of all coral colonies on the Rigs-to-Reefs structures in deeper water may be 

due to physical disturbance upon removal of the platform and additional disturbance during 

deployment of the structures at their new site. 

The above data suggest that Rigs-to-Reefs structures do function as a substrate for coral 

settlement. The probability of continued growth of the corals, however, varies by speciesspecifically between structure types. Toppling did not appear to enhance development of 

hermatypic coral populations, increase coral abundances in general, or create a three-dimensional 

reef-like habitat (as observed by ROV) which could promote demersal fish community 

development.
1

1.0 INTRODUCTION

1.1 GENERAL BACKGROUND: PLATFORMS IN THE NORTHERN GULF OF MEXICO 

AND THE FLOWER GARDEN BANKS

At present, there are approximately 3,200 oil and gas production platforms operating in the 

northern Gulf of Mexico (GOM) Dauterive, 2000 a,b; Francois, 1993; Knott, 1995). The shelf 

there is almost entirely soft sediment, and the platforms provide hard substratum, reaching 

through shallow water, where little exists or has existed for tens of thousands of years (Blum et 

al., 2001; Curray, 1965a, b; also see Blum et al. 1998; Frost, 1977; Schroeder et al., 1995). 

There are many banks in this region, 37 or so; some are composed of calcium carbonate and 

house coral populations (Hickerson et al., 2006; Rezak et al., 1985; Schmahl et al., 2005; 

Schmahl and Hickerson, 2006a, b), but most are drowned reefs from the Pleistocene epoch 

(Aharon, 2003; Rezak et al., 1985; Winker and Buffler, 1988). The platforms have provided new 

habitat for a variety of epibenthic fauna and flora (Adams, 1996; Boland, 2002; Bright et al., 

1991; Driessen, 1989; Gallaway and Lewbel, 1981), extending from the surface to a depth of 

hundreds of meters. They represent oases of hard substratum (Shinn, 1973, 1974; Shinn and 

Wicklund, 1989) which have been colonized by demersal fish (Childs, 1998; Love et al., 2000; 

Rooker et al., 1997; Schroeder et al., 2000; Sonnier et al., 1996; Winfield, 1973), and many 

pelagic and reef-associated fish were also attracted to these structures (McDonough and Cowan, 

2006; Stanley and Wilson, 1997, 2000). Before the introduction of platforms, hard substratum

was limited to scattered banks and shoals, mostly in deep water (Rezak et al., 1985). Hardbottom organisms were restricted to those banks. These platforms represent a novel 

development for the GOM and have provided thousands of artificial islands. 

The Flower Garden Banks (FGB) are a set of coral reefs located on two salt diapirs ~180 km 

southeast of Galveston, Texas and are among the most isolated coral reefs in the western Atlantic 

(Brazeau et al., 2005; Rezak et al., 1985; Sammarco et al., 2004; Sammarco and Brazeau, 2001;

Snell et al., 1998). Other shallow coral reefs in shallow water include, off of Tampico, Mexico–

the Isla de Lobos reefs, and in southern Florida–the Florida Keys, U.S. (Sammarco et al., 2012a).

Separating these reefs on the continental shelf are large areas of soft bottom. Soft bottom is 

generally unsuitable for coral larval settlement and growth (Hunte and Wittenberg, 1992; Miller 

et al., 2009; Sammarco, 1980, 1991); thus, natural settlement sites for shallow-water corals in the 

northern GOM are relatively scarce. Coral colonization of such habitats is achieved via larval 

dispersal, and larvae can travel hundreds to thousands of km to settle on reefs like the FGB 

(Richmond 1982, 1987) or on platforms in the northern GOM. 

1.2 CORALS IN THE NORTHERN GULF OF MEXICO

It is now known that oil and gas platforms have substantial coral communities both in the 

area of the FGB (Sammarco and Atchison, 2002; Sammarco et al., 2003, 2004) and over the 

entire northern GOM (Sammarco et al., 2012a; Sammarco, 2013), from the Matagorda Island 

(MI) lease sector, south of Corpus Christi, Texas to the Mississippi Canyon (MC) and Main Pass 

(MP) lease sectors, south of Mobile, Alabama. Coral species richness and abundance is 

moderately high there (up to 11 species), with corals exhibiting species-specific depth 

distributions. Recent broad-scale surveys have demonstrated that corals have colonized 
2

platforms throughout the northern continental shelf in the western and central GOM, particularly 

near the shelf edge. In addition, the FGB are now known to serve as a larval source for the 

platform colonization and most likely vice versa (Atchison, 2005; Atchison et al., 2004a, b, 

2005; Sammarco et al., 2012b). The platforms are clearly facilitating "island-hopping" 

(Futuyma, 1998) by coral larvae as a dispersal mechanism (Atchison, 2005; Atchison et al., 

2004a, b, 2008; Bright et al., 1991; Sammarco et al., 2004, 2005, 2012b; G. Boland, pers. 

comm., unpub. data; K. Deslarzes, pers. comm).

1.3 THE RIGS-TO-REEFS PROGRAM

Federal legislation requires that all offshore platforms be removed within one year after 

cessation of all production on a lease block (BOEM, 2012; Velazquez, 2010). In the mid-1970s, 

however, the U.S. Department of the Interior’s then Minerals Management Service (MMS), now 

the Bureau of Ocean Energy Management (BOEM), entered into a joint agreement with each of 

the Gulf States entitled the Rigs-to-Reefs Program. Under this agreement, platform owners (oil

and gas companies) were allowed to donate platforms into a program where they would remain 

on the shelf either in their original location, or could be towed to a designated artificial reef site 

within the EEZ, joining other “reefed” platforms. There are a number of such sites along the 

continental shelf of the northern GOM (Louisiana Department of Wildlife and Fisheries, 2012; 

Texas Parks and Wildlife, 2012), designed primarily to enhance the development of recreational 

and commercial fish populations (Baine, 2001; Dauterive, 1999; Pitcher and Seaman, 2000). 

Several options were available for the reefing of platforms, including a) cut at a depth of >26 m 

and leave in place, with the upper part of the platform being retrieved and returned to shore; b) 

cut similarly, but the upper portion may be toppled to create a supplementary artificial reef on 

the shelf floor; c) cut 3.5 m below the surface of the sediment and move the platform to a 

designated “Artificial Reef Zone” on the shelf, to be toppled there. The program has been used 

extensively in the northern GOM (Dauterive, 2000 a, b), particularly in Louisiana and Texas

(Kaiser and Pulsipher, 2005), and has enjoyed much success in terms of recreational fishing (see 

Frumkes, 2002). This artificial reef system is also being used as a model for use of postproduction platforms in other countries (e.g., Cripps and Aabel, 2002). 

The degree of coral community development on the deeper portions of the platforms could 

play an important role in enhancing reef development on artificial reefs in the GOM, if known 

before making a decision to “reef” the platform. If so, these would add environmental value to 

platforms cut at 26 m. Data collected by dive teams indicated that some hermatypic (Sammarco 

et al., 2012a; G.S. Boland, current study) and ahermatypic (Sammarco et al., 2011; J.K. Reed, 

pers. comm.; also see Reed, 2002) corals occur below 33 m on the platforms. Because of the 

difficulty of using divers at such depths, ROVs were used extensively in this study for surveying

in deeper water. 

In light of this information, the following questions arise: To what extent are these lower 

portions colonized by scleractinian corals, and by what species? Are they hermatypic or 

ahermatypic corals? Are corals–shallow- and deep-water–surviving the relocation and depthhabitat change to deeper, cooler, darker waters? That is, is the Rigs-to-Reef Program being 

effective with respect to benthic reef community development? 
3

1.4 OBJECTIVES

The objectives of this study are as follows: 

• To examine coral community development at >33 m depth using a Remotely Operated 

Vehicle (ROV) on select platforms in the northern GOM, comparing active, standing, 

producing platforms with toppled Rigs-to-Reefs platforms; 

• To examine comparative species-specific trends in densities of the coral populations; and 

• To define comparative depth-distributions for hermatypic compared with ahermatypic 

corals on platforms in this system. 

1.5 DURATION

Because of field trip delays caused by poor weather conditions, this study ran for ~5 years, 

including data collection through to data analysis, graphic analysis, and writing, from 2007 

through 2012. A variety of delays, some caused by hurricanes, resulted in cruise delays and 

extended the study beyond the projected original date of completion. The first two years were 

spent surveying platforms. Distribution and abundance data regarding adult coral populations 

were analyzed. Various reports were prepared during the course of the study and submitted to 

BOEM. 

5

2.0 MATERIALS AND METHODS

2.1 STUDY SITES

The study focused on the High Island and West Cameron lease areas, because this is where 

high densities of corals are known to occur on platforms (Sammarco et al., 2004, 2012a). It is 

also where a number of Rigs-to-Reefs zones are located. The two standing platforms studied 

were those known to possess marked coral community development in shallow water (Table 1). 

Such communities occur on platforms generally on the outer 25% of the shelf, primarily in the 

central and western portions of the northern GOM (i.e., south of Port Arthur, Texas and 

Terrebonne Bay, Louisiana; Sammarco et al. 2012a). Five toppled platforms were also assessed 

within areas designated as Rigs-to-Reefs zones (see Figure 1 and Table 1 for details of names 

and locations). We accessed platforms by a charter vessel (M/V Fling, Freeport, Texas). 

2.2 ROV SURVEYS

Surveys were performed using a Deep Ocean Engineering Phantom 2 ROV, a SeaBotix 

LBV-300 ROV, and a Video-Ray. Both vehicles had red laser beams directed downward or 

forward, respectively, in parallel, a certain distance apart to be used as a linear scale reference for 

size. On standing platforms, surveys were run from 20 m depth to the bottom of the platforms, a 

maximum of 110 m. Vertical pilings on the down-current side of the platform were surveyed 

along with the outer portions of two sets of horizontal struts, usually at depths of 13–17 m, and 

21–27 m. This side of the platform was surveyed for safety purposes: to help keep the ROV on 

the outside of the structure and to avoid tangling the ROV umbilical in the jacket of the platform. 

On reefed platforms, the minimum depth surveyed was ~25m. 

ROV videos and still photos were taken during each excursion. Videos were processed in the 

laboratory using Adobe® Premiere® CS5 on a Dell Precision T-3400, dual core graphics 

computer. The video was stopped every 2 m of surveyed substratum and the associated still was 

analyzed for the presence of scleractinian corals. Counts of corals were taken and recorded in a

Microsoft® Excel® spreadsheet (Sammarco, 2012a, 2013). The total for each taxon was tallied 

for each quadrat, quantified on the computer screen with a grid and standardized to a spatial 

scale. Densities were standardized to no./10 m2

. Relative abundance was also calculated for 

each group for the platform, and depth distribution was determined. Horizontal struts included 

many more quadrats than vertical pilings; thus, their area was estimated and standardized for, 

prior to calculating densities of any species. Colony counts of all coral species were also taken 

for each quadrat to provide density data. These procedures were followed for both standing and 

toppled platforms. Data were logged using Microsoft® Excel®. 
6

Small squares represent other standing, producing platforms. Standing platforms designated by 

vertical trapezoid; toppled platforms designated by a horizontal one. Location of the East and West 

FGB are also shown.

Figure 1. Map of platforms surveyed in this study for scleractinian corals. 
7

Table 1.

List of oil and gas platforms on the continental shelf of the northern GOM surveyed during this study 

Platform Type

Platform 

Code Owner Latitude Longitude

Standing 

Production

Platforms

HI-A-376 Apache Oil 27.96197 -93.67089

WC-643-A Chevron-Texaco Oil 27.98137 -93.03419

Toppled 

Rigs-to-Reefs

Platforms

EC-273-A Texas Parks and 

Wildlife Dept. 28.2550 -92.3930

HI-A-271 Texas Parks and 

Wildlife Dept. 28.364160 -93.785320

HI-A-281 Texas Parks and 

Wildlife Dept. 28.439490 -93.716940

HI-A-355 Texas Parks and 

Wildlife Dept. 28.0414 -93.7086

HI-A-492 Texas Parks and 

Wildlife Dept. 28.2266 -94.0583

2.3 STATISTICAL ANALYSES

Data were analyzed by both parametric and non-parametric statistical techniques using Rohlf 

and Slice’s (1999) BIOMStat V3.3. Procedures included basic descriptive statistical 

calculations, RxC G-tests, Kruskal-Wallis tests, and the Mann-Whitney U-tests (see Sokal and 

Rohlf, 1981). Statistical results, including data transformations, are provided in figure legends. 

Density data were transformed by square root (Y + 0.5) in graphs for normalization purposes, to 

facilitate visualization of differences discerned. 

2.4 TECHNICAL CONSIDERATIONS AND ALTERATION OF ORIGINAL PLAN

Survey plans needed to be altered due to a variety of factors. Poor or severe weather 

conditions caused truncation or cancellation and re-scheduling of cruises in the interests of safety 

of the scientific and ship’s crew. This resulted in changes in survey dates and number of 

platforms to be surveyed. Statistical analyses were adjusted accordingly to accommodate 

changes in experimental design. 

9

3.0 RESULTS

3.1 CORAL SPECIES COMPOSITION

Four species of coral were encountered during this study. One was a zooxanthellate, 

hermatypic coral, Madracis decactis (Lyman 1859). Two were azooxanthellate, ahermatypic 

corals, Tubastraea coccinea (Lesson 1829) and Phyllangia americana (Edwards and Haime 

1849). The fourth was a facultative zooxanthellate coral, Oculina diffusa (Lamarck 1816). T.

coccinea is an Indo-Pacific coral, introduced into the western Atlantic in the early 1940s (Cairns, 

2000; Humann and DeLoach, 2002). It has since spread unchecked throughout the tropical and 

sub-tropical western Atlantic; it arrived in the GOM in the 1990s (Fenner, 2001; Fenner and 

Banks, 2004). The other three species are common in the GOM and Caribbean regions. 

3.2 CORAL DENSITY

Average total coral density on the standing platforms was approximately 20/10m2, while that 

on the toppled Rigs-to-Reefs structures was ~90/10m2 (Figure 2). Despite this apparent 

difference in densities, high variances resulted in their being no significant difference in total 

coral density between these two types of structures. 

Significant differences did appear, however, between these two types of platform structures 

when individual species densities were considered. In both M. decactis and T. coccinea, coral 

densities were significantly higher on the toppled Rigs-to-Reefs platforms than on the standing 

production platforms (Figure 3). 

The trends evident in M. decactis and T. coccinea were not paralleled, however, by the other 

two scleractinian coral species assessed in this study. Densities of P. americana were ~4/10m2

on standing platforms and were significantly higher than on toppled Rigs-to-Reefs structures 

(Figure 4). Densities of O. diffusa upon first look appeared to have a similar pattern, but there 

was no significant difference in the two average densities. This was due to high variances in the 

data. 
10

Note: Mean shown along with 95% confidence limits. Coral density transformed by square root (Y+0.5) for 

normalization purposes (Sokal and Rohlf, 1981).

Figure 2. Graph showing the total coral density in no./10m2 on standing and toppled Rigs-to-Reefs 

platforms on the northern GOM outer continental shelf. 
11

Note: Mean plus 95% confidence limits shown. Coral density transformed by square root (Y+0.5) for 

normalization purposes (Sokal and Rohlf, 1981).

Figure 3. Graph showing coral density in no./10m2 in two scleractinian species: Tubastraea 

coccinea (upper graph), an azooxanthellate coral, and Madracis decactis (lower), a 

zooxanthellate one. 
12

Note: Mean plus 95% confidence limits shown. Coral density transformed by square root 

(Y+0.5) for normalization purposes (Sokal and Rohlf, 1981).

Figure 4. Graph depicting coral density in no./10m2 in two azooxanthellate 

scleractinian coral species: Phyllangia americana (upper graph) and 

Oculina diffusa (lower). 
13

3.3 CORAL DEPTH DISTRIBUTION

Depth distribution of total corals revealed different patterns of distribution between the two 

sets of platform types. The standing platforms exhibited a tri-modal distribution, with relative 

abundances of corals peaking at 30, 40, and 75 m depth (Figure 5). The depth range of the corals

extended to 95 m, near the bottom of the jackets. However, on the Rigs-to-Reefs structures,

densities were restricted to shallower depths, exhibiting a unimodal pattern, with a peak at 30-

35 m depth. The depth range of the corals on these structures extended only to 55 m. 

The only hermatypic coral found on the platforms surveyed was M. decactis, a 

zooxanthellate coral. On both the standing platforms and the Rigs-to-Reefs structures, the 

relative abundances of this species of coral peaked at 35–40 m depth (Figure 6). In both cases, 

this accounted for a total of ~75–80% of all of the colonies observed. The depth of M. decactis

extended slightly deeper on standing platforms to 50 m when compared to the Rigs-to-Reefs 

structures (40 m). This species is distributed shallow, most likely due to its need for light –

irrespective of type of structure. It would appear that any hermatypic coral colonies present on 

the standing platforms that were introduced into waters > 40 m did not survive the reefing 

process. 

The highest proportions of populations of T. coccinea were found in relatively shallow water, 

exhibiting a unimodal distribution peaking at depths of 35–40 m or less (Figure 7). There was, 

however, a significant difference in depth range and distribution between the Rigs-to-Reefs 

platform structures and the standing platforms. These corals extended much deeper on the 

standing platforms than on the Rigs-to-Reefs ones, although with low relative abundances. 

O. diffusa also showed a deeper depth distribution on the standing platforms than on the 

Rigs-to-Reefs ones, but in a more pronounced fashion (Figure 8). On the standing platforms, the 

depth of this species extended to 95 m. In this case, corals were azooxanthellate. It exhibited a 

bimodal distribution, with peaks at 60 and 75 m. On the Rigs-to-Reefs structures, the 

distribution range was shallower. The maximum depth extension was 55 m, with a clear 

unimodal peak occurring at 40 m, accounting for ~60% of the corals observed.

The depth distributions of P. americana varied in a fashion similar to those of O. diffusa. 

That is, the standing platforms exhibited a significantly deeper range of distribution than the 

Rigs-to-Reefs structures (Figure 9). Populations on both types of structures had unimodal 

distributions, with the Rigs-to-Reefs populations peaking at 40 m depth, while the standing 

platform populations peaked at 75 m depth. 
14

Figure 5. Depth distribution of all scleractinian corals. Corals distributed significantly deeper on the 

standing platforms than on the toppled Rigs-to-Reefs structures (p < 0.001, RxC 

frequency G-test).
15

Figure 6. Depth distribution of Madracis decactis. No significant difference between depth 

distributions of this species between the two sets of structures (p > 0.05, RxC frequency 

G-test). 
16

Figure 7. Depth distribution of Tubastraea coccinea. Highly significant difference between depth 

distributions of this species between the two sets of structures (p < 0.001, RxC frequency 

G-test). 
17

Figure 8. Depth distribution of Oculina diffusa. Corals distributed significantly deeper on the 

standing platforms than on the toppled Rigs-to-Reefs structures (p < 0.001, RxC frequency 

G-test). 
18

Figure 9. Depth distribution of Phyllangia americana. Corals distributed significantly deeper on the 

standing platforms than on the toppled Rigs-to-Reefs structures (p < 0.001, RxC 

frequency G-test).
19

4.0 DISCUSSION

4.1 OVERALL DENSITY COMPARISONS

The finding that there is no significant change in overall coral density between standing and 

toppled platforms is important, because, on the surface, it suggests that there is no particular 

advantage or disadvantage to toppling platforms and allowing the benthic community, 

particularly corals, to develop on them as artificial reefs. The rationale behind this lack of 

change, however, lies in the details of the response of different coral species to this event. That 

is, there has been a species-specific response to the toppling, and the responses were not parallel. 

4.2 MADRACIS DECACTIS

First, the reason that M. decactis densities do not vary between the two structure types relates 

to its biology and resultant depth distribution, which are essentially the same on both structure 

types. M. decactis is a zooxanthellate, hermatypic coral and requires light for colony survival 

and growth. Whether the platform structure is upright or on its side, the colonies will survive 

only in shallow water. Those colonies existing on standing platforms that were transported into 

deeper, darker, cooler water upon toppling will not have survived. Those larvae that settled in 

shallower water would have a higher probability of survival than in deeper water. Thus, there is 

a nil effect on changes in the overall density of the coral M. decactis upon toppling of a platform. 

4.3 TUBASTRAEA COCCINEA

T. coccinea populations seem to thrive more on toppled platforms than on standing ones. 

This seems logical, because this species is known to do particularly well not only on artificial 

substrata such as platforms, but also in disturbed habitats (Byers, 2002; Sheehy and Vik, 2010). 

A toppled platform may be considered a disturbed habitat for several reasons. First, the mode of 

severing from seafloor of a standing platform is often explosives set around the base of the major 

support pilings. This would dislodge numerous sessile and vagile epibenthic organisms from the 

jacket and would kill many of the demersal, pelagic, and reef-associated fish in the area due to 

shock. Thus, the toppled platform will have a great deal of newly available space for settlement 

by incoming larvae or expansion by robust surviving species. Second, upon impacting the 

seafloor, the toppled platform will disturb the sediment there, resulting in high sedimentation on 

the platform and additional mortality of both shallow and deeper water organisms, freeing up 

additional space for colonization or growth by T. coccinea. 

4.4 Oculina diffusa

Although O. diffusa populations did not vary in size between standing and toppled platform 

structures, there was a major shift in its populations to shallower water after toppling. As 

mentioned above, this species is a facultative zooxanthellate one (Cairns, 1999). However, in 

this case, no colonies were observed to have pigmentation and they did not appear to harbor 

substantial zooxanthellar populations. Thus, light is not believed to have played an important 

role in this distribution change. It is more likely that the platform removal process played a 

greater role. Of all species encountered here, O. diffusa was the most fragile. This was because, 

unlike the other species, it has a thin, delicate branching morphology. Explosive removal of the 
20

platform would have caused severe damage to the O. diffusa colonies on it. By comparison, both

M. decactis and Phyllangia americana are massive in form. Although T. coccinea also has a 

branching format, its branches are quite short, and colonies are stronger and more resistant to 

breakage. 

4.5 PHYLLANGIA AMERICANA

P. americana’s populations appear to suffer significantly upon toppling of a platform. Not 

only did the populations shrink dramatically in size, they were also restricted to shallower water. 

Again, this is an azooxanthellate species (Cairns, 2000) and does not require light; thus, the loss 

of depth range is not due to loss of light with depth. 

4.6 NO EVIDENCE FOR ENHANCEMENT OF CORAL COMMUNITY STRUCTURE 

AND RESULTANT FISH HABITAT

Although there is no overall change in coral density between standing platforms and toppled 

Rigs-to-Reefs structures, significant changes in coral community structure do result from the

toppling procedure. In addition, underlying those changes are species-specific responses. Some 

coral population densities do not change after this event; others do. Some do not change their 

depth distribution; others do. It is clear that toppling of platforms dramatically affects the coral 

communities on the platforms, changing the species composition and diversity with depth. At 

this point, however, it seems clear that the toppling of platforms does not enhance the 

development of community structure. 

One of the major purposes of artificial reefs programs, and particularly the Rigs-to-Reefs 

program, is to facilitate development of fisheries, particularly recreational, in the offshore 

environment (see above). It is well known that coral development on hard bottom promotes fish 

population growth by providing three-dimensional relief, habitat, and refuge (Lindberg et al., 

2006; Peterson et al., 2000). The lack of three-dimensional structure provided by a welldeveloped coral community would not provide those habitat characteristics known to enhance 

fish community development. In addition, it would fail to provide critical habitat cues for 

settlement of the larvae of demersal and reef fish (Breitburg et al., 1995). Any procedure that 

removes corals and three-dimensional relief from these platforms will detract from their 

suitability as fish habitat. 
21

5.0 CONCLUSIONS

The conclusions of this study are as follows:

• With respect to all corals considered together, there was no significant difference 

between corals found on standing platforms compared with the toppled Rigs-to-Reefs

platforms. 

• Densities of the hermatypic/zooxanthellate coral Madracis decactis were significantly

higher on the Rigs-to-Reefs structures than on the standing platforms. 

• Densities of the ahermatypic/azooxanthellate coral Tubastraea coccinea were also found 

to be significantly higher on the Rigs-to-Reefs structures than on the standing platforms. 

• Densities of the ahermatypic/azooxanthellate coral Phyllangia americana, however, were 

found to be significantly higher on the standing platforms than on the Rigs-to-Reefs 

structures. 

• Likewise, densities of the ahermatypic/azooxanthellate coral Oculina diffusa were not 

found to be significantly different from the standing platforms. 

• These opposing patterns caused the overall lack of difference in coral density between the 

two types of structures. 

• Overall, corals were found to be distributed to significantly deeper depths on standing 

platforms than on Rigs-to-Reefs structures.

• There was no significant difference between depth distributions in M. decactis between 

the two types of structures. All corals of this species were found to occur in depths of 

<50 m. This was most likely due to this species being zooxanthellate and requiring light 

for survival and growth. 

• T. coccinea, O. diffusa, and P. americana were all found to be distributed significantly 

more deeply on standing platforms than on Rigs-to-Reefs structures. 

• It is believed that the reduction in density of many corals on the Rigs-to-Reefs structures 

in deeper water may be due to physical disturbance on removal of the platform and 

deployment of the structures at their new site.

23

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