Ecological Performance

of OCS Platforms

as Fish Habitat off California

Pacific Outer Continental Shelf Region OCS Study

U.S. Department of Interior

Minerals Management Service

Pacific OCS Region

Ecological Performance

of OCS Platforms

as Fish Habitat off California

Authored by:

Milton S. Love

Donna M. Schroeder

Submitted by:

Marine Science Institute

University of California

Santa Barbara, CA 93106

Prepared under:

MMS Cooperative Agreement No. 1435-01-03-CA-72694

U.S. Department of Interior

Minerals Management Service Camarillo

Pacific OCS Region March 2006

OCS Study
Disclaimer 

This report had been reviewed by the Pacific Outer Continental Shelf Region, Minerals 

Management Service, U.S. Department of the Interior and approved for publication. The 

opinions, findings, conclusions, or recommendations in this report are those of the 

authors, and do not necessarily reflect the views and policies of the Minerals 

Management Service. Mention of trade names or commercial products does not constitute 

an endorsement or recommendation for use. This report has not been edited for 

conformity with Minerals Management Service editorial standards. 

Availability 

Available for viewing and in PDF at: 

www.lovelab.id.ucsb.edu 

Minerals Management Service 

Pacific OCS Region 

770 Paseo Camarillo 

Camarillo, CA 93010 

805-389-7621 

Milton Love 

Marine Science Institute 

University of California, Santa Barbara 

Santa Barbara, CA 93106 

805-893-2935 

Suggested citation 

Love, M.S. and D.M. Schroeder. Ecological performance of OCS platforms as fish 

habitat off California. MMS OCS Study 2005-005. Marine Science Institute, University 

of California, Santa Barbara, California. MMS Cooperative Agreement Number 1435-01-

03-CA-72694. 
Table of Contents

Technical Summary i

Study Product iv

Executive Summary vi

Task 1: 1

Distribution of Bocaccio (Sebastes paucispinis) and Cowcod 

(S. levis) around Oil Platforms and Natural Outcrops off 

California with Implications for Larval Production

Task 2: 13

Ecological Performance of Young-of-the-Year Blue Rockfish 

(Sebastes mystinus) Associated with Oil Platforms and Natural 

Reefs in California as Measured by Daily Growth Rates

Task 3: 28

Comparative Predation Rate on Painted Greenling Between 

Natural Habitat and an Offshore Oil Platform
i

Technical Summary

Study Title: Ecological Performance of OCS Platforms as Fish Habitat off California

Report Title: Ecological Performance of OCS Platforms as Fish Habitat off California

Contract Number: 1435-01-03-CA-72694

Sponsoring OCS Region: Pacific

Applicable Planning Area: Central and Southern California

Fiscal Years of Project Funding: FY2003-FY2005

Completion Date of the Report: March 2006

Costs: $175,000

Cumulative Project Cost: $175,000

Principal Investigator: Milton Love

Key Words: oil platforms, platforms, California, rockfishes, Sebastes, ecological performance, decommissioning, platform decommissioning, bocaccio, cowcod, blue rockfish, Sebastes paucispinis, Sebastes levis, 

Sebastes mystinus

Background and Objectives:

There are 27 oil and gas platforms in the waters off California. All platforms have a finite economic life 

and the life spans of some California platform may be nearing an end. Once an industrial decision is made 

to cease oil and gas production at a platform, managers must decide what to do with the structure, a process 

known as decommissioning. During the decommissioning process, the MMS conducts detailed environmental reviews of any proposed projects to evaluate the impacts from platform removal on regional fish 

populations. Removing platforms removes potentially useful habitat, kills numerous fishes, and may have 

adverse impacts on regional populations of rockfishes and other species on the Pacific OCS. Thus, the assessing of habitat quality greatly bears upon platform decommissioning issues, as questions about Essential 

Fish Habitat and the ecological role of Pacific OCS platforms are still unresolved.

Previous federal government funded investigations found that platform fish assemblages are similar to 

those of natural reefs and differences were due almost entirely to the greater numbers of more species of fishes 

around platforms, rather than differences in species composition. At least 85 species of fish were observed at 

platforms and 94 species at the outcrops. Rockfishes dominated both habitats, comprising 89.7% of all fishes at 

platforms and 92.5% at outcrops. Almost all of the more abundant species that the researchers observed were 

more common around platforms. Species that were more common at one or more platforms than at natural 

reefs included cowcod and bocaccio (young-of-the-year (YOY), juvenile, and adult), copper, greenspotted, 

greenstriped, YOY widow, vermilion, canary and flag rockfishes and YOY juvenile and adult lingcod.

In particular, these studies found that (1) platforms tended to have higher abundances of large fishes, particularly of such economically important species as cowcod, bocaccio, and lingcod, than did most or all natural 

reefs and that (2) in general, platforms harbored higher densities of young-of-the-year (YOY) rockfishes than 

did nearby natural outcrops or, indeed, most other outcrops surveyed in central and southern California. 

However, less well known is how important these fishes are on a regional basis and how well they fare 

around these artificial habitats compared to natural reefs. This is often referred to as “ecological performance.” Among the key issues in the Pacific OCS platform reefing debate is defining the ecological performance of fishes, and by extension the role that these fishes may play in the recovery of fish populations.

Thus, the goal of this study was to determine certain aspects of the ecological performance of fishes 

living on offshore platforms compared to those living on natural reefs. 
ii

Description:

The comparative ecological performance of fishes at oil platforms and natural outcrops was measured 

through three tasks.

Task One: Larval production of bocaccio and cowcod at platforms and natural reefs

In this task, the amount of larvae per unit area produced by bocaccio and cowcod was estimated and 

compared with that produced by both species on natural outcrops in the same general area. This was done 

by determining the densities and sizes of bocaccio and cowcod at platforms and natural reefs and, knowing 

the size-specific fecundity of each species, computing the larval production of the fishes at each location.

Task Two: Ecological performance of young-of-the-year blue rockfish associated with oil 

platforms and natural reefs. 

Comparing the growth rates of fishes living around platforms with those on natural reefs is one method of contrasting the overall health of these animals. If growth rates of juvenile rockfish at platforms are 

similar (or better) than those from reefs, it implies that the young rockfishes at the platforms are at least as 

likely to grow and develop as those from reefs. In this task, the daily growth rates of young-of-the-year blue 

rockfish living around three platforms and three natural outcrops were compared.

Task Three: Predation Rate on Painted Greenling Between Natural Habitat and an Offshore Oil 

Platform.

Another measure of ecological performance is mortality rate. In this task we compared the mortality 

rate of young painted greenling at a platform with that of fish at a natural outcrop. 

Significant Results:

Task 1: In this study, we used data from manned submersible studies conducted between 1995 and 

2002 around seven platforms and over 80 deeper water reefs in southern and central California. We focused 

our analyses on bocaccio and cowcod, species that had been economically important to recreational and 

commercial fishermen and were declared over-fished by the National Marine Fisheries Service.

Overall, we observed 1,054 bocaccio of all sizes at platforms and 976 at reefs, and 125 cowcod at platforms and 134 at reefs. Among bocaccio estimated to be mature, we observed 313 bocaccio at platforms and 

313 at natural reefs and 38 mature cowcod at platforms and 61 at reefs. Adult fishes were patchily distributed 

among both platforms and natural reefs. Fifty-seven percent of the natural reefs had mature bocaccio, as 

did four of seven platforms. Forty-two percent of natural reefs harbored mature cowcod, as did two of five 

platforms. Two platforms, Gail and Hidalgo, harbored relatively high densities of both species. Individuals 

of both species tended to be larger at the platforms, particularly those inhabiting Platform Gail.

Because both densities and mean lengths of both species were higher at Platform Gail than at any 

natural reef, their potential larval production at that platform was far higher than at any natural reef. We 

estimated that if Platform Gail (whose footprint is 0.5337 ha) were removed, it would be the equivalent of 

removing 12.57 ha of average-producing natural habitat in southern California for cowcod, or equivalent to 

removing 29.24 ha of average-producing natural habitat in southern California for bocaccio.

This study demonstrated that, in some instances, fishes at platforms provide significant amounts of 

larvae, numbers that may be of regional importance.

Task 2: The daily growth rates of YOY blue rockfish collected during the fall of 1999 at three platforms 

and three natural reefs were compared. There was no evidence that fish at platforms grew more slowly than 

those living on natural reefs. Comparison of growth rates between fishes living at Platform Holly and Naples 

Reef implied that YOY blue rockfish may, in some instances, grow faster at platforms.
iii

Task 3: This research was conducted at two sites, Platform Holly and Naples Reef, that are located near 

each other in the Santa Barbara Channel using the painted greenling, Oxylebius pictus (Hexagrammidae), 

a small reef fish common to offshore platforms and kelp beds off California. The experiment showed that 

juvenile painted greenling residing at Naples Reef experienced a relative predation rate more than 2.5 times 

greater than the estimated rate experienced by juveniles residing at Platform Holly. This is likely due to the 

relatively low density of predators that live in platform midwaters compared to natural reefs. This implies 

that the large numbers of young fishes living around the platforms of southern California have lower mortality rates due to predation, than do these fishes inhabiting natural reefs.
iv

STUDY PRODUCTS

PaperS

2005 Love, M. S. D. M. Schroeder, and W. H. Lenarz. Distribution of bocaccio (Sebastes paucispinis) and 

cowcod (Sebastes levis) around oil platforms and natural outcrops off California with Implications for 

larval production. Bull. Mar. Sci. 77:397-408.

2005 Love, M. S., E. Brothers, D. M. Schroeder, and W. H. Lenarz. Ecological performance of young-of-the 

year blue rockfish (Sebastes mystinus) associated with oil platforms and natural reefs in California as 

measured by daily growth rates. Bull. Mar. Sci. Submitted for Publication.

PAPERS PRESENTED

Presentations by Milton Love:

Oil platforms as fish habitat. Minerals Management Service Symposium, Oceanology International, New 

Orleans, Louisiana. May 2003.

Fish Assemblages of southern California oil platforms. Mineral Management Service, Decommissioning 

Offshore Platforms and Pipelines: Environmental Studies Workshop, Catalina Island. October 2003.

Fish Assemblages of southern California oil platforms. Cabrillo Marine Aquarium, San Pedro, CA. November 2003.

Oil platforms as fish habitat. Pt. Lobos Association, Monterey, California. January 2004.

For now we see through a glass, darkly: a potpourri of observations on the fish assemblages of oil/gas platforms and natural reefs in southern California. Western Groundfish Conference, Victoria, British Columbia. February 2004.

Oil platforms as fish habitat. Centro de Investigacion Cientifica y de Educacion Superior Ensenada (CICESE), 

Ensenada, Mexico. March 2004.

The role of oil and gas platforms in fish production and aggregation in southern and central California. 

Friends of Moss Landing, Moss Landing, California. July 2004.

The role of oil and gas platforms in fish production and aggregation in southern and central California. 

University of Alaska, Juneau. July 2004.

The role of oil and gas platforms in fish production and aggregation in southern and central California. 

Plains Oil Company, Santa Maria, CA. Oct. 2004.

The role of oil and gas platforms in fish production and aggregation in southern and central California. 

Ocean Explorers, 27 January 2005.

The role of oil and gas platforms in fish production and aggregation in southern and central California. 

Environmental Defense, Oakland, CA. 24 March 2005.

More fun than working for a living – tales of 10 years of research on the fishes of California oil platforms, 

CARAH, Biloxi, 11 April 2005.

An overview of fish assemblages of offshore oil and gas platforms and natural reefs in southern and central 

California. American Fisheries Society, Anchorage, Alaska, 11 September 2005.
v

Presentations by Donna Schroeder:

Comparative juvenile reef fish recruitment and mortality between offshore oil/gas platforms and natural 

reefs. CARAH, Biloxi, Miss.12 April 2005.

Patterns in juvenile reef fish recruitment to offshore oil/gas platforms and natural reefs in relation to upwelling. American Fisheries Society, Anchorage, Alaska, 11 September 2005.

POSTERS 

Emery, B.M., M.M. Nishimoto L. Washburn, M. Love, 2003, “Alternative fate estimation of oil rig bocaccio 

recruits using HF radar,” Eastern Pacific Ocean Conference, Catalina Island, CA, 24-27 Sept., poster 

presentation

Emery, B., M. Nishimoto, L. Washburn, and M. Love. Do offshore platforms affect the fate of recruiting 

bocaccio? An analysis based on HF radar derived surface trajectories. Western Groundfish Conference, 

Victoria, British Columbia, Feb. 2004.
vi

Ecological Performance of 

OCS Platforms as Fish Habitat off California

Executive Summary

Information Needed

There are 27 oil and gas platforms in the waters off California. These platforms are located between 1.2 

and 10.5 miles from shore and at depths ranging from 11 to 363 m (35-1,198 ft.). All platforms have a finite 

economic life and the life spans of some California platform may be nearing an end. Once an industrial 

decision is made to cease oil and gas production at a platform, managers must decide what to do with the 

structure, a process known as decommissioning. Regarding oil and gas platforms, the Minerals Management Service (MMS) defines decommissioning as the process of ending operations and returning the lease 

or pipeline right-of-way to a condition that meets the requirements of the regulations. The MMS works to 

ensure that wells are plugged to prevent pollution; that pipelines are decommissioned to prevent seepage of 

hydrocarbons and removed to minimize conflicts with other uses of the Outer Continental Shelf (OCS); and 

that all sites are cleared of obstructions to minimize use conflicts. During the decommissioning process, the 

MMS conducts detailed environmental reviews of any proposed projects to evaluate the impacts from platform removal on regional fish populations. Removing platforms removes potentially useful habitat, kills numerous fishes, and may have adverse impacts on regional populations of rockfishes and other species on the 

Pacific OCS. Thus, the assessing of habitat quality greatly bears upon platform decommissioning issues, as 

questions about Essential Fish Habitat and the ecological role of Pacific OCS platforms are still unresolved 

Several MMS- and USGS-funded investigations and a number of pilot studies have been completed 

and provide background for the present effort. The habitat value of a number of platforms on the Pacific 

OCS was determined under OCS Study MMS 99-0015, The Ecological Role of Natural Reefs and Oil and Gas 

Production Platforms on Rocky Reef Fishes in Southern California [available as a pdf at www.lovelab.id.ucsb.

edu]. In that study, researchers from the University of California at Santa Barbara (UCSB) compared fish 

assemblages from eight platforms and many natural outcrops throughout southern California. 

Overall, the study found that platform fish assemblages are similar to those of natural reefs and differences were due almost entirely to the greater numbers of more species of fishes around platforms, rather 

than differences in species composition. At least 85 species of fish were observed at platforms and 94 species 

at the outcrops. Rockfishes dominated both habitats, comprising 89.7% of all fishes at platforms and 92.5% 

at outcrops. Almost all of the more abundant species that the researchers observed were more common 

around platforms. Species that were more common at one or more platforms than at natural reefs included 

cowcod and bocaccio (young-of-the-year (YOY), juvenile, and adult), copper, greenspotted, greenstriped, 

YOY widow, vermilion, canary and flag rockfishes and YOY juvenile and adult lingcod.

Two of the major conclusions from this study were:

1) Platforms tended to have higher abundances of large fishes, particularly of such economically important species as cowcod, bocaccio, and lingcod, than did most or all natural reefs. It is likely that this is 

due to the relatively low fishing effort around many platforms in southern California, thus the platforms 

are acting as de facto marine protected areas. The study compared densities of all rockfishes (of all sizes), all 

rockfishes greater than or equal to 30 cm, and adult bocaccio and cowcod that they observed at platforms 

and at natural outcrops. In most instances, fishes 30 cm or larger were less abundant, or sometimes absent, 

from many natural reefs compared to most platforms. Platform Gail, in particular, held some of the highest 

densities of the important but severely depleted cowcod and bocaccio that were seen anywhere during the 
vii

observations. The study speculated that platforms, such as Platform Gail, might be a major source of cowcod 

and bocaccio larvae and thus might be important fish habitat on a regional level. 

2) The research also found evidence that, in general, platforms harbored higher densities of youngof-the-year (YOY) rockfishes than did nearby natural outcrops or, indeed, most other outcrops surveyed in 

central and southern California. The study pointed out that platforms occupy more of the water column 

than do most natural outcrops and presettlement pelagic juvenile rockfishes are much more likely to encounter these tall structures than the relatively low-lying natural structures. The researchers also observed 

that many of the major predators of young rockfishes are species that live and stay close to the bottom and 

that, in general, these species do not ascend the platform jacket up into the water column, and thus are absent from the platform midwaters. 

However, while it is undisputed that large numbers of both YOY and adult fishes populate platform 

habitats, less well known is how important these fishes are on a regional basis and how well they fare around 

these artificial habitats compared to natural reefs. This is often referred to as “ecological performance.” 

Among the key issues in the Pacific OCS platform reefing debate is defining the ecological performance of 

fishes, and by extension the role that these fishes may play in the recovery of fish populations.

One way to determine the ecological performance and relative importance of a platform as fish habitat 

is to compare the amount of larvae produced by fishes at platforms with that produced on natural outcrops 

in the same general area. This may indicate whether a platform is a significant source of larvae when compared to natural reefs 

Regarding YOY rockfishes, some observers have suggested that, while settlement of young rockfish 

may be high around some platforms, these structures may not supply suitable habitat or sufficient food for 

adequate growth and maturity. One way of addressing these concerns is to examine the health of young 

rockfishes around platforms and compare this to the health of young rockfish of the same species living on 

natural reefs. Comparing the growth rates of fishes living around platforms with those on natural reefs is 

one method of contrasting the overall health of these animals. If growth rates of juvenile rockfish at platforms are similar (or better) than those from reefs, it implies that the young rockfishes at the platforms are 

at least as likely to grow and develop as those from reefs.

A third way of approaching ecological performance is by comparing the mortality rate of young fishes 

at platforms with those at natural reefs. If survival and mortality rates of at platforms are similar (or better 

in the case of survival) than those from reefs, it implies that fishes at the platforms are at least as likely to 

survive to adulthood or to enter a fishery as those from natural reefs.

Thus, the goal of this study was to determine certain aspects of the ecological performance of fishes 

living on offshore platforms compared to those living on natural reefs. 

Research Summary

Task 1. Distribution of bocaccio (Sebastes paucispinis) and cowcod (Sebastes levis) Around Oil 

Platforms and Natural Outcrops off California with Implications for Larval Production

In this study, we used data from manned submersible studies conducted between 1995 and 2002 around 

seven platforms and over 80 deeper water reefs in southern and central California. We focused our analyses 

on bocaccio and cowcod, species that had been economically important to recreational and commercial 

fishermen and were declared over-fished by the National Marine Fisheries Service. 

We considered a reef site appropriate for inclusion in this study if it was in a depth of least 60 m for 

bocaccio and 80 m for cowcod and was categorized as “rocky high relief,” “rocky low relief,” or “boulders.” 

Based on these parameters, 63 reefs and seven platforms (Gail, Grace, Harvest, Hermosa, Hidalgo, Holly, and 
viii

Irene) were used in the bocaccio analyses and 52 reefs and five platforms (Gail, Grace, Harvest, Hermosa, 

and Hidalgo) in the cowcod dataset. At each survey site, we computed the adult densities and the length frequencies of both species. From these densities and length frequencies and from published data on fecundity 

at length, we then computed the potential larval production of each species at each site. In general, because 

densities of both species were quite low or nonexistent, particularly at natural reefs, there was little or no 

potential larval production. 

Overall, we observed 1,054 bocaccio of all sizes at platforms and 976 at reefs, and 125 cowcod at platforms and 134 at reefs. Among bocaccio estimated to be mature, we observed 313 bocaccio at platforms and 

313 at natural reefs and 38 mature cowcod at platforms and 61 at reefs. Adult fishes were patchily distributed 

among both platforms and natural reefs. Fifty-seven percent of the natural reefs had mature bocaccio, as 

did four of seven platforms. Forty-two percent of natural reefs harbored mature cowcod, as did two of five 

platforms. Two platforms, Gail and Hidalgo, harbored relatively high densities of both species. Individuals 

of both species tended to be larger at the platforms, particularly those inhabiting Platform Gail. 

Because both densities and mean lengths of both species were higher at Platform Gail than at any 

natural reef, their potential larval production at that platform was far higher than at any natural reef. We 

estimated that if Platform Gail (whose footprint is 0.5337 ha) were removed, it would be the equivalent of 

removing 12.57 ha of average-producing natural habitat in southern California for cowcod, or equivalent to 

removing 29.24 ha of average-producing natural habitat in southern California for bocaccio.

This study demonstrated that, in some instances, fishes at platforms provide significant amounts of 

larvae, numbers that may be of regional importance.

Task 2. Ecological Performance of Young-of-the-Year Blue Rockfish (Sebastes mystinus) Associated 

with Oil Platforms and Natural Reefs in California as Measured by Daily Growth Rates

In this study, we compared the daily growth rates of YOY blue rockfish collected during the fall of 1999 

at three platforms and three natural reefs. Comparisons were made between fish at Platform Irene and a reef 

at Santa Rosa Island (both are in the California Current and are 84 km apart), Platform Holly and Naples Reef 

(in the same water mass and located with 8 km of each other) and Platform Gilda and Anacapa Island (in 

different water masses and within 16 km of each other). Collections were made during one day at each site. In 

the laboratory, each fish was measured (standard length, SL) to the nearest millimeter and its sagittal otoliths 

removed. A mid-sagittal section was prepared to preserve both the otolith margin and as well as early growth 

around the primordium. Microincrements were counted with the aid of a video microscope (to about 1500x 

magnification). The typical counting path was from the primordium to the dorsal otolith margin. 

We collected a total of 187 blue rockfish; collections ranged in number from 15 fish at Santa Rosa Island to 39 fish at Naples Reef. Fish were sampled between 15 September and 23 November 1999. Calculated 

mean daily growth rates of individuals ranged from 0.27 to 0.46 mm/day and mean growth rates at each 

site varied between 0.30 and 0.38 mm/day. Mean growth rates were statistically different among most sites. 

Growth rates were significantly different between fish living at Platform Holly and Naples Reef and those 

living at Platform Gilda and Anacapa Island. In both instances, fish grew more rapidly at the platforms. 

However, because fish at Gilda and Anacapa were collected two months apart, growth comparisons were not 

considered valid. There was no statistical difference between blue rockfish growth rates at Platform Irene 

and Santa Rosa Island. We combined the growth rates of fish from Platform Irene and Holly and compared 

the mean growth rate with that of fish living at Naples Reef and Santa Rosa Island. Blue rockfish living at the 

platforms grew 0.016mm/day more quickly.

In this study there was no evidence that fish at platforms grew more slowly than those living on natural 

reefs. Comparison of growth rates between fishes living at Platform Holly and Naples Reef implied that YOY 

blue rockfish may, in some instances, grow faster at platforms.
ix

Task 3. Comparative Predation Rate on Painted Greenling Between Natural Habitat and an 

Offshore Oil Platform

We conducted this study at two sites, Platform Holly and Naples Reef that are located near each other 

in the Santa Barbara Channel. We conducted our study at Platform Holly on the shallowest (9 m) horizontal 

crossbeams and at Naples Reef in 10 m depth. Our model organism, the painted greenling, Oxylebius pictus (Hexagrammidae), is a small reef fish common to offshore platforms and kelp beds off California. We 

employed a tethering approach to compare relative predation rates on juvenile painted greenling between 

the platform and natural habitat. This benthic fish lacks a swim bladder, and its most frequently observed 

daytime behavior is to simply rest on the bottom, making tethering a suitable experimental technique.

Relative predation rate on painted greenling was significantly higher at Naples Reef than at Platform 

Holly. Naples Reef also had higher painted greenling density and higher predator density. We also found that 

the number of predator species was much greater at Naples Reef than at Platform Holly. 

The tethering experiment shows that juvenile painted greenling residing at Naples Reef experienced a 

relative predation rate more than 2.5 times greater than the estimated rate experienced by juveniles residing 

at Platform Holly. This is likely due to the relatively low density of predators that live in platform midwaters 

compared to natural reefs. This implies that the large numbers of young fishes living around the platforms of 

southern California have lower mortality rates due to predation, than do these fishes inhabiting natural reefs. 

Conclusions

We conducted three studies that measured and compared various aspects of the ecological performance of fishes living on platforms and natural reefs. From this research, it is apparent that fishes living 

around platforms perform at least as well as those inhabiting natural reefs and, in some instances, better. 

For instance, it is clear that Platform Gail, and to a lesser extent Platform Hidalgo, are very important 

regional sources of larvae for both cowcod and bocaccio. This is likely to remain the case for as long as these 

platforms act as de facto marine reserves and for as long as stocks of these two species remain depressed. 

Parenthetically, our unpublished density patterns of other species, for instance greenspotted, greenblotched, 

and flag rockfishes, strongly imply that platforms are also major exporters of these larvae.

Our research also has considerable bearing on the issue of the extremely heavy densities of youngof-the-year rockfishes living around oil platforms. In many years, platform midwaters harbor tens or even 

hundreds of thousands of young blue, olive, squarespot, widow, and yellowtail rockfish and bocaccio. Some 

observers have questioned the fate of these fishes, implying that their mortality rates might be higher than 

congeners living on natural reefs. Our research demonstrates that, in some instances, young rockfishes living 

around oil platforms suffer less mortality than those living over natural reefs and there is no evidence that 

mortality rates of fishes living around oil platforms is greater than that of natural reef fishes.

While these studies were limited in scope, we consider our results to be sufficiently strong to demonstrate the importance of some artificial structure in driving marine population dynamics. It is now arguable 

that fishes living around oil platforms may be quite important to regional fish production.
1

Task 1: Distribution of Bocaccio (Sebastes paucispinis) and Cowcod 

(S. levis) around Oil Platforms and Natural Outcrops off California with 

Implications for Larval Production

Milton S. Love, Donna M. Schroeder, and William H. Lenarz

Abstract

There is increasing evidence that some California oil platforms form important habitats for a 

number of economically important fishes. We asked to what extent might platforms be important 

as producers of larvae of several overfished species (bocaccio, Sebastes paucispinis Ayres, 1854 and 

cowcod, S. levis Eigenmann and Eigenmann, 1889) on a local or regional basis. We compared adult 

densities and potential larval production of these species at platforms and natural outcrops in California. Densities of mature bocaccio and cowcod were highly variable among survey sites, but were 

generally very low at both natural reefs and platforms. However, the mean densities for both species 

were higher around platforms than at natural reefs. Two of the three platforms (Gail and Hidalgo) 

that harbored mature bocaccio had larger mature individuals than did any natural reef. Platform 

Gail had by far the highest densities of both mature bocaccio and cowcod of any natural or humanmade habitat and the potential larval production of both species at Platform Gail was much higher 

than at any other site surveyed. We estimated the removal of Platform Gail would be the equivalent 

of removing 12.57 ha of average- producing natural habitat in southern California for cowcod or 

29.24 ha of average-producing natural habitat for bocaccio. These results may have implications for 

the platform decommissioning process. 

Introduction

There are 27 oil and gas platforms off the coast of southern and central California. Located in both 

state and federal waters, these structures are situated in bottom depths ranging from 11 to 363 m and can 

have footprints as large as 10,606 m2 (Love et al., 2003). While all of these platforms are currently either 

pumping oil or gas or are being used as transfer stations for these products, platforms have a finite economic 

life span. Once an industrial decision is made to cease oil and gas production, managers must decide what 

to do with the structure, a process known as decommissioning. Decommissioning is a complex process, involving state and federal agencies, corporate entities, and such stakeholders as recreational and commercial 

fishermen and non-consumptive users (Schroeder and Love, 2004). Ultimately, a decommissioned platform 

could be left in place, removed to some point below the sea surface, toppled to lie on the sea floor, or totally 

removed (Schroeder and Love, 2004).

One issue in the decommissioning process is the role that platforms may play as fish habitat. There 

is increasing evidence that some California platforms form important habitats for many economically important fishes. This is particularly true of the rockfishes (genus Sebastes) that often comprise over 90% of 

all fishes observed around platforms in southern and central California waters (Love et al., 2003). Platform 

habitat may serve at least two functions for these fishes. First, the midwaters of many platforms serve as 

nursery habitat for a suite of rockfishes and other fish species, often harboring higher densities of these 

species than do nearby natural outcrops (Love et al., 1999; Carr et al., 2003; Love et al., 2003). Compared to 

most natural reefs, a platform’s size, structural complexity, and high vertical profile probably provide pelagic 
2

juvenile rockfishes and larvae of other species with a relatively strong stimulus to trigger settlement. In addition, most platforms have few large fishes in the midwaters and thus predation on young fishes is likely to 

be low (Schroeder et al., 2000; Love et al., 2003). Platform bottoms, where the jacket and conductor pipes 

meet the seafloor, may harbor high densities of subadult and adult fishes, again usually comprised primarily 

of rockfishes (Love et al., 1999; Carr et el., 2003; Love et al., 2003). The high densities of larger fishes at the 

platform bottoms is due to both acceptable habitat and because some platforms are rarely fished and thus 

act as de facto marine reserves (Schroeder and Love, 2002; Love et al., 2003). 

Off southern California, fishing pressure by both recreational and commercial fishermen on natural 

outcrop species has been very heavy for many decades (Love et al., 1998, 2002). In particular, the adults of 

economically important species such as bocaccio (Sebastes paucispinis Ayres, 1854) and cowcod (S. levis Eigenmann and Eigenmann, 1889) are now uncommon or even absent on many natural outcrops in southern 

California (Love et al., 1998, 2003; Love and Yoklavich, unpubl. data). 

Given the depleted state of many rockfish species on natural reefs (seven species have been declared 

overfished by NOAA Fisheries), and the relative abundance of some of these species on some platforms, we 

asked to what extent might platforms be important as producers of larvae on a local, or even regional, basis? 

That is, how important are adult populations of rockfishes on platforms? To help answer that question, we 

conducted a pilot study that focused on cowcod and bocaccio (Fig. 1), two species declared overfished by 

NOAA Fisheries and the two overfished species that are most abundant as adults around California platforms. We compared adult densities and potential larval export of these species at both offshore platforms 

and natural outcrops in central and southern California. A knowledge of the relative importance of these 

human-made structures as fish habitat could play a role in decommissioning decisions. 

Figure 1. a) Adult bocaccio (Sebastes paucispinis) and b) cowcod (S. levis) at the bottom of Platform Gail, eastern Santa 

Barbara Channel. Note the undercut area below the bottom-most cross beam, this provides shelter for both species.
3

Materials and Methods

Fish surveys

Between 1995 and 2002, we surveyed platforms sited over a wide range of bottom depths, ranging between 29 and 224 m, and sited from north of Point Arguello to off Long Beach, southern California. Most 

of our platform surveys were conducted at seven structures (Platforms Irene, Hidalgo, Harvest, Hermosa, 

Holly, Grace, and Gail) located in the Santa Barbara Channel and Santa Maria Basin. In addition, we surveyed over 80 deeper-water outcrops (many in the vicinity of platforms), in waters between 30 and 360 m 

deep. Most of these deeper-water natural sites were visited once, a few were surveyed during as many as four 

years and, one outcrop, North Reef near Platform Hidalgo, was sampled annually. 

We surveyed fish assemblages using the Delta submersible, a 4.6-meter, 2-person vessel, operated by 

Delta Oceanographics of Oxnard, California. Aboard the Delta, we conducted belt transects about two meters from the substrata, while the submersible maintained a speed of about 0.5 knots. At the platforms, 

transects were made around the bottom of the platform and around each set of crossbeams to a minimum 

depth of 20-30 m below the surface (e. g., midwater habitat). The belt transect was also used to sample the 

shell mounds surrounding the platforms and natural rock outcrops. The shell mounds and outcrops were 

sampled in consistently the same fashion as the platform method described above.

Submersible surveys were conducted during daylight hours between one hour after sunrise and two 

hours before sunset. During each transect, observations were taken from one viewing port on the starboard 

side of the submersible. An externally mounted hi-8 mm or digital video camera with associated lights 

filmed the same viewing fields as seen by the observers. The observer identified, counted, and estimated the 

lengths of all fishes and verbally recorded those data on the video. All fishes within 2 m of the submarine 

were counted. All fish larger than the size of first maturity were observed within 2 m of the bottom (none 

were seen in midwater transects). Thus, densities were calculated as fish per m2. Fish lengths were estimated 

using a pair of parallel lasers mounted on either side of the external video camera. The projected reference 

points were 20 cm apart and were visible both to the observer and the video camera. An environmental 

monitoring system aboard the submarine continuously recorded date and time, depth and altitude of the 

vessel above the sea floor. The environmental data were overlaid on the original videotape upon completion 

of each survey.

Transect videos were reviewed aboard the research vessel or in the laboratory. Field observations were 

transcribed into a database. For each fish, we recorded the following information: 1) species (if known); 2) 

its estimated total length; and 3) in the surveys over natural reefs, the habitat it occupied (e.g., rock, sand, 

mud, cobble, or boulder).

Many years of experience along the Pacific Coast have shown that if the Delta is moving at a constant 

and slow rate of speed, as in these surveys, there is very little obvious effect on demersal rockfishes (M. Love, 

pers. obs., V. O’Connell, per. obs., Alaska Department of Fish and Game, and M. Yoklavich, NOAA Fisheries, 

pers. obs.). Certainly, we noticed virtually no movement at all from most of the fishes in this study as the 

research submersible passed by.

Data analyses

While our surveys estimated size for most observed fish, we were not able to estimate sex ratios. Cowcod are not sexually dimorphic and it seemed reasonable to assume an equal female to male ratio. Bocaccio 

are sexually dimorphic and sex ratios of commercial catches are not 1:1 and are related to size. We obtained 

data to estimate length-specific sex ratios for bocaccio from Mark Wilkins (NOAA Fisheries, AFSC, Seattle, WA), in the form of length compositions by sex from the triennial bottom trawl survey conducted by 

the AFSC. The trawl survey uses finer meshed nets than commercial operations and all captured fish are 
4

sampled for length compositions. Commercial operations also discard some fish due to market demand or 

regulations. 

Our submersible survey estimated fish size to the nearest 5 cm. Observers were not able to estimate 

size for a small proportion of fish (0.006 for bocaccio and 0.048 for cowcod). These fish were assumed to 

be smaller than the size of maturity and not included in the estimates. The trawl survey measured size to 

the nearest 2 cm. We converted the trawl survey length compositions to 5 cm intervals. NOAA Fisheries 

estimated the total abundance of bocaccio off southern and central California in numbers of fish by size 

and sex for each year of the survey. The estimates of total numbers of fish varied considerably among years. 

Since growth is sexually dimorphic for bocaccio and there is considerable variation in year-class strength, we 

decided it would be best to calculate the ratios of females to total fish at each year and then average the ratios, 

in order to give equal weight to each year’s estimates. The results indicated that ratios varied without trend 

through 60 cm and then rapidly increased to 1 (all females) at 70 cm. We concluded that because females 

grow faster and to larger size than males it would be reasonable to apply the average female to total fish ratio, 

0.44, between 25 and 60 cm, the observed value, 0.82, for 65 cm, and 1 for larger fish (Table 1).

We used estimates of maturity schedules for bocaccio and cowcod obtained from the study area (M. 

Love, unpubl. data). We believe it was more appropriate to use these data than other published results from 

fish that were collected from more northern waters. These data are summarized in Love et al. (1990), but the 

published results were not in sufficient detail for the purpose of the present study. We converted to maturity 

data in 1-5 cm intervals. We estimated maturity schedules for combined sexes by sum for each sex of proportion mature weighted by proportion for that sex (Table 2).

We used the size fecundity estimates of Love et al. (1990) to estimate number of eggs of mature females 

as these were derived from fishes collected from the study area. The bocaccio estimates are similar to those 

estimated by Phillips (1964) from fish that were collected from more northern waters.

For bocaccio, Fecundity = 1.15 (L^3.2696), and for cowcod, Fecundity = 1.702(L^3.1542), where L is 

total length in centimeters.

To calculate the number of larvae that could be produced by resident bocaccio and cowcod at a given 

area, we first segregated all observed fish during a survey into size classes. For bocaccio, we estimated the 

number of females in each size class using the sex ratio data in Table 1 (cowcod are assumed to have a 1:1 

ratio). Next, for each species in each size class, we used maturity schedules (Table 2) to estimate the proportion of mature females in each class. Length-fecundity equations were then used to determine the number 

of larvae each female could produce. Larval production is therefore linked to fish density and rates are scaled 

appropriately across sites.

We estimated area surveyed by multiplying transect distance by 2, because survey width was 2 m, and 

then calculated densities per area for each species and size. Transect lengths were estimated using a pair of 

parallel lasers mounted on either side of the external video camera. The projected reference points were 20 

cm apart and were visible both to the observer and the video camera. Transect lengths were computed by 

counting the number of 20 cm laser segments in 15 s subsamples (one per min) throughout the transect, 

calculating speed based on those counts and averaging them over the whole transect, and multiplying that 

average speed by the transect duration. The 15 s subsamples were made during the first 15 s of each minute 

of the transect in which the laser points were visible. We first summed all data for a dive. Sometimes there 

was more than one dive for a station in a year. In this case, we averaged the results of the dives for estimates 

of the station-year. In many cases a station was surveyed in more than one year and we averaged the results 

over years for each of those stations. We also grouped stations that were adjacent to each other.

Estimates of footprint areas for the platforms come from Love et al. (2003). We estimated area of natural reefs needed to produce the same amount of larvae as an average platform by multiplying the average 

area of a platform by the ratio of average platform production to average reef production. 
5

Table 1. Estimated ratio of female to total bocaccio, in 5 cm intervals, in southern California.

Female:Total

Size cm Observed Estimated

25 0.40 0.44

30 0.45 0.44

35 0.48 0.44

40 0.45 0.44

45 0.55 0.44

50 0.34 0.44

55 0.40 0.44

60 0.48 0.44

65 0.82 0.82

70 1.00 1.00

75 1.00 1.00

Table 2. Maturity schedules, in 5 cm intervals, estimated for bocaccio and cowcod in southern California.

Bocaccio Cowcod

Size cm Female Male Combined Female Male Combined

30 0.00 0.00 0.00 0.00 0.00 0.00

35 0.22 0.33 0.29 0.00 0.07 0.03

40 0.78 0.92 0.86 0.02 0.35 0.19

45 0.99 1 1 0.64 0.70 0.67

50 1.00 1 1 0.95 1.00 0.97

55 1.00 1 1 1.00 1.00 1

Results

In comparing larval production, fish densities, and ultimately habitat equivalence between platforms 

and natural outcrops we restricted our analyses to habitats and depths most likely to harbor adult cowcod 

and bocaccio (based on data from Love et al., 2002 and Yoklavich et al., 2000). A site was considered to be 

appropriate habitat and retained for analyses if any starting depth was ≥ 60 m for bocaccio or ≥ 80 m for 

cowcod and if any patch bottom in that site was classified as “rocky high relief”, “rocky low relief”, or “boulders”. Based on these parameters, 63 reefs and seven platforms (Gail, Grace, Harvest, Hermosa, Hidalgo, 

Holly, and Irene) were used in the bocaccio dataset and 52 reefs and five platforms (Gail, Grace, Harvest, 

Hermosa, and Hidalgo) in the cowcod dataset. 

Overall, we observed 1054 bocaccio of all sizes at platforms and 976 at reefs, and 125 cowcod at platforms and 134 at reefs. Among bocaccio estimated to be mature, we observed 313 individuals around platforms and 313 at reefs, and 38 mature cowcod at platforms and 61 at reefs. Adult fishes were patchily distributed among both platforms and natural reefs. Fifty-seven percent of surveyed natural reefs (36 out of 

63) with appropriate habitat (as defined above) had mature bocaccio on them and 42% (23 out of 54) of 
6

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Figure 3. Densities of mature cowcod at five platforms and 52 natural reefs in the southern 

California Bight and off central California, 1995-2002. Included are sites that were classified 

as “rocky high relief”, “rocky low relief”, or “boulders” and in depths of 80 m or more.

Figure 2. Densities of mature bocaccio at seven platforms and 63 natural reefs in the southern California Bight and off central California, 1995-2002. Included are sites that were classified as “rocky high relief”, “rocky low relief”, or “boulders” and in depths of 60 m or more.
7

surveyed natural reefs with appropriate habitat had adult cowcod on them. Platforms had nearly identical 

ratios as at natural reefs, as 57% (4 out of 7) had adult bocaccio and 40% (2 out of 5) had adult cowcod.

Densities of mature bocaccio and cowcod were highly variable among survey sites, but were generally very low at both natural reefs and platform habitats (Figs. 2 and 3). The mean density on natural reefs 

for bocaccio and cowcod was 0.15 per 100 m2 (SE 0.03) and 0.04 per 100 m2 (SE 0.01) respectively. Higher 

densities of mature fishes on natural reefs tended to occur in more remote areas (Figs. 2 and 3). The mean 

density around platforms for bocaccio and cowcod was higher, 1.25 per 100 m2 (SE 1.16) and 0.21 per 100 

m2 (SE 0.19), respectively. While densities were much lower on reefs than on natural reefs at most sizes, relative numbers of medium-size fish tended to be higher on reefs than on platforms (Fig. 4). 

For both species, the larger average adult size at platform habitat proportionally increased the potential 

larval production at platforms compared to natural reefs, especially for bocaccio (Fig. 5). Mean (±1SE) larval production (no. eggs/m2

) at platform sites was 2883 (2597) for bocaccio and 743 (676) for cowcod, while 

mean production for natural outcrop sites was much less, 254 (58) for bocaccio and 132 (34) for cowcod.

Of particular interest, Platform Gail had by far the highest densities of mature bocaccio (8.17 per 100 

m2 compared to 1.46 for the highest density on a natural reef) and cowcod (1.00 per 100 m2 compared to a 

maximum of 0.23 for natural reefs) of any natural or human-made habitat in this study (Figs. 2 and 3). Platform Gail also had the highest potential larval production values, topping out at 18,397 eggs/m2

 for bocaccio 

and 3440 eggs/m2 for cowcod (Fig. 5). This means that if Platform Gail (whose footprint is 0.5337 ha) were 

removed during decommissioning it would be the equivalent of removing 12.57 ha of average-producing 

natural habitat in southern California for cowcod, or equivalent to removing 29.24 ha of average-producing 

natural habitat in southern California for bocaccio.

Discussion

Our study provides the first published evidence, based on direct underwater observations, for intense 

depletion of both adult bocaccio and cowcod on many natural structures in southern California (previous 

evidence came from fishery-dependent data, e.g., Love et al., 1998 and Butler et al., 2003; and larval or juvenile surveys, e.g., Love, et al., 1998; Moser et al., 2000; Butler et al., 2003). It is likely that these low densities 

were due both to overfishing and to poor larval and pelagic juvenile survivorship (Love et al., 2002; Love et 

al., 2003). With the exception of Platforms Gail and Hidalgo, densities were also low for adults of both species at platforms.

Platform Gail harbored unusually high densities of adult bocaccio and cowcod. The high densities of 

both species at Platform Gail are likely due to its status as a de facto marine protected area (Schroeder and 

Love, 2002), a bottom depth that is commonly occupied by adult bocaccio and cowcod, and the presence 

of a bottom crossbeam that provides a sheltering area for both species (Fig. 1). It is probable that the adult 

bocaccio and cowcod living at Platform Gail were derived from young-of-the-year (YOY) that had recruited 

from the plankton to that platform’s jacket, shell mound, or pipeline. During nine years of surveys, we have 

on several occasions observed relatively high densities of YOY bocaccio recruiting to the midwaters of Platform Gail and YOY cowcod to its shell mound and pipeline (Love et al., 2003; Love and York, in press). In 

addition, our research indicates that both species not only have recruited as YOYs to the jacket or surrounding structures, these year classes also have remained at the platform over succeeding years (Love et al., 2003; 

Love, unpubl. data). 

The densities of bocaccio and cowcod we observed at Platform Gail were not only far higher than those 

of any other platform or natural reef in southern California, they were higher than four of five deeper-water 

sites surveyed in Soquel Canyon in central California and were surpassed only by an outcrop that likely has 

never been fished (Yoklavich et al., 2000). This natural refuge provides a comparison from deeper waters 
8





     



















     

















     



















     















.UMBERPERM

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Figure 4. Average densities by size of bocaccio and cowcod observed at natural reefs and platforms. Data from all dives 

in suitable habitat and all fish sizes are included.
9

off central California, as most of the abundant species at the Soquel Submarine Canyon site (e.g., bocaccio, 

cowcod, lingcod, greenspotted, and greenblotched rockfishes) are also those most characteristic of the jacket 

bottom at Platform Gail (Love et al., 2003). Yoklavich et al. (2000) make the point that the Soquel Canyon 

site may be an important site for larval production, noting that “These areas appear to function as a natural 

harvest refugium, potentially contributing new recruits to adjacent fished areas” through larval export. The 

high densities and relatively large size of fishes at Platform Gail translate into substantially higher potential 

larval export per unit area for that platform than for any other site surveyed in our study.

On a regional basis, how important is larval bocaccio and cowcod production at Platform Gail? First, 

from our research submersible surveys it is clear that there is little larval production from bocaccio and cowcod that live on natural outcrops in the Santa Barbara Channel. With the exception of the carbonate feature 

Twelve Mile Reef (Fig. 2), located in the center of the Santa Barbara Channel, much of the sea floor in the 

Channel (at depths frequented by adult bocaccio and cowcod) is composed of soft substrata (G. Cochrane, 

United States Geological Survey, pers. comm.). The Twelve Mile Reef was at one time the major Santa Barbara Channel fishing ground for a number of rockfish species, including both cowcod and bocaccio (M. 

Kronman, Santa Barbara Harbor Department, pers. comm.; M. Love, unpubl. data). However, during our 

surveys we observed no adult bocaccio and cowcod on that outcrop. Other, and smaller, Santa Barbara 

Channel features (Fig. 2) also harbored few or no adults of these species. 

0 5000 10,000 15,000 20,000

Bocaccio

Natural outcrops 

Platforms 

Hidalgo

Grace

Gail

Irene

0 5000 10,000 15,000 20,000

Bocaccio

Natural outcrops 

Hidalgo

Grace

Gail

Irene

0 1000 2000 3000 4000

Cowcod

Hidalgo

Gail

0 1000 2000 3000 4000

Cowcod

Hidalgo

Gail

Potential larval production

(no eggs/m2)

Figure 5. Comparison of potential larval production (no. eggs/m2) of sites that had at least one mature bocaccio (40 

natural reefs and four platforms) or one mature cowcod (25 natural reefs and two platforms). Bars representing platforms are darkened. Platforms in the appropriate depth range that did not harbor mature cowcod included Grace, 

Harvest, Hermosa, and for bocaccio, Harvest, Hermosa and Holly.
10

Rockfish larvae may be partially retained in the east Santa Barbara Channel, where Platform Gail is 

sited, through oceanographic fronts and eddies (Nishimoto, 2000). Under certain conditions, larvae generated within the Santa Barbara Channel may also be entrained in seasonal gyres located in the western Channel (Nishimoto and Washburn, 2002). Thus local sources of larvae may be critical to the maintenance of 

Santa Barbara Channel rockfish populations and Platform Gail, apparently one of the major producers of 

bocaccio and cowcod larvae in the Channel, may be quite important. It is interesting to note that some of the 

highest densities of YOY bocaccio we have observed anywhere in our studies were found at platforms Gilda 

and Grace, which are located within a few kilometers of Platform Gail (M. Love, unpubl. data). Thus, there 

is a possibility that these young fishes were generated as larvae at the latter structure. 

While platform fish assemblages vary with platform bottom depth, it is clear that many of the platforms 

off southern California, regardless of bottom depth, act as de facto marine reserves. At these platforms, fish 

species in addition to bocaccio and cowcod have also responded to protection from fishing as densities of 

other economically important species (e. g., brown, Sebastes auriculatus, copper, Sebastes caurinus, flag, S. 

rubrivinctus, greenblotched, S. rosenblatti, and greenspotted, S. chlorostictus, rockfishes, and lingcod, Ophiodon elongatus) are higher at platforms than at most or all southern California natural reefs (Schroeder and 

Love, 2002; Love et al., 2003). It is likely that larval production of these species, per unit area, will also be 

higher at platforms than at most natural habitats. Thus, the potential importance of platforms as marine refugia may be large given the generally very low densities of adults of a number of rockfish species on natural 

reefs in southern California. If future platform decommissioning involves retaining some of the platform 

jacket in place, it important to note that the high fish densities we observed would only be maintained if the 

platforms remained protected from fishing. 

Clearly, southern and central California platforms vary widely in size, bottom depth, and fish assemblages. For instance, of the platforms we surveyed, Platform Gail, and to a lesser extent, Hidalgo, were unique 

in having high densities of mature bocaccio and cowcod. Given these variables, it would be premature to 

judge the importance of all of these structures as fish habitat. In addition, all of our surveys were conducted in the fall and we do not know whether adult cowcod and bocaccio remain at any of these structures 

throughout the year, although the interannual persistence of high densities of large bocaccio and cowcod at 

Platform Gail lends credence to this hypothesis. However, it is clear from our analysis that platforms can be 

important habitat for adult fishes, particularly of species that have been heavily exploited. Given the high 

densities of adults of many species of economically important rockfishes and lingcod around platforms and 

the low densities of these same species on many or most natural reefs in southern California, it is quite possible that some platforms are of regional significance as sources of larval production and export for some 

species. The extent of this significance should play an important role in determining the preferred option of 

future decommissioning activities (removal or leave-in-place) once oil and gas production ceases. 
11

Acknowledgments

We would like to thank A. Bull, L. Thorsteinson, and G. Steinbach for their support of this research. 

This research was funded by the Minerals Management Service (Contract Number 1435-01-03-CA-72694 

OCS/MMS 2004-005), Biological Resources Division, U. S. Geological Survey (National Offshore Environmental Studies Program 1445-CA-0995-0386) based on an information need identified by the Minerals 

Management Service’s Pacific OCS Regions, and by the California Artificial Reef Enhancement Program 

(CARE). A. MacCall recommended the NOAA triennial trawl survey for use in our bocaccio sex ratio analysis. We thank the pilots of the submersible Delta, C. Ijames, J. Lily, and D. Slater, for their very professional 

handling of the technical aspects of that survey, as well as the crews of the R/V Cavalier and R/V Valero. R. 

Lea, M. Nishimoto, L. Snook, R. Starr, and M. Yoklavich conducted some of the surveys and L. Snook managed the survey data. 

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___________ and M. S. Love. 2002. Recreational fishing and marine fish populations in California. CalCOFI Rep. 43:182-190.

___________ and __________. 2004. Ecological and political issues surrounding decommissioning of offshore oil facilities in the Southern California Bight. Ocean Coast. Manage. 47:21-48.

Yoklavich, M. M., H. G. Greene, G. M. Cailliet, D. E. Sullivan, R. N. Lea, and M. S. Love. 2000. Habitat associations of deep-water rockfishes in a submarine canyon: an example of a natural refuge. Fish. Bull. 

98:625-641.

Addresses

(M.L., D. S.) Marine Science Institute, University of California, Santa Barbara, California 93106. (W. H. L.) 

P.O. Box 251, Kentfield, California 94914.

Corresponding Author

(M.L.) and E-mail: <love@lifesci.ucsb.edu>.
13

Task 2: Ecological Performance of Young-of-the-Year Blue Rockfish 

(Sebastes mystinus) Associated with Oil Platforms and Natural Reefs in 

California as Measured by Daily Growth Rates

Milton S. Love, Edward Brothers, Donna M. Schroeder, William H. Lenarz

Abstract

Some decommissioning alternatives regarding the fate of offshore oil platforms include leaving 

some or all of the platform structure on the seabed as an artificial reef. Questions surrounding the 

utility of such structures as productive fish habitat have not been resolved. Despite the high numbers 

of fishes that can reside around offshore platforms, little is known about the comparative ecological 

performance between natural and artificial habitats, where higher growth, reproduction or survival 

rates would be beneficial to regional fish populations. To address this need, we analyzed the otolith 

microstructure of 187 young-of-the-year (YOY) blue rockfish (Sebastes mystinus) to compare mean 

daily growth rate and birth date distribution among three platforms and three natural reefs in the 

Santa Barbara Channel region. Paired natural and platform sample sites were selected to include a 

range of oceanographic conditions present in the region. We found a significant though modest lunar pattern in birth dates where blue rockfish produced (or successfully recruited) fewer larvae in the 

week following the full moon. Channel-wide patterns of birth date distribution were present. Sites 

located in the west channel region included a range of birth dates (January through March) whereas 

those in the east channel were limited to January and February. Mean growth rates were significantly 

different across sites (ANOVA, F = 71, 96, df = 5, 180, P< 0.01). At the one site pair where comparisons were valid, YOY rockfish growth rates were significantly higher at the platform habitat. This 

study demonstrates that, as measured by daily growth rates, blue rockfish living around oil and gas 

production platforms may perform at least as well as those fish living on natural reefs, and supports 

previous research implying that some platforms may benefit regional fish populations.

Introduction

All 27 oil and gas platforms located off the coast of southern and central California will at 

some time reach the end of their economic life spans. California platforms are located in both state 

and federal waters and are sited in bottom depths ranging from 11 to 363 m. The largest of these 

structures has a footprint of 10,606 m2 (Love et al., 2003). While platform operators are responsible for the costs of disposition of an uneconomical platform, managers must decide what to do 

with that structure, a process known as decommissioning. Ultimately, a decommissioned platform 

could be left in place, removed to some point below the sea surface, toppled to lie on the sea floor, 

or totally removed (Schroeder and Love, 2004). Decommissioning can be a lengthy process, as it 

requires input from state and federal agencies, corporate entities, and such stakeholders as recreational and commercial fishermen, and non-consumptive users. It may involve extensive analyses 

of the biological, economic, and other sociological impacts of retaining or removing these structures (Schroeder and Love, 2004).

Since 1995, much of our research has focused on the potential biological impacts of platforms 

as fish habitats. A number of economically important fishes, particularly rockfishes (genus Sebastes), 

are found at high densities around some California platforms. Rockfishes, a group that dominates 
14

many of the shelf and slope habitats of southern and central California (Love et al., 2002), often 

comprise over 90% of all fishes observed around these structures (Love et al., 2003). 

Platform midwaters (and the bottoms of some shallow-water structures) often act as nursery grounds for rockfishes and other species; densities of many rockfish species are almost always 

higher than at nearby natural outcrops (Love et al., 1999; Carr et al., 2003; Love et al., 2003). It is 

likely that, compared to most natural reefs, a platform’s size, structural complexity, and high vertical profile provides pelagic juvenile rockfishes and larvae of other species with a relatively strong 

stimulus to trigger settlement (Carr et al., 2003). While we have observed young-of-the-year (YOY) 

of at least 28 rockfish species at platforms, five rockfish species (bocaccio, Sebastes paucispinis Ayres, 

1854; widow, Sebastes entomelas Jordan and Gilbert, 1880; squarespot, Sebastes hopkinsi Cramer, 1895; olive, Sebastes serranoides Eigenmann and Eigenmann, 1890; and blue, Sebastes mystinus Jordan and Gilbert, 

1881) recruit to many of these structures in particularly large numbers (Love et al., 2003; Carr et 

al., 2003). As an example, we estimated that during 2003, Platform Grace (located in the Santa Barbara 

Channel) harbored more than 350,000 YOY bocaccio (Love et al., in press). Young-of-the-year rockfish recruitment to both platforms and natural reefs varies greatly from year to year.

The high densities of YOY rockfishes inhabiting platform midwaters notwithstanding, there remain 

questions regarding the importance of these young fishes to regional fish populations. The relative ecological contribution fishes living around artificial and natural habitats (as measured by fish growth, survival, and 

reproduction) is a major determinant of that habitat’s importance (Carr and Hixon, 1997; Carr et al., 2003; 

Mason, 2003). In 1999, a relatively strong recruitment of YOY blue rockfish to both platforms and natural 

reefs in southern and central California allowed us to compare daily growth rates of these young fishes.

Methods

Sample Collections

During the fall of 1999, YOY blue rockfish were collected by pole spear at three platforms and three 

natural reefs (Fig. 1). We selected our collection sites based on where YOY blue rockfish were sufficiently 

abundant to allow for inter-site growth rate comparisons. Comparisons were made between fish at Platform 

Irene and a reef at Santa Rosa Island (both are in the California Current and are 84 km apart), Platform 

Holly and Naples Reef (in the same water mass and located with 8 km of each other) and Platform Gilda and 

Anacapa Island (in different water masses and within 16 km of each other). Collections were made during 

one day at each site.

In the laboratory, each fish was measured (standard length, SL) to the nearest millimeter and its sagittal otoliths removed. After removal, sagittae were cleaned in water, soaked in a solution of 50% household 

bleach and 50% water for a few minutes, then rinsed in water, air dried and stored in vials. Prior to their 

being read, each otolith was embedded in Spurr’s low viscosity epoxy medium before wet grinding with 800 

grit aluminum oxide. A mid-sagittal section was prepared to preserve both the otolith margin and as well 

as early growth around the primordium. The final curvilinear (convex) surface was produced by applying 

variable pressure and tilt during the grinding process. The samples were polished with diamond paste (3 

um followed by 1 um). Microincrements were counted with the aid of a video microscope (to about 1500x 

magnification). The typical counting path was from the primordium to the dorsal otolith margin. Methods 

generally followed techniques outlined in Brothers (1987) and Ralston et al. (1996). 

We did not validate the daily nature of the sagittal increments. However, in the eastern Pacific increments in rockfish otoliths have been determined to be produced daily in the YOYs of black (Sebastes melanops Girard, 1856) (Yoklavich and Boehlert, 1987), pygmy (Sebastes wilsoni Gilbert, 1915) (Laidig et al., 

2004), shortbelly (Sebastes jordani Gilbert, 1896) and brown (Sebastes auriculatus Girard, 1854) rockfishes 
15

(Laidig et al., 1991). In the western Pacific, similar studies have shown this to be true for Sebastes thompsoni

(Jordan and Hubbs, 1925) (Kokita and Omori, 1998) and Sebastes inermis Cuvier, 1829 (Plaza et al., 2001). 

Increments were inferred to be daily in studies of larval, pelagic juvenile, or benthic YOY blue (Yoklavich et 

al., 1996), stripetail (Sebastes saxicola Gilbert, 1890) and greenstriped (Sebastes elongatus Ayres, 1859) rockfishes and cowcod (Sebastes levis Eigenmann and Eigenmann, 1889) (Johnson et al., 2001), and widow and 

yellowtail (Sebastes flavidus Ayres, 1862) rockfishes, bocaccio and chilipepper (Sebastes goodei Hilgendorf, 

1880) (Woodbury and Ralston, 1991).

Analytic Methods

We estimated average growth (mm/day) of each fish by dividing its standard length by its age. This 

metric includes both growth prior to, and after, benthic settlement. Since most blue rockfish settle between 

early May and late June (Love et al., 2002) and our collections were made from September to November, our 

measurement of total growth includes a significant period of benthic growth. 

We used one-way ANOVA’s to test the hypotheses that mean birthdate (day of the year), age, standard 

length, and growth did not vary among sites. ANOVA’s are robust to minor deviations from the assumption 

that observations are normally distributed. We compared the results of the ANOVA’s using untransformed 

data with ANOVA’s using log-transformed data and the F values were very close. We chose to use untransformed data for the analyses. We used Scheffé’s Post Hoc Test to contrast means of the six sites. 

Pt. Conception 

Pt. Arguello 

Santa

Barbara

Ventura 

San Miguel 

Island Santa Rosa 

Island

Santa Cruz 

Island

Anacapa

Island

Irene

Gilda

Holly

Naples

Reef

31

31

31 15

40

39

X

Santa Barbara Channel 

C A L I F O R N I A 

Area Detailed

Figure 1. Location and number of young-of-the-year blue rockfish collected at six sites in southern and central California, September-November 1999. Platform and natural reef comparison pairs are noted by stars, triangles, and squares.
16

Results

We collected a total of 187 blue rockfish; collections ranged in number from 15 fish at Santa Rosa 

Island to 39 fish at Naples Reef (Table 1, Fig. 1). Fish were sampled between 15 September and 23 November 1999. Otoliths of all fish collected, except for one individual from Naples Reef, were readable and were 

included in this study. 

Lengths of individual fish sampled ranged from 6.7-11.1 cm and mean lengths of fish sampled at each 

site ranged from 8.1 to 10.0 cm (Fig. 2, Table 1). The ANOVA showed that lengths were significantly different 

among sites (F = 31.69, df = 5,180, P ≤0.01). Scheffé’s contrasts were significant for the site pairs Platform 

Holly – Naples Reef and Platform Gilda – Anacapa Island (Table 2). Mean lengths were not statistically different at the site pair Platform Irene – Santa Rosa Island (Table 2). Young-of-the-year blue rockfish ranged in 

age from 199 to 316 days and mean ages of fish at each site varied between 215 to 301 days (Table 1, Fig. 3). 

Mean ages were statistically different between almost all sites (one-way ANOVA, F = 205.60, df = 5,180, 

P≤0.01, and Scheffé’s test Table 2); a notable exception was the site pair Platform Irene – Santa Rosa Island.

Back-calculated birth or parturition dates (Ralston et al. 1996) for individual fish ranged from early January to mid-March (Table 1, Fig. 4). While mean parturition dates varied considerably between sites (F = 22.20, 

df = 5,180, P≤0.01), they were similar for the site pairs Platform Irene-Santa Rosa Island (mean = mid-February) and Platform Holly-Naples Reef (mean = late January) (Scheffé’s Test P>0.05, Table 2). In both instances 

mean birth dates were within four days of each other (Table 1). Patterns of birth dates were very similar at 

Platform Irene and Santa Rosa Island; parturition in March was only observed in fish inhabiting these two 

sites (Fig. 4). Mean birth dates were 18 days apart for fish at Platform Gilda and Anacapa Island (Scheffé’s Test 

P<0.05, Table 2).

Table 1. Data on collections and biological parameters of blue rockfish used in the study. Minimum and maximum values 

in parentheses. Note that the otoliths of one of the fishes collected from Naples Reef was not readable, thus the total usable 

fish from that site is 39.

Date Mean Mean Mean Mean Growth

Site n of Sample Date of Birth Age (days) Length (SL cm) (mm/day)

Platform Irene 31 16-Nov-99 18Feb(5Jan-16Mar) 270(245-315) 8.6(7.4-10.5) 0.32(0.27-0.39)

Santa Rosa I. 15 16-Nov-99 14Feb(10Jan-21Mar) 275 (240-310) 8.5(6.7-9.5) 0.31(0.27-0.36)

Platform Holly 31 15-Oct-99 26Jan(9Jan-19Feb) 262(238-279) 10.0(8.7-11.1) 0.38(0.33-0.46)

Naples Reef 40 25-Sep-99 30Jan(3Jan-16Feb) 240(221-265) 8.6(7.5-9.7) 0.36(0.33-0.40)

Platform Gilda 39 15-Sep-99 12Feb(13Jan-28Feb) 215(199-245) 8.1(6.9-10.3) 0.38(0.33-0.44)

Anacapa I. 31 23-Nov-99 25Jan(11Jan-18Feb) 301(278-316) 9.0(7.9-9.4) 0.30(0.28-0.33)

 There was some indication that the timing of larval release was related to moon phase (Rayleigh’s Test, 

Table 3, Fig. 5). While at most sites parturition dates were statistically uniform over a lunar cycle, when data 

was pooled it appeared that few larvae were extruded during the week following the full moon.

Calculated mean daily growth rates of individuals ranged from 0.27 to 0.46 mm/day and mean growth 

rates at each site varied between 0.30 and 0.38 mm/day (Table 1, Fig. 6). Mean growth rates were statistically 

different among most sites (ANOVA, F = 71.96, df = 5,180, P≤0.01). Growth rates were significantly different between fish living at Platform Holly and Naples Reef and those living at Platform Gilda and Anacapa 

Island (Table 2). In both instances, fish grew more rapidly at the platforms. There was no statistical difference between blue rockfish growth rates at Platform Irene and Santa Rosa Island (Scheffé’s Test P>0.05, 

Table 2).
17

Table 2. Differences (column – row), as contrasted by Scheffé’s Post Hoc Test, between pairs of sites in mean length, mean age, 

mean birth date, and mean growth of YOY blue rockfish. Fish collected between September and November 1999. *= P≤0.05; 

**=P≤0.01. Platform and natural reef site pair contrasts are denoted in bold.

Mean Length (mm, SL)

 Naples Platform Platform Platform Anacapa

 Reef Gilda Irene Holly Island

Platform Gilda 0.51

Platform Irene -0.03 -0.54

Platform Holly -1.46** -1.97** -1.43**

Anacapa Island -0.37 -0.88** -0.34 1.09**

Santa Rosa Island 0.07 -0.44 0.10 1.53** 0.44

Mean Age (days)

 Naples Platform Platform Platform Anacapa

 Reef Gilda Irene Holly Island

Platform Gilda 23.33**

Platform Irene -32.44** -55.77**

Platform Holly -24.05** -47.38** 8.39

Anacapa Island -63.15** -86.48** -30.71** -39.10**

Santa Rosa Island -36.85** -60.18** -4.41 -12.80 26.30**

Mean Birth Date (day of year)

 Naples Platform Platform Platform Anacapa

 Reef Gilda Irene Holly Island

Platform Gilda -13.33**

Platform Irene -19.56** -6.23

Platform Holly 4.05 17.38** 23.61** 

Anacapa Island 4.15 17.48** 23.71** 0.10

Santa Rosa Island -15.15** -1.82 4.41 -19.20** -19.30**

Mean Growth Rate (mm/day)

 Naples Platform Platform Platform Anacapa

 Reef Gilda Irene Holly Island

Platform Gilda -0.015

Platform Irene 0.042** 0.057**

Platform Holly -0.023* -0.008 -0.065**

Anacapa Island 0.064** 0.078** 0.022* 0.086**

Santa Rosa Island 0.052** 0.067** 0.010 -0.074** -0.012

We wished to make an overall comparison of growth rates of blue rockfish combining all platform data 

and comparing that to all natural reef data. However, because fish at Platform Gilda and Anacapa Island 

were sampled two months apart and because growth rates may decline later in the season (thus complicating 

inter-site comparisons), in these comparisons we did not include data from these two sites. We combined 

the growth rates of fish from Platform Irene and Holly and compared the mean growth rate with that of fish 

living at Naples Reef and Santa Rosa Island. Blue rockfish living at the platforms grew 0.016mm/day more 

quickly (Scheffé’s Test, P<0.05).
18 Number of Fish 

Number of Fish 

Standard Length (cm) 

Standard Length (cm) 

Platform Irene 

Anacapa Island 

0

2

4

6

8

10

12

0

2

4

6

8

10

12

14

16

18

6 7 8 9 10 11

6 7 8 9 10 11

X

X

Number of Fish

Standard Length (cm)

Santa Rosa Island

0

1

2

3

4

5

6

7

6 7 8 9 10 11

X

Number of Fish

Standard Length (cm)

Platform Holly

0

1

2

3

4

5

6

7

8

9

6 7 8 9 10 11

X

Number of Fish

Standard Length (cm)

Naples Reef

0

5

10

15

20

25

6 7 8 9 10 11

X

Number of Fish

Standard Length (cm)

Platform Gilda

0

2

4

6

8

10

12

6 7 8 9 10 11

X

Figure 2. Size distribution of young-of-the-year blue rockfish collected at six sites in southern and central California, 

September-November 1999.
19

Number of Fish 

Age (days) 

Platform Irene 

0

2

4

6

8

10

12

14

195 215 235 255 275 295 315

X

Number of Fish

Age (day)

Platform Holly

0

2

4

6

8

10

12

14

16

195 215 235 255 275 295 315

X

Number of Fish

Age (days)

Santa Rosa Island

0

1

2

3

4

5

195 215 235 255 275 295 315

X

Number of Fish

Age (days)

Naples Reef

0

2

4

6

8

10

12

14

16

195 205 215 225 235 245 255 265 275 285 295 305 315

X

Number of Fish 

Age (days) 

Anacapa Island 

0

2

4

6

8

10

12

14

195 215 235 255 275 295 315

X

Number of Fish

Age (days)

Platform Gilda

0

2

4

6

8

10

12

14

16

18

195 215 235 255 275 295 315

X

Figure 3. Age distribution of young-of-the-year blue rockfish collected at six sites in southern and central California, 

September-November 1999.
20Number of Fish 

Month of Birth 

Platform Irene 

0

1

2

3

4

5

6

7

8

 January February March

X

Number of Fish

Month of Birth

Santa Rosa Island

0.5

1.5

2.5

3.5

4.5

0

1.0

2.0

3.0

4.0

 January February March 

X

Number of Fish

Month of Birth

Platform Holly

10

12

0

2

4

6

8

 January February March 

X

Number of Fish

Month of Birth

Naples Reef

10

12

14

16

18

20

0

2

4

6

8

 January February March 

X

Number of Fish

Month of Birth

Platform Gilda

0

2

4

6

8

10

12

14

 January February March 

X

Number of Fish 

Month of Birth 

Anacapa Island 

0

2

4

6

8

10

12

14

 January February March

X

Figure 4. Parturition date distribution of young-of-the-year blue rockfish collected at six sites in southern and central 

California, September-November 1999.
21

.UMBEROF&ISH

$AYOF,UNAR-ONTH

0OOLED3ITES

0

2

4

6

8

10

12

14

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29

NEWMOON

FULLMOON

Figure 5. Distribution of parturition over the lunar month, as back-calculated from otoliths of young-of-the-year blue 

rockfish. Three lunar cycles are pooled. Arrow indicates the mean lunar day of parturition. New moon (filled circle) 

was set as the first day, full moon (open circle) as the 15th day. 

Table 3. Statistics of Rayleigh’s test (Z) for distributions of parturition dates for blue rockfish in 1999.

Site Z Significance Level

Platform Irene 0.61 ns

Santa Rosa I. 1.60 ns

Platform Holly 2.51 ns

Naples Reef 2.97 ns

Platform Gilda 4.77 0.01

Anacapa I. 1.07 ns

Pooled Data 3.51 0.05

Discussion

 For blue rockfish, we identified a number of patterns in growth and birth date distribution across the 

sampling region. 

First, we observed a significant, albeit subtle, lunar pattern in blue rockfish birth dates. In 1999, blue 

rockfish appeared to produce fewer larvae in the week following the full moon. Because parturition has not 

been observed in Eastern Pacific rockfishes, it is not possible to compare our data with direct observations 

of the timing of blue rockfish (or any other rockfish species) larval release. Thus we do not know if the pattern we observed represents reduced parturition in the third lunar quarter or if there is no lunar parturition 

pattern, but rather increased larval mortality (e.g., from increased predation or lowered food availability) on 

those larvae produced during that time. Studies of tropical reef fishes, by both direct observation and backcalculations from daily growth rings, demonstrate that spawning by many species is closely linked to lunar 

phase (Thresher, 1984; Robertson et al., 1988; Sponaugle and Cowen, 1994). However, studies on gray snapper (Lutjanus griseus (Linnaeus, 1758)) imply that lunar spawning patterns may occur in some years but not 

in others (Denit and Sponaugle, 2004). Only one other study has examined lunar parturition periodicity in 

rockfishes. Pastén et al. (2003), using back calculations of daily growth rings of Sebastes inermis collected off 

Japan, found that parturition occurred significantly more often around the new and full moons. 
22Number of Fish 

Number of Fish 

Growth (mm/days) 

Growth (mm/days) 

Platform Irene 

Anacapa Island 

0

1

2

3

4

5

6

7

0

2

4

6

8

10

12

14

16

0.27 0.29 0.31 0.33 0.35 0.37 0.39 0.41 0.43 0.45

0.27 0.29 0.31 0.33 0.35 0.37 0.39 0.41 0.43 0.45

X

X

Number of Fish

Growth (mm/days)

Platform Holly

0

1

2

3

4

5

6

7

0.27 0.29 0.31 0.33 0.35 0.37 0.39 0.41 0.43 0.45

X

Number of Fish 

Growth (mm/days) 

Platform Gilda 

0

1

2

3

4

5

6

7

8

0.27 0.29 0.31 0.33 0.35 0.37 0.39 0.41 0.43 0.45

X

Number of Fish

Growth (mm/days)

Santa Rosa Island

0

1

2

3

4

5

0.27 0.29 0.31 0.33 0.35 0.37 0.39 0.41 0.43 0.45

X

Number of Fish 

Growth (mm/days) 

Naples Reef 

0

2

4

6

8

10

12

0.27 0.29 0.31 0.33 0.35 0.37 0.39 0.41 0.43 0.45

X

Figure 6. Distribution of growth rates of young-of-the-year blue rockfish collected at six sites in southern and central 

California, September-November 1999.
23

There is some evidence that the YOY blue rockfish living at Platform Irene and Santa Rosa Island and 

those living at Platform Holly and Naples Reef either came from similar source populations or had larvae 

and pelagic juveniles that had experienced similar pelagic environmental conditions. Evidence is strongest 

for fish living at Platform Irene and Santa Rosa Island. Fish at these two sites had statistically identical 

mean birth dates (Platform Irene, 18 February; Santa Rosa Island, 14 February) and mean lengths (Platform 

Irene, 8.6 mm; Santa Rosa Island, 8.5 mm), and they were the only sites harboring fish that had been born 

in March (Tables 1, 2; Figure 4). Fish at Platform Holly and Naples Reef had non-significantly different 

birth dates (Platform Holly, 25 January; Naples Reef, 30 January). Interestingly, Platform Gilda and Anacapa 

Island, although relatively near each other, harbored blue rockfish YOYs that were quite dissimilar in all 

measured variables (Tables 1, 2; Figures 2-4). Platform Irene and Santa Rosa Island are often strongly bathed 

by the California Current and it is likely that in 1999 larvae produced off central California were carried 

southwards to both platform and island. The source populations for the other sites are more problematic. 

For instance, while we have little data on current patterns in the eastern Santa Barbara Channel (site of 

Platform Gilda and Anacapa Island) in 1999, research on the source populations of 2004 YOY bocaccio living around Platform Gilda implies that pelagic juveniles were brought to that site on currents flowing from 

several directions (M. Nishimoto et al., 2005).

The growth rates of YOY blue rockfish in our studies (with mean per site ranging from 0.30 to 0.38 

mm/day) are similar to those found in other young rockfishes. Summarizing a range of rockfish studies of 

both larval and pelagic juvenile phases, including both field and laboratory studies, Love et al. (1991) calculated a mean growth rate of 0.29 mm/day in field studies and 0.27 mm/day in laboratory experiments. 

However, only one other study, Johnson et al (2001), examined the daily growth rates of benthic YOY rockfishes integrated over the life of the animals from larval extrusion to post settlement. In that study, growth 

rates were 0.17 mm/day in greenstriped rockfish (Sebastes elongatus Ayres, 1859), 0.25 mm/day in cowcod 

(Sebastes levis (Eigenmann and Eigenmann, 1889)), and 0.32 mm/day in stripetail rockfish (Sebastes saxicola 

(Gilbert, 1890)). The growth rates of young rockfishes are low compared to a wide range of other fishes 

(e.g., European hake, Merluccius merluccius (Linnaeus, 1758), 0.71-0.74 mm/day, Kacher and Amara 2005; 

gray snapper, Lutjanus griseus (Linnaeus, 1758), 0.62-0.88 mm/day, Denit and Sponaugle, 2004; greater amberjack, Seriola dumerili (Risso, 1810), 1.65-2.00 mm/day, Wells and Rooker 2002; blue marlin, Makaira 

nigricans Lacepede, 1802, 16.6 mm/day).

The two-month disparity in collection dates among locations complicated our comparing the growth 

rates of fish from Platform Gilda and Anacapa Island and from comparing growth rates among all sites. 

Nevertheless, it is clear that there is no evidence that fish at platforms grew more slowly than those living 

on natural reefs. Comparison of growth rates between fishes living at Platform Holly and Naples Reef implied that YOY blue rockfish may, in some instances, grow faster at platforms. Growth might be accelerated 

at platforms for several reasons. Young-of-the-year blue rockfish are midwater feeders; they prey on such 

zooplankters as barnacle larvae, larvaceans, and copepods (Gaines and Roughgarden, 1987; Singer, 1985) 

and zooplankton density may be greater around Platform Holly than over Naples Reef. The zooplankton 

living in the nearshore waters flowing along and over the extensive kelp beds and rock reefs associated with 

Naples Reef may have been exposed to many more predators than organisms inhabiting the open ocean waters flowing into the platform jacket. In addition, the waters inside the platform may harbor higher densities 

of barnacle and other invertebrate larvae because the platform jacket is heavily encrusted with these forms 

(Love et al., 2003). Lastly, compared to natural reefs, the midwaters of most California platforms harbor 

low densities of those piscivorous fishes (e.g., kelp bass, Paralabax clathratus (Girard, 1854); cabezon, Scorpaenichthys marmoratus (Ayres, 1854); Pacific barracuda, Sphyraena argentea (Girard, 1854) and yellowtail, 

Seriola lalandi Valenciennes, 1833) that are common on natural reefs. As an example, predation rates on 

small fishes at Platform Holly are significantly lower than those at Naples Reef (Schroeder et al., 2005). This 
24

may allow YOY blue rockfish (and other YOY rockfishes) inhabiting the platform midwaters to spend more 

time on foraging activities rather than predator avoidance.

While such environmental factors as hypoxia, salinity, and the varied nutritional values of prey can 

influence growth rates of the early life stages of marine fishes (Lee et al., 1994; McNatt and Rice, 2004; Wuenschel et al., 2004), in ocean waters temperature and food availability are usually the most important parameters (Boehlert and Yoklavich, 1983; Denit and Sponaugle, 2004; Kurita et al., 2004; Bacon et al., 2005). In 

black rockfish (Sebastes melanops Girard, 1856), a species closely related to blue rockfish, water temperature 

was perhaps the major factor affecting growth rates (Boehlert and Yoklavich, 1983). We did not see a clear 

temperature effect on growth rate in our study. This was particularly apparent when comparing growth 

rates of fish living at Platform Irene, Santa Rosa and Anacapa islands. All of the fish from these sites were 

taken in November (thus we measured growth over the same period) and growth rates were not significantly 

different. However, these sites reside in two different oceanographic regimes; Platform Irene and Santa Rosa 

Island are in the cold California Current and Anacapa Island in the warmer waters of the southern California Bight. In 1999, from July (when blue rockfish begin to settle out of the plankton) to November (when 

the fish were captured), mean temperatures off Point Arguello (near Platform Irene) were about 3° colder 

(13.8°C, s.d. 1.1°C) than near Anacapa Island (17.0°C, s.d. 0.8°C) (data from Point Arguello, Arguello MidShelf Mooring, 34.5202, -120.6970; Anacapa Island data from the Anacapa Mid-Shelf Mooring, 34.0542, 

-119.3035, B. Emery, University of California, Santa Barbara, pers. comm.). Thus water temperature, at least 

as experienced by the fish our samples, may not have been the major factor in growth rates. Another possibility is that while temperature was important, differences in food availability may have been sufficiently 

large that they swamped any temperature effect.

It has been argued that if the YOY rockfishes that recruit to California oil and gas platforms experience 

relatively poor ecological performance, compared to fish living on natural reefs, these structures might act as 

biological sinks (Krop, 1997). This would occur because those young fishes that recruited to these structures 

would be less likely to survive to adulthood than individuals that settled on natural reefs. On the other hand, 

as noted by Carr et al., (2003), “If species perform equivalently or better on artificial reefs than they do on 

natural reefs, it is most likely that presence of an artificial habitat will benefit the regional population of that 

species.” Although limited in scope, the current study demonstrates that, as measured by daily growth rates, 

young blue rockfish living around oil and gas platforms performed at least as well as those fish living on 

natural reefs. Several previous studies have examined the fate of YOY rockfishes that recruited to California 

platforms. Love et al. (in press) presented data showing the survival and ultimate maturation of a year-class 

of YOY bocaccio at a platform in the Santa Barbara Channel. In a separate study, YOY bocaccio tagged at 

several Channel platforms were recovered on natural reefs as adults years later as much as 150 km from the 

tagging sites (Hartmann, 1987). Our current study supports previous research implying that at least some 

platforms may benefit regional fish populations.

Acknowledgments

We thank A. Bull, L. Thorsteinson, and G. Steinbach for their support of this research. This 

research was funded by the Minerals Management Service (Contract Number 1435-01-03-CA-72694 

OCS/MMS 2004-005), Biological Resources Division, U. S. Geological Survey (National Offshore 

Environmental Studies Program 1445-CA-0995-0386) based on an information need identified 

by the Minerals Management Service’s Pacific OCS Regions, and by the California Artificial Reef 

Enhancement Program (CARE). We are also indebted to S. Clark, E. Hessell, S. Mau, and C. Thompson 

for helping to collect the specimens. M. Nishimoto, B. Emery, and L. Washburn provided much needed information on the oceanographic regimes of our study area.
25

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27

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Addresses

(M.L., D. S.) Marine Science Institute, University of California, Santa Barbara, California 93106. (E. B.) EFS 

Consultants, 3 Sunset West, Ithaca, New York 14850. (W. H. L.) P.O. Box 251, Kentfield, California 94914. 

Corresponding Author

(M.L.) and E-mail: <love@lifesci.ucsb.edu>.
28

Task 3: Comparative Predation Rate on Painted Greenling Between Natural 

Habitat and an Offshore Oil Platform

Donna M. Schroeder and Milton S. Love

Abstract

Off southern California, decisions regarding decommissioning of offshore oil platforms will 

rely in part on the ecological performance of platforms as juvenile fish habitat. Here we use a tethering experiment to determine the relative risk of predation for a small benthic fish, the painted 

greenling (Oxylebius pictus), between a natural reef and an oil platform. We found that juvenile 

painted greenling at the natural reef suffered a predation rate more than 2.5 times greater than 

platform juveniles. Platform habitat had lower painted greenling density (t = -3.941, p = 0.017 ) and 

lower piscivore density ( t = -3.965, p = 0.017), but there was no significant difference among habitats in the ratio of predators to prey (Mann-Whitney U = 3.00, p = 0.513). The number of predator 

species was greater at the natural reef (7 reef species vs 1 platform species), which suggests that relative predation risk among habitat types may be governed by the habitat-specific suite of predators. 

Decommissioning decisions can alter platform fish assemblages, including the piscivore assemblage. 

Thus, the ecological performance of platform structure as juvenile fish habitat should be considered 

a dynamic property that will be impacted by the decommissioning process.

Introduction

In decommissioning offshore oil platforms, options that leave some or all of the platform structure on 

the seabed (“rigs-to-reefs”) engender a great deal of controversy (Schroeder and Love 2004). Assessment of 

the quality of platform habitat bears greatly upon decommissioning decisions, because such information 

aids in understanding the ecological role of platforms at a regional scale. Thus, measuring the comparative 

ecological performance (i.e., a species’ reproduction, growth and survivorship rates) between natural and 

platform habitats has been identified as a priority research area by marine scientists and resource managers 

(Manago and Williams 1998, Holbrook et al. 2000). 

For example, in certain years, many offshore platforms host large numbers of juvenile fishes (Schroeder et al. 2000, Love et al. in press). These observations might imply that, for these species, platforms are 

high quality habitat. However, the ultimate fate of these juveniles remains unclear. Specifically, it is unknown 

whether these fish may suffer higher rates of mortality than they would at natural habitats. 

In this study, we estimate the relative predation rate of a juvenile fish between natural habitat and an 

offshore oil platform. We used a tethering technique because, despite inherent artifacts of a tethering approach (Peterson and Black 1994, Halpin 2000), the technique is useful to evaluate relative differences in 

predation rate at small scales (Aronson et al. 2001). Further, tethering is a more direct approach than other 

field techniques, such as visual surveys or tag and recapture, where changes in fish abundance may be due to 

a combination of immigration and mortality rates. To provide insight in explaining potential differences in 

predation rate, we also compared densities of painted greenling and piscivores between habitat types. Studies that investigate comparative ecological performance among natural and artificial habitats are useful in 

fisheries management and restoration ecology, where use of artificial reefs plays an increasingly important 

role (Powers et al. 2003).
29

Methods

We conducted the study at two sites located in the Santa Barbara Channel off southern California. Platform Holly (34.3897 N, 119.9055 W) is located in 64 m of water, and 2.9 km offshore of the mainland. The 

subsurface platform structure (called the jacket) is supported by steel tubular legs anchored in the seafloor. 

Steel horizontal, diagonal, and oblique crossbeams extend both around the perimeter of the jacket and reach 

inside and across the platform. We conducted our study at Platform Holly on the shallowest (9 m) horizontal crossbeams. In depths less than 10 m, the platform structure is annually cleaned of sessile organisms. At 

the time of the study, the substrate was encrusted with a dead “turf” matrix (old byssal threads, decaying 

holdfasts, etc), red algae, bryozoans, and a number of small mussels. Naples Reef (34.4222 N, 119.9523 W) is 

located approximately 1 km offshore the mainland coast, with a depth range of 6.5 to 18 m. Here sedimentary rock outcrops form a series of ridges and pinnacles that frequently exceed a meter or more in height, 

and provide numerous caves and crevices for shelter-seeking fishes. We conducted our study in a high relief 

area at 10 m depth. In certain years, giant kelp, Macrocystis pyrifera, forms extensive stands on the reef.

Our model organism, the painted greenling, Oxylebius pictus (Hexagrammidae), is a small reef fish 

common to offshore platforms and kelp beds off California. We employed a tethering approach to compare 

relative predation rates on juvenile painted greenling between a natural and an artificial (platform) habitat. 

This benthic fish lacks a swim bladder, and its most frequently observed daytime behavior is to simply rest 

on the bottom, making tethering a suitable experimental technique.

The tethering apparatus consisted of two 30 m horizontal lines secured to the rock or platform substrate. Two lines were used for ease of deployment, and were laid end-to-end to simulate a continuous line. 

To these lines we attached ten 0.5 m monofilament tethers (“2 lb” test) spaced approximately 5 m apart, with 

a swivel at the origin and a small, 0.6 cm hook at the terminus. 

Scuba divers used hand nets to collect juveniles between 6 and 9 cm total length. After capture, divers attached the fish to the monofilament line by inserting the small hook into the dorsal musculature just 

posterior of the soft dorsal fin. Painted greenling were then placed into a small underwater holding chamber 

for approximately ten to thirty minutes so that the fish could acclimate and all fish could be released at the 

same time. All sets occurred between 1000 and 1400 hrs. After a one hour time period, divers enumerated 

the number of fish losses and released the remaining fish back into the field. We deployed 5 sets of ten tethers 

at each site. Tethering trials were conducted between November 15 and December 10, 2003.

To detect potential tethering artifacts that may occur with this experimental protocol, we installed an 

underwater video system on Naples Reef to observe the activity of a tethered painted greenling and its interactions with other organisms during a one hour set that was distinct from the experimental sets. 

We tested the hypothesis of no difference in mean fish losses between platform and natural habitat 

using a two-tailed t-test (n=5). Fish losses were recorded as the proportion of fish remaining after each set. 

Prior to statistical analysis, we arcsine transformed the data to improve normality.

We used scuba to visually survey the fish assemblages at Platform Holly and Naples Reef. At each 

habitat type, density (individuals per hectare), mean size (total length) and species composition of reef 

fishes were estimated in shallow portions of platforms (0 to 9 m depth) and natural reefs (7 to 15 m). We 

performed three surveys during the Fall of 2003. All divers performing visual counts had completed previous training in size estimation. In each platform survey, scuba divers swam a pattern which incorporated all 

four corner legs as well as major horizontal crossbeams and portions inside the platform jacket. Natural reef 

surveys consisted of divers recording observations along eight haphazardly placed 30 m length x 2 m width 

x 2 m height belt transects. We tested the hypothesis of no difference in mean painted greenling density (no. 

fish/1000m3), mean predator density (no. fish/1000m3), and mean ratio of predators to painted greenling 

density, between platform and natural habitat using two-tailed t-tests (n=3). For each fish assemblage, we 
30

determined the proportion of potential predators by referring to Quast (1968), who enumerated the percent 

occurrence of perciform fishes in gut contents of kelp bed fishes. 

Results

Experiments employing tethering techniques must include a thorough assessment of any treatment 

artifacts, particularly those relating to fish behavior. Diver observations and video tape at the natural reef 

revealed that after tethered painted greenling were released from the underwater holding chamber they either 

directly settled down on the substrate, or swam to the end of the tether, stopped, and then either settle to the 

bottom or swam to the nearest shelter (typically a patch of red algae or large cobbles). Fish did not exhibit 

sustained struggling behavior or other apparent distress signs that might have attracted predators. At the platform, diver observations recorded that tethered greenling behaved much in the same way as those on the natural reef, except that there were fewer refuges for the fish, and most remained exposed. Currents were stronger 

at Platform Holly than at Naples Reef, and tethered fish generally swam in the downstream direction.

During the videotaped session at Naples Reef, we observed three predator strikes on two different 

painted greenling. One kelp bass, Paralabrax clathratus, made two strikes (one successful) on an untethered 

painted greenling (which was positioned near the tethered fish), and one sheephead, Semicossyphus pulcher, 

made one successful strike on the tethered fish. Based on this video tape record and diver observations, we 

assumed that fish losses are able to approximate the relative predation rate between habitat types.

Relative predation rate on painted greenling was significantly higher at Naples Reef than at Platform 

Holly (Fig. 1). Naples Reef also had higher painted greenling density (t = -3.941, p = 0.017 ) and higher predator density (all species combined, t = -3.965, p = 0.017; Fig. 1). The number of predator species was much 

greater at Naples Reef than at Platform Holly (Table 1). In the test comparing the mean ratio of predators to 

prey between habitats, unequal variances required the use of a nonparametric test; there was no significant 

difference between means (Mann-Whitney U = 3.00, p = 0.513; Fig. 1). 

Table 1. Mean (1SE) density (no. fish/1000 m3) of each predator species between habitat types.

Common name Scientific name Naples Reef Platform Holly

Lingcod Ophiodon elongatus 0.3 (0.3) –

Kelp bass Paralabrax clathratus 5.7 (1.9) –

Barred sand bass Paralabrax nebulifer 0.3 (0.3) –

Cabezon Scorpaenichthys marmoratus 0.3 (0.3) 3.0 (2.5)

Kelp rockfish Sebastes atrovirens 1.7 (0.9) –

Gopher rockfish Sebastes carnatus 0.7 (0.3) –

Sheephead Semicossyphus pulcher 20.0 (2.5) –
31

Discussion

The tethering experiment shows that juvenile painted greenling residing at Naples Reef experienced a 

relative predation rate more than 2.5 times greater than the estimated rate experienced by juveniles residing 

at Platform Holly. How can we interpret this result to actual differences in predation among habitat types? 

It is important to note that experimental predation rates are generally not equivalent to natural predation 

rates; the later tends to be much lower (Linehan et al. 2001). Next, we consider whether common difficulties in using tethering as an experimental approach could negate the general result of higher predation on 

juveniles at the natural reef. 

Differences in the escape response between tethered and untethered prey could limit generality of experimental conclusions. For example, some studies found that highly mobile prey made poor experimental 

subjects because, when tethered, they behaved in a manner that attracted predators (e.g., Zimmer-Faust et al. 

1994). Painted greenling are benthic fish and tethering did not produce obvious anomalous behavior. When 

tethering trials were terminated, remaining fish were observed quiescent on the bottom much like freeranging fish. Further, the video tape record of an untethered fish being eaten by a piscivore before a nearby 

tethered fish supports the supposition that behavior was not dramatically altered in this experiment. 

Another criticism of tethering is that the technique promotes predation by novel species. In particular, 

tethered fish may be more susceptible to predation by aggressive invertebrates such as crabs (Curran and 

Able 1998, Linehan et al 2001, Adams et al. 2004). Page et al. (1998) show that Platform Holly hosts large 

densities rock crabs (Cancer spp.). If these crabs preyed upon tethered painted greenling, it would not have 

changed the conclusion of a lower relative predation rate at Platform Holly compared to Naples Reef.

20

40

60

80

10

20

30

0.1

0.2

0.3

0.4

0.2

0.4

0.6

0.8

*

*

*

B

C D

A

Platform

Holly

Naples

Reef

Platform

Holly

Naples

Reef

Platform

Holly

Naples

Reef

Platform

Holly

Naples

Reef

Mean (1SE) proportion

fish losses

Mean (1SE) predator

density (no. fish / 1000 m3

)

Mean (1SE) painted greenling

density (no. fish / 1000 m3

)

Mean (1SE) ratio of 

predators to painted greenling

Fig. 1. Comparison of (A) mean proportion of fish losses (representing relative predation risk; n = 5), (B) painted 

greenling density (n = 3), (C) predator density (n = 3), and (D) predator to prey ratio (n = 3), between Naples Reef and 

Platform Holly. Significance levels of p < 0.05 are indicated by asterisks.
32

An implicit assumption of tethering experiments is that second-order interactions do not affect results 

(Peterson and Black, 1994, Halpin 2000). That is, any artifact of tethering prey is equivalent across treatments. 

Problems arise if there is an interaction between habitat and the degree to which a tethering artifact is expressed. 

For instance, Adams et al. (2004) show that tethered prey often became entangled in seagrass (further restricting prey movement and increasing risk) but not in other habitat types. In this study, fish may have more easily 

broken free from their tethers at Platform Holly than at Naples Reef due to higher current flow and presence 

of sharp-edged mussel shells which could increase the chance of line abrasion. Escaped fish would be recorded 

as fish losses and artificially enhance estimates of predation rate. But if either of these interactions occurred, it 

would have not altered the general conclusion of a higher relative predation rate at Naples Reef.

A number of studies show that mortality rates of juvenile reef fishes are higher when predatory fishes 

are abundant (Connell 1996, Beets 1997, Stewart and Jones 2001, Webster 2002, but see also Nakamura and 

Sano 2004). Connell (1997) experimentally demonstrated that tethered juvenile fishes on small artificial reefs, 

where there are few predators, also had higher survivorship rates than juveniles on natural reefs. Caution 

must be used in interpreting these results as a large number of predators does not necessarily translate into 

higher mortality; there must be an increase in encounter, strike or success rate (Holling 1959). A higher risk of 

predation at Naples Reef corresponded to a higher predator density, and a higher number of predator species. 

However, the ratio of predator to juvenile painted greenling were not different between habitats (although 

statistical power was weak). This result suggests that the relative predation risk among habitat types may 

depend in part upon the habitat-specific suite of predators. Fish assemblages (including piscivore density) 

show a great deal of variability among platforms and decommissioning decisions can affect these assemblages 

(Schroeder and Love 2004). Thus, the ecological performance of platform structure as juvenile fish habitat 

should be considered a dynamic property that will be impacted by the decommissioning process.

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