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. 
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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.
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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.