OCS Study 

•

MMS 2005-001 

Role of Food Subsidies and Habitat 

Structure in Influencing Benthic 

Communities of Shell Mounds at Sites 

of Existing and Former Offshore Oil 

Platforms 

Final Technical Summary 

Final Study Report 

U.S. Department of the Interior 

Minerals Management Service

Pacific OCS Region 

OCS Study 

MMS 2005-001 

Role of Food Subsidies and Habitat 

Structure in Influencing Benthic 

Communities of Shell Mounds at Sites 

of Existing and Former Offshore Oil 

Platforms 

Final Technical Summary 

Final Study Report 

Authors 

Henry M. Page 

Jenifer Dugan 

James J. Childress 

Principal Investigators 

Prepared under MMS Cooperative 

Agreement No. 14-35-0001-31063 

by 

Coastal Marine Institute 

Marine Science Institute 

University of California 

Santa Barbara, CA 93106-6150 

 

 

U.S. Department of the Interior 

Minerals Management Service Camarillo

Pacific OCS Region February 2005
 

Disclaimer

This report has 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 of Report

Extra copies of the report may be obtained from: 

U.S. Dept. of the Interior 

Minerals Management Service 

Pacific OCS Region 

770 Paseo Camarillo 

Camarillo, CA 93010 

Phone: 805-389-7621 

A PDF file of this report is available at: 

http://www.coastalresearchcenter.ucsb.edu/CMI/ 

Suggested Citation 

The suggested citation for this report is: 

Page, H. M., J. Dugan, and J. Childress. Role of food subsidies and habitat structure in influencing 

benthic communities of shell mounds at sites of existing and former offshore oil platforms. MMS 

OCS Study 2005-001. Coastal Research Center, Marine Science Institute, University of California, 

Santa Barbara, California. MMS Cooperative Agreement Number 14-35-0001-31063. 32 pages. 
Table of Contents 

FINAL TECHNICAL SUMMARY....................................................................... 1 

FINAL STUDY REPORT...................................................................................... 5 

 Introduction....................................................................................................... 5 

Materials and Methods..................................................................................... 7 

 Study Sites..................................................................................................... 7 

 Distribution and abundance of mobile macroinvertebrates .......................... 8 

 Transect Sampling............................................................................. 8 

 Trap Sampling................................................................................... 9 

 Historical photographs..................................................................... 10 

 Statistical analyses ........................................................................................ 11 

Results................................................................................................................ 11 

 Distribution and abundance of macroinvertebrates....................................... 11 

 Population structures of benthic macroinvertebrates .................................... 13 

 Distribution and abundance of crabs............................................................. 15 

 Comparison and current and historical macroinvertebrate abundance ......... 16 

Discussion........................................................................................................... 18 

 Structure of shell mound communities ......................................................... 18 

 Effects of the allochthonous food subsidy and platform structure................ 19 

 Effects of benthic substrate composition ...................................................... 20 

 Temporal changes following platform removal............................................ 20 

Acknowledgments ............................................................................................. 22 

 References.......................................................................................................... 23 

 

 
List of Tables

Table 1. Details of study sites .................................................................................. 8 

Table 2. Summary timetable for field sampling....................................................... 10 

Table 3. Results of One-way ANOVA on transect data .......................................... 13 

Table 4. Results of repeated measures Two-way ANOVA ..................................... 16 

Table 5. Species and densities of benthic macroinvertebrates................................. 17 

List of Figures 

Figure 1. Formation and topography of shell mounds............................................. 6 

Figure 2. Study region and location of oil platforms ............................................... 7 

Figure 3. Densities of selected echinoderms and mollusks ..................................... 12 

Figure 4. Size frequency distribution on shallow shell mounds .............................. 14 

Figure 5. Regressions of body dry weight versus shell length................................. 14 

Figure 6. Mean CPU of the crabs in traps deployed ................................................ 15 

Figure 7. Mean CPU of invertebrates other than crabs in traps deployed ............... 17 

 

 

 
Role of Food Subsidies and Habitat Structure 

1 

FINAL TECHNICAL SUMMARY 

STUDY TITLE: Habitat Value of Shell Mounds to Ecologically and Commercially 

Important Benthic Crustaceans 

REPORT TITLE: Role of Food Subsidies and Habitat Structure in Influencing Benthic 

Communities of Shell Mounds at Sites of Existing and Former Offshore Oil Platforms 

CONTRACT NUMBER: 14-35-0001-31063 

SPONSORING OCS REGION: Pacific 

APPLICABLE PLANNING AREA(S): Southern California 

FISCAL YEAR(S) OF PROJECT FUNDING: FY 00/01, FY 01/02 

COMPLETION DATE OF REPORT: April 2004

COSTS: FY 00/01 - $32,231; FY 01/02 - $25,613 

CUMULATIVE PROJECT COST: $57,844 

PROJECT MANAGER: Russell J. Schmitt

AFFILIATION: Coastal Research Center, Marine Science Institute, University of 

California, Santa Barbara, CA 93106 

PRINCIPAL INVESTIGATORS: 1Henry M. Page, 1Jenifer Dugan, 1James J. Childress 

ADDRESSES: 1Marine Science Institute, University of California, Santa Barbara 

KEY WORDS: benthic community, oil platform, shell mound, artificial reef, trophic 

subsidy, allochthonous input

BACKGROUND: Offshore oil and gas platforms are among the largest artificial structures in 

the marine environment. Off the coast of southern and central California, oil platforms provide 

hard substrate for the attachment of sessile and semi-mobile organisms. The principal 

components of this assemblage at depths of <15 m are mussels. Waves/swell, storm events and 

platform cleaning dislodge clumps of the mussel community, which fall to the seafloor. This 

“faunal litterfall” provides a food subsidy to benthic consumers and alters the physical 

characteristics of the seafloor by creating a hard substrate mound. The phenomenon of shell 

mound formation has been documented at most oil platforms off the coast of California. 

During platform decommissioning, obsolete structures are removed. If the platform structure is 

removed, the deposition of faunal litterfall ceases, but the hard substrate habitat provided by the 

shell mound remains. We predicted that the removal of platforms and the subsequent loss of 
Final Technical Summary – Page and Dugan 

2

food subsidy and potential recruitment habitat would dramatically alter the shell mound 

community through changes in species composition and trophic structure, and a reduction in the 

densities and sizes of shell mound associated species. 

OBJECTIVES: To test the predictions above, we compared the abundance and population 

structures of mobile macroinvertebrates on shell mounds beneath existing platform sites, shell 

mounds from which the overlying platform structure was removed 4 to 5 years prior to data 

collection, and adjacent soft bottom areas. 

DESCRIPTION: This study was conducted in the Santa Barbara Channel at three shell 

mounds beneath existing offshore oil platforms (Gina, Houchin, and Hogan), four shell mounds 

without overlying platforms (the former sites of Platforms Hazel, Hilda, Hope, and Heidi), and 

five soft-bottom sites. We used three approaches to explore the effect of the presence of the 

platform on the distribution and abundance of mobile benthic invertebrate species. First, at the 

shallow sites (depth <35 m), which included shell mound-only (Hazel, Hilda), shell mound 

under platform, (Gina) and soft bottom habitats (SB-Gina), we sampled invertebrates within 

band transects using SCUBA. Second, at the shallow and deep sites, which included shell 

mound only (Hazel, Hilda, Heidi, Hope), shell mound under platform (Houchin, Hogan), and 

soft bottom sites, we sampled commercially important crabs (primarily Cancer spp.) as well as 

other mobile macroinvertebrate species using baited commercial traps. Finally, we recorded 

species present and estimated densities of macroinvertebrates in historical photographs taken at 

shell mound Hilda prior to platform removal. 

SIGNIFICANT CONCLUSIONS: Our results suggest that the structure of shell mound 

communities is strongly influenced by the presence of the platform structure and the food 

subsidies provided by the clumps of mussels and associated organisms that continually slough 

from the platform to the seafloor. Predatory and omnivorous seastars (Pisaster spp., Asterina 

miniata), in particular, were much less abundant and of a smaller size at the shell mound-only 

sites. However, the relative abundance (as CPUE) of commercially important crabs, Cancer 

antennarius and C. anthonyi, did not differ among shell mound sites. These crabs are predators 

and carnivorous scavengers likely consume components of faunal litterfall when available, but 

have greater mobility than other invertebrate taxa of shell mounds, and can forage over a larger 

area both on and off the mounds. Populations of two macroinvertebrate detritivores on shell 

mounds, Parastichopus parvimensis and P. californicus, also appeared to be little affected by 

the removal of the platform structure. We hypothesize that the food subsidy provided by the 

dislodged organisms that fall from platform structures to the shell mounds exerts bottom up 

control upon populations of predatory and omnivorous sea stars. The platform structure could 

also affect the abundance and size structure of benthic organisms through the provision of 

recruitment habitat. 

There was no overlap in species composition of mobile macroinvertebrates between benthic 

communities on the shell mound under Platform Gina and the adjacent soft bottom locations. 

Thus, our results also suggest that the presence or absence of hard substrate was an important 

factor in determining the distribution of shell mound associated taxa 
Role of Food Subsidies and Habitat Structure 

3 

The relative effect of the platform on the benthic community appeared to vary among mound 

species in relation to trophic level, degree of mobility, and substrate preference. Mobile crabs 

(e.g.., Cancer antennarius) and the sea cucumber, Parastichopus parvimensis, which prefer 

hard substrate, were seemingly least affected, but given the estimated sedimentation rate at 

mound-only sites (Hazel and Hilda, 0.9 to 1.4 mm year-1; Heidi and Hope, 1.5 to 2.0 mm year1

), these sites may no longer provide suitable habitat in a few years. Thus, while the presence of 

the platform structure may enhance secondary productivity in the benthic community, this 

effect may disappear rapidly (<5 yr in this case) after the platform is removed. 

STUDY RESULTS: Densities of mobile macroinvertebrate species varied by trophic level and 

taxa among shell mound sites. Density of predatory echinoderms was highest on the mound 

under Platform Gina. The predatory sea stars, Pisaster giganteus and P. ochraceus, were found 

only on the shell mound under Platform Gina. Density of the predatory-omnivorous sea star, 

Asterina miniata, was an order of magnitude higher on the shell mound under Platform Gina 

than at the shell mound-only sites Hazel and Hilda. Similarly, densities of the cowry, Cypraea 

spadicea, a carnivorous scavenger, were 5-15 times higher on the shell mound under Platform 

Gina than at mound-only sites. In contrast, the density of the deposit-feeding sea cucumber, 

Parastichopus parvimensis, did not differ significantly among platform and mound-only sites. 

There was no overlap in species composition between the macroinvertebrate community at the 

mound under Platform Gina and the adjacent soft bottom. Species composition on the soft 

bottom was typical of sandy and muddy bottom and included the echinoderms Luidia foliolata

and Lytechinus anamesus, and the mollusks Cancellaria cooperi, Megasurcula carpenteriana, 

M. stearsiana, and Polinices lewisii. 

We compared the population structures of the more abundant species (Asterina miniata, 

Parastichopus parvimensis, P. californicus, Cypraea spadicea) among sites. The population 

structure of Asterina miniata differed among sites. Mean size (as 

arm radius) of this sea star was significantly larger on the mound under Platform Gina than at 

shell mound-only Hazel and Hilda. Mean length of P. parvimensis was significantly greater at 

Hazel than at Hilda, but neither mean differed significantly from the mean length of individuals 

on the mound under Gina. There was no difference in mean lengths of the predatory cowry, 

Cypraea spadicea, among sites. However, there was a significant difference in the relationship 

between dry body weight and shell length. Cowries were heavier for a given shell length on the 

mound under Platform Gina compared with mound-only sites Hazel and Hilda. 

Four species of brachyuran decapods, the cancrid crabs, Cancer antennarius, C. anthonyi, C. 

productus, and the majid crab, Loxorynchis grandis, were caught in traps. Only four

individuals of C. productus were caught in traps during this study (in 2000) and were not 

included in the analyses. The mean CPUE of all crab species did not vary over time within 

each sampling season. The CPUE of Cancer antennarius varied among habitat type in both 

2000 and 2001. In 2000, the mean CPUE of C. antennarius on the shallow and deep shell 

mounds was significantly higher than at the shallow soft bottom sites. In 2001, patterns of 

abundance of C. antennarius were similar to the previous year, but overall CPUE was higher. 

Mean CPUE at the shell mounds was again significantly higher than at the soft bottom sites. 

In contrast, the CPUE of Cancer anthonyi was more variable among habitats. In 2000, there 

was a trend of higher CPUE on deep shell mound-only compared with shallow soft bottom and 
Final Technical Summary – Page and Dugan 

4

shallow mound-only sites. In 2001, we observed a trend of higher CPUE on soft bottom, 

compared with mounds under platforms and deep and shallow shell mound-only sites. The 

CPUE of the majid crab, Loxorhynchus grandis, was generally much lower than for Cancer

spp. and no patterns were evident among habitats 

In addition to Cancer spp. and Loxorhynchus grandis, several other species of mobile 

invertebrates were present in traps during sampling in 2000 and 2001. The most abundant were 

the sea stars, Asterina miniata, which were caught in traps at shallow and deep depths on shell 

mounds, but not on soft bottom, and Pisaster giganteus, which were present in traps only at 

mounds under platforms. 

The species composition and ranges of densities of macroinvertebrates on the mound under 

Platform Hilda prior to platform removal, estimated from historical photographs, were 

comparable to those found by us on the mound under Platform Gina. 

STUDY PRODUCTS: 

Presentations: 

Bomkamp, R. E., H. M. Page, and J. E. Dugan. 2002. Habitat value of shell mounds at existing 

and former offshore oil platform sites to mobile benthic invertebrates. Studies 

Presentation, MMS, Camarillo. 

Bomkamp, R. E., H. M. Page, and J. E. Dugan. 2001. Habitat value of shell mounds at existing 

and former offshore oil platform sites to mobile benthic invertebrates. Western Society 

of Naturalists, Ventura. 

Page, H. M. 2001. Ecology of offshore oil platforms and mussel mounds in the Santa Barbara 

Channel. Invited seminar. Bodega Marine Laboratory. 

Publications: 

Bomkamp, R. E. 2003. Distribution and abundance of mobile benthic macroinvertebrates and 

fishes on shell mounds and existing and former offshore oil platform sites, Masters 

Thesis, University of California, Santa Barbara. 

Bomkamp, R.E., H. M. Page, and J.E. Dugan. Role of food subsidies and habitat structure in 

influencing benthic communities of shell mounds at sites of existing and former 

offshore oil platforms. Marine Biology 146: 201-211. 
Role of Food Subsidies and Habitat Structure 

5 

FINAL STUDY REPORT 

INTRODUCTION 

Two important controls on the structure of benthic communities are the physical characteristics 

of the habitat and food availability. Artificial structures, such as constructed reefs and offshore 

platforms, can directly influence the species composition, distribution, and abundance of local 

invertebrate and fish fauna through the provision of hard substrate habitat and topographic relief 

(Wolfson et al. 1979; Davis et al. 1982; Bohnsack 1989; Herrnkind et al. 1997). The 

aggregation of mobile invertebrates and fishes on and around artificial structures, which are 

typically located on soft bottom, is well documented and usually attributed to the attraction 

and/or possible production of reef-associated species at or on the structures. At the same time, 

these structures often provide food chain support for higher level consumers and may indirectly 

subsidize the adjoining soft bottom benthic community through the production and export of 

organic matter (Wolfson et al. 1979, Page et al. 1999). Food availability has been shown to 

exert a strong influence on the species composition, densities, and growth rates of consumers in 

many marine habitats (Bustamante et al. 1995, Polis et al. 1996, Dahlhoff and Menge 1996, 

Link and Almeida 2002, Dugan et al. 2003). Such bottom-up controls are considered an 

important organizing force in marine ecosystems (e.g., Menge 1992, Bustamante and Branch 

1996). 

Offshore oil and gas platforms are among the largest artificial structures in the marine 

environment. Off the coast of southern and central California, oil platforms provide hard 

substrate for the attachment of sessile and semi-mobile organisms that are not typical of the soft 

bottom habitat over which platforms are installed (Wolfson et al. 1979; Page and Hubbard 

1987; Page et al. 1999). The principal components of this assemblage at depths of <15 m are 

mussels (Mytilus californianus, M. galloprovincialis), with encrusting bivalves (e.g., Chama 

arcana, Crassodoma giganteus), barnacles (e.g., Megabalanus californicus), and anemones 

(e.g., Metridium senile) predominating deeper (Page et al. 1999). Waves/swell, storm events 

and platform cleaning dislodge clumps of the mussel community, which fall to the seafloor 

(Fig. 1a). Page et al. (1999) estimated that from 47 to 1031 kg wet weight mussel community 

wk-1 fell from vertical conductor pipes, a fraction of the platform structure (area = 3710 m2), to 

the seafloor beneath Platform Holly. Wolfson et al. (1979) estimated faunal litterfall from 

Platform Eva (Huntington Beach, California) to be from 678 to 1169 kg wet weight mussels 

wk-1. 

This “faunal litterfall” alters the benthos in two distinct ways. First, allochthonous organic 

matter falling to the seafloor provides a food subsidy to benthic consumers (Fig. 1a). Second, 

the input of mussel shells alters the physical characteristics of the seafloor by creating hard 

substrate habitat and altering bottom topography (Fig. 1b). The resulting shell mound habitat 

differs dramatically from the surrounding soft bottom, rising up to 8.5 m above the surrounding 

seafloor and measuring up to 70 m across (deWit 2001). The phenomenon of shell mound 

formation has been documented at many offshore oil platforms off the coast of California (MEC 

2003). 

During platform decommissioning, obsolete structures are removed, or if they are destined to 

serve as artificial reefs, they may be cropped or toppled in place or towed and submerged in a 
Final Study Report – Page and Dugan 

6

different location (reviewed in Love et al. 2003). If the platform structure is removed, the 

deposition of faunal litterfall ceases, but the hard substrate habitat provided by the shell mound 

remains. 

Figure 1. Diagram illustrating the fall of mussels and other organisms from the structure of offshore oil platforms 

to the seafloor and the formation and topography of shell mounds. a) Platform with shell mound showing [1] 

encrusting invertebrates [2] detaching from platform structure and falling to the seafloor, where soft tissue is 

consumed by benthic invertebrates, leaving [3] a mound of empty shells surrounded by [4] soft bottom. b) 

Topography of shell mound Hilda, which is typical of shell mounds in this study. c) Cross section of the shell 

mound beneath Platform Gina, also typical of shell mounds in this study.

We used the “natural” experimental opportunity provided by the removal of platform structures 

to explore the effects of substrate characteristics and food subsidies on the distribution and 

abundance of mobile epibenthic invertebrates. We predicted that the removal of platforms (and 

loss of food subsidy and potential recruitment habitat) would dramatically alter the shell mound 

community through a change in species composition and trophic structure, and a reduction in 

the densities and sizes of shell mound associated species. To investigate this prediction, we 

compared the abundance and population structures of mobile macroinvertebrates on shell 

mounds beneath existing platform sites, shell mounds from which the overlying platform 

structure was removed 4 to 5 years prior to data collection, and soft bottom sites. We used a 

combination of transect sampling using SCUBA at shallow sites, trapping at shallow and deep 

sites, and historical photographs of mound habitat. Results suggest that platforms alter benthic 

communities both through food subsidies and habitat modification. Although the shell mound 

habitat remains, our results suggest that the associated benthic community is profoundly altered 

by removal of the platform structure. 

2

1

3 4

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a.

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Role of Food Subsidies and Habitat Structure 

7 

MATERIALS AND METHODS 

Study sites 

This study was conducted in the Santa Barbara Channel at three shell mounds beneath existing 

offshore oil platforms (Gina, Houchin, and Hogan), four shell mounds without overlying 

platforms (the former sites of Platforms Hazel, Hilda, Hope, and Heidi), and five soft-bottom 

sites (Table 1, Fig. 2). All shell mounds except Gina are located offshore of Summerland, 

California. Shell mounds Hazel and Hilda are located in water depths of ~30 m. Platforms 

Houchin and Hogan and shell mounds Hope and Heidi are arranged in a line oriented northeastsouthwest along a depth gradient of ~40 to 50 m. Platform Gina is located in a water depth of 

29 m offshore of Oxnard, California, 33 km southeast of Platform Hogan and is the only extant 

platform in which the shell mound is easily accessible using conventional SCUBA (Table 1). 

Soft-bottom sites were established within 1 km of each of the four shell mound-only sites and 

Platform Gina (Fig. 2). 

Figure 2. Map showing the study region and the locations of the oil platform, shell mound-only, and soft bottom 

study sites in the Santa Barbara Channel. 

The shell mounds are composed of clay and drill cuttings covered by a 0.3-2.1 m thick layer of 

empty shells (de Witt 2001). The shell mounds vary from 45 to 84 m in diameter and extend 

from 4 to 9 m above the seafloor (Table 1, Fig. 1b). The platform structure at the sites of 

former Platforms Hazel, Hilda, Heidi, and Hope, was removed ~5 years prior to our study. A 

thin layer of sediment covers these mounds (de Wit 2001). 

San Francisco

Santa Barbara

San Diego

Pacific Ocean

N

100 km

Carpinteria

Oxnard

Pacific Ocean

10 km

Houchin Heidi

Hazel Hilda

Hope

Gina

Hogan

N

Mound under platform

Mound-only

Soft bottom

San Francisco

Santa Barbara

San Diego

Pacific Ocean

N

100 km

Carpinteria

Oxnard

Pacific Ocean

10 km

Houchin Heidi

Hazel Hilda

Hope

Gina

Hogan

N

Mound under platform

Mound-only

Soft bottom
Final Study Report – Page and Dugan 

8

Table 1. Details of study sites, including habitat type and name, depth class, depth, distance from shore, year of 

platform installation and removal. Data from de Wit (2001) and MEC (2003). 

Habitat 

Depth 

Class Depth 

(m) 

Distance 

from shore 

(km) 

Year 

install

ed 

Year 

removed 

Mound 

height 

(m) 

Mound 

dimension

s (m) 

Moundonly 

 

Hazel Shallow 31 3.4 1958 1996 8.5 66 x 72 

Hilda Shallow 34 3 1960 1996 6.7 66 x 72 

Heidi Deep 40 4.8 1966 1996 7.3 60 x 75 

Hope Deep 42 5.2 1965 1996 6.7 51 x 60 

 

Mound 

under 

 

platform 

Gina Shallow 29 6.9 1980 -- 3.9 45 x 63 

Hogan Deep 47 6.9 1967 -- 7.8 78 x 78 

Houchin Deep 50 7.6 1968 -- 6.3 84 x 84 

 

Soft bottom 

SB-Gina Shallow 30 6.9 -- -- -- -- 

SB-1 Shallow 30 ~ 3 -- -- -- -- 

SB-2 Shallow 30 ~ 3 -- -- -- -- 

SB-3 Deep 41 ~ 5 -- -- -- -- 

SB-4 Deep 41 ~ 6 -- -- -- -- 

Distribution and abundance of mobile macroinvertebrates on shell mounds 

We used three approaches to explore the effect of the presence of the platform on the 

distribution and abundance of mobile benthic invertebrate species. First, at the shallow sites 

(depth <35 m), which included shell mound-only (Hazel, Hilda), shell mound under platform, 

(Gina) and soft bottom habitats (SB-Gina), we sampled invertebrates within band transects 

using SCUBA (Table 2). Second, at the shallow and deep sites, which included shell mound 

only (Hazel, Hilda, Heidi, Hope), shell mound under platform (Houchin, Hogan), and soft 

bottom sites (SB-1, 2, 3, 4), we sampled commercially important crabs (primarily Cancer spp.) 

as well as other mobile macroinvertebrate species using baited commercial traps (Fathoms 

PlusTM) (Table 2). Finally, we recorded species present and estimated densities of 

macroinvertebrates in historical photographs taken at shell mound Hilda prior to platform 

removal. These approaches are elaborated on below. 

Transect sampling 

We sampled invertebrates in 1 x 20 m band transects at mound-only sites, Hazel (n=4) and 

Hilda (n=3), and in 1 x 10 m transects at Platform Gina (n=3). Transect length was shorter at 

Gina because of the much higher density of organisms at this site. To expedite sampling at 
Role of Food Subsidies and Habitat Structure 

9 

depths of 30-33 m in poor visibility (< 2 m), transects were situated in a radial design with 

equal angular spacing from a central reference point (i.e., base of marker buoy chain). Divers 

collected all mobile benthic macroinvertebrates encountered along the transects. Invertebrates 

were returned to the boat for identification and measurement to the nearest millimeter. Sea stars 

were measured from the center of the disk to the tip of the longest arm, sea cucumbers were 

measured on the longitudinal axis while contracted, and gastropods were measured along the 

longest axis of the shell. 

To compensate for over sampling at the center due to the radial transect design, organism 

densities in the band transects were weighted according to proximity to the center. Band 

transects were divided into four segments of equal length. Organism densities in each segment 

were multiplied by weighting factors assuming an imaginary wedge-shaped transect with the 

same area as the band transect. Weighting factors, determined by dividing the areas of each 

segment of a hypothetical wedge-shaped transect by the area of the corresponding segment in 

the band transect, ranged from 0.25 to 1.75. 

In addition, we compared the body weight of the chestnut cowry, Cypraea spadicea, among the 

shallow shell mounds (Hazel, Hilda, and Platform Gina) since body weight may reflect 

nutritional condition (e.g., Mann 1978, Shriver et al. 2002). The soft tissues of individuals of a 

range of shell-lengths were excised and oven dried at 60° C to a constant weight. Dry body 

weight was regressed against shell length for each site. 

Trap sampling 

We sampled commercially important crabs and other macroinvertebrates using baited 

commercial traps (Fathoms Plus). Sampling was conducted from September through December 

because crabs were found in highest abundance at this time of year in a previous study (Page et 

al. 1999). In 2000, we sampled the shallow and deep shell mound-only (Hazel, Hilda, Hope, 

Heidi) and soft-bottom (SB-1, SB-2) sites 4x from November through December. In 2001, we 

again sampled the shell mound-only sites (Hazel, Hilda, Hope, Heidi), but also the deep mound 

under platform (Hogan, Houchin) and soft-bottom sites (SB-3, SB-4) 4x from September 

through November. The shallow mound under platform site at Gina was not sampled because 

of logistical difficulties. Consistent trap placement at mound-only and soft bottom stations was 

achieved using GPS (Furuno FBX-2). 
Final Study Report – Page and Dugan 

10

Table 2. Summary timetable for the field sampling of shell mound sites by transect and trap. Transect sampling 

was conducted only at the shallow sites. 

Transect sampling 

 

Trap sampling 

Category 2000 2001 2000 2001 

Mound-only 

Hazel X X X 

Hilda X X X 

Heidi X X 

Hope X X 

 

Mound under 

platform 

Gina X 

Hogan X 

Houchin X 

 

Soft bottom 

SB-Gina X 

SB-1 X 

SB-2 X 

SB-3 X 

SB-4 X 

Traps were baited with a total of 1.4 kg of coarsely chopped mackerel, Scomber japonicus, 

enclosed in rigid plastic mesh (8 mm) containers. Traps (n=2 per site) were retrieved after a 24 

hour soak time. Captured crabs were identified to species and counted. Trapping results are 

expressed as catch per unit effort (CPUE), which is the mean number of crabs per trap per site 

captured after 24 hours, and compared over time in a repeated measures design (Zar 1984). 

Historical photographs 

There were no sampling data available on shell mound communities prior to the removal of 

Platforms Hazel, Hilda, Heidi, and Hope in 1996. However, color photographs, taken in 1976 

to document fauna associated with the shell mound at Platform Hilda, were used to record 

species present and to estimate historical densities of mobile benthic invertebrate species on the 

shell mound beneath this platform prior to platform removal. This information was useful for 

comparisons with data from the existing shallow platform site (Gina) and published data. Only 

photographs taken perpendicular to the bottom and covering an area of ~70 cm2 were used in 

the analyses (n=5). 
Role of Food Subsidies and Habitat Structure 

11 

Statistical analyses 

All statistical analyses were carried out using JMP 4.04 and SPSS 11.5. The effect of site on 

density and mean size of mobile benthic invertebrates sampled by band transect was evaluated 

using One-way ANOVA. The effect of site on the relationship between body weight and shell 

length of Cypraea spadicea was explored using a t-test of homogeneity of slopes. The effect of 

site on the abundance of crabs, as CPUE, was evaluated using repeated measures ANOVA; this 

statistic was appropriate since our study involved repeated sampling of the same study sites. 

Crab abundance data were log transformed [x’ = log10(x+1)] prior to analysis to correct for 

heteroscedasticity (Zar, 1984). 

RESULTS

Distribution and abundance of macroinvertebrates: transect sampling at shallow sites 

Densities of mobile macroinvertebrate species varied by trophic level and taxa among shell 

mound sites (Table 3, Fig. 3). Density of predatory echinoderms was highest on the mound 

under Platform Gina. The predatory sea stars, Pisaster giganteus (0.89 ± 0.17 m-2, x ± 1 SE) 

and P. ochraceus (0.05 ± 0.03 m-2), were found only on the shell mound under Platform Gina. 

One individual of a congener, P. brevispinus, was found at Hilda (0.03 ± 0.03 m-2). Density of 

the predatory-omnivorous sea star, Asterina miniata, was an order of magnitude higher on the 

shell mound under Platform Gina (2.54 ± 0.53 m-2) than at the shell mound-only sites (Hazel, 

0.34 ± 0.14 m-2; Hilda, 0.12 ± 0.02 m-2, P = 0.0012, F = 20.43, df = 2, 7, One-way ANOVA: 

Table 3, Fig. 3). 

Similarly, densities of the cowry, Cypraea spadicea, a carnivorous scavenger, were 5-15 times 

higher on the shell mound under Platform Gina (0.45 ± 0.21 m-2) than at mound-only sites 

(Hazel, 0.08 ± 0.08 m-2; Hilda, 0.03 ± 0.03 m-2, P = 0.1: Table 3, Fig. 3). The density of C. 

spadicea under Platform Gina was significantly higher than on shell mound-only Hazel and 

Hilda when data for the latter two sites were pooled (P < 0.05, t = 2.66, df = 8, Student’s t-test). 

The gastropods, Conus californicus, a predator, and Kelletia kelletii, a carnivorous scavenger, 

occurred in low densities (--) at the shell mound-only sites and were not found on the mound 

under Platform Gina; the omnivorous gastropod, Megathura crenulata, and sea urchin, 

Strongylocentrotus fransicanus, were found only on the mound under Platform Gina (0.6 ± 0.1 

m

-2) (Fig. 3). 

In contrast, the density of the deposit-feeding sea cucumber, Parastichopus parvimensis, did not 

differ significantly among platform and mound-only sites (Table 3, Fig. 3). However, density 

of a congener, P. californicus, were significantly higher under Platform Gina (0.74 ± 0.03 m-2) 

compared to mound-only Hazel (0.05 ± 0.05 m-2) and Hilda (0.02 ± 0.02 m-2, P < 0.05, F = 

6.18, df = 2, 7, One-way ANOVA: Table 3, Fig. 3). 

 
Final Study Report – Page and Dugan 

12

Figure 3. Densities of selected a) echinoderms and b) mollusks on shallow mound-only (Hazel and Hilda), shallow 

mound under an existing platform (Gina), and soft bottom sites (SB-Gina). Mean values ± 1 SE, n = 3-4 transects 

per site. Note difference in y-axis scale among panels. 

There was no overlap in species composition between the macroinvertebrate community at the 

mound under Platform Gina and the adjacent soft bottom (Fig. 3). Species composition on the 

soft bottom was typical of sandy and muddy bottom and included the echinoderms Luidia 

foliolata (0.02 m-2) and Lytechinus anamesus (1.2 m-2), and the mollusks Cancellaria cooperi

(0.02 m-2), Megasurcula carpenteriana (0.05 m-2), M. stearsiana (0.02 m-2), and Polinices 

lewisii (0.02 m-2). 

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Hazel Hilda Gina SB-Gina

D

e

n

sity

N( o. m-2)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

a. Echinoderms

b. Mollusks

0 0 0000

0 0 00 0000

Megathura crenulata

Cypraea spadicea

Kelletia kelletii

Conus californicus

Site

mound only (platform)

Asterina miniata

Parastichopus californicus

Parastichopus parvimensis

Pisaster giganteus

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Hazel Hilda Gina SB-Gina

D

e

n

sity

N( o. m-2)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

a. Echinoderms

b. Mollusks

0 0 0000

0 0 00 0000

Megathura crenulata

Cypraea spadicea

Kelletia kelletii

Conus californicus

Site

mound only (platform)

Asterina miniata

Parastichopus californicus

Parastichopus parvimensis

Pisaster giganteus
Role of Food Subsidies and Habitat Structure 

13 

Table 3. Results of One-way ANOVA on transect data evaluating the effect of site on the densities of mobile 

benthic mollusks and echinoderms. 

Species F P df 

Cypraea spadicea 3.20 0.10 2, 7 

Kelletia kelletii 1.64 0.26 2, 7 

Parastichopus californicus 6.18 < 0.05 2, 7 

P. parvimensis 0.55 0.60 2, 7 

Asterina miniata 20.43 < 0.001 2, 7 

Pisaster giganteus 32.85 < 0.001 2, 7 

Population structures of benthic macroinvertebrates 

We compared the population structures of the more abundant species (Asterina miniata, 

Parastichopus parvimensis, P. californicus, Cypraea spadicea) among sites (Fig. 4). The 

population structure of Asterina miniata differed among sites (Fig. 4a). Mean size (as 

arm radius) of this sea star was significantly larger on the mound under Platform Gina (87.3 ± 

1.3 mm) than at shell mound-only Hazel (48.1 ± 2.1 mm) and Hilda (50.4 ± 1.2 mm; P < 

0.0001, F = 301.29, df = 2, 284, One-way ANOVA, P < 0.05, Tukey post hoc test: Fig. 4a). 

Mean length (contracted) of the deposit feeder, Parastichopus parvimensis, also varied 

significantly among sites (Gina, 110.4 ± 6.9 mm; Hazel, 128.4 ± 4.9 mm; Hilda, 92.7 ± 3.8 mm; 

P <0.0001, F = 16.96, df = 2, 176, One-way ANOVA: Fig. 4b). Mean length of P. 

parvimensis was significantly greater at Hazel than at Hilda (P < 0.05, Tukey post hoc test), but 

neither mean was significantly different from the 

mean length of individuals on the mound under Gina (P > 0.05, Tukey post hoc test). The mean 

length of P. californicus was significantly greater at Platform Gina (162.4 ± 5.7 mm) than at 

Hilda (114.0 ± 13.7 mm; P = 0.002, t = 3.26, df = 52, t-test). 

There was no difference in mean lengths of the predatory cowry, Cypraea spadicea, among 

sites (Fig. 4c). However, there was a significant difference in the relationship between dry body 

weight and shell length (P < 0.001, t = -3.918, df =19, t-test of homogeneity of slopes: Fig. 5). 

Cowries were heavier for a given shell length on the mound under Platform Gina compared 

with mound-only sites Hazel and Hilda. 
Final Study Report – Page and Dugan 

14

Figure 4. Size frequency distribution of a) Asterina miniata, b) Parastichopus parvimensis, and c) Cypraea 

spadicea on the shallow shell mounds. Note difference in axis scales and units among panels. 

Figure 5. Regressions of body dry weight versus shell length for Cypraea spadicea sampled from shell mounds at 

former platform sites Hazel and Hilda (●, y = 0.13x - 3.88, r2

 = 0.77, n = 9) and from the shell mound beneath 

platform Gina (○, y = 0.24 x – 7.84, r2

 = 0.91, n = 12). 

0.0

0.1

0.2

0.3

0.4

Frequency

0.0

0.1

0.2

0.3

0.4

0 20 40 60 80 100 120 140

0.0

0.1

0.2

0.3

0.4

Hazel

n=46

Hilda

n=115

Gina

n=126

Arm radius (mm)

a. Asterina miniata

Contracted length (mm)

0 50 100 150 200 250 300

0.00

0.05

0.10

0.15

0.20

0.00

0.05

0.10

0.15

0.20

0.00

0.05

0.10

0.15

0.20

Hazel

n=56

Hilda

n=95

Gina

n=28

b. Parastichopus parvimensis

Shell length (mm)

0.0

0.1

0.2

0.3

0.4

0.0

0.1

0.2

0.3

0.4

30 35 40 45 50 55 60 65 70

0.0

0.1

0.2

0.3

0.4

Hazel

n =12

Hilda

n =13 

Gina

n = 24

c. Cypraea spadicea

Shell length (mm)

34 36 38 40 42 44 46 48 50 52 54

Dry weight (g)

0

1

2

3

4

5

6
Role of Food Subsidies and Habitat Structure 

15 

Distribution and abundance of crabs: trap sampling 

Four species of brachyuran decapods, the cancrid crabs, Cancer antennarius, C. anthonyi, C. 

productus, and the majid crab, Loxorynchis grandis, were caught in traps (Fig. 6). Only four

individuals of C. productus were caught in traps during this study (in 2000) and were not 

included in the analyses. The mean CPUE of all crab species did not vary over time within 

each sampling season (P > 0.05, Repeated measures ANOVA: Table 4). 

Figure 6. Mean CPUE of the crabs, Cancer antennarius, C. anthonyi, and Loxorhynchus grandis in traps 

deployed in a) 2000 and b) 2001. n = 2 sites per habitat type (4 sampling dates per year). 

The CPUE of Cancer antennarius varied among habitat type in both 2000 and 2001 (P < 0.05, 

Repeated measures ANOVA: Table 4, Fig. 6). In 2000, the mean CPUE of C. antennarius on 

the shallow (3.4 ± 0.8 crabs trap-1) and deep (3.8 ± 0.7 crabs trap-1) shell mounds was 

significantly greater than at the shallow soft bottom sites SB-1 and SB-2 (0.4 ± 0.3 crabs trap-1; 

P < 0.05, post hoc paired t-test with Bonferroni correction). In 2001, patterns of abundance of 

C. antennarius were similar to the previous year, but overall CPUE was higher, ranging from 

2.4 to 10.2 crabs trap-1 (Fig. 6b). Mean CPUE at the shell mounds was again significantly 

higher than at the soft bottom sites (p< 0.05, post hoc paired t-test with Bonferroni correction). 

0

2

4

6

8

10

12

14 a. 2000

Shallow soft bottom

Mound under platform

0

2

4

6

8

10

12

14

Deep mound

Shallow mound

Deep soft bottom

b. 2001

Mean CPUE

0 0 n/a

n/a n/a

Cancer antennarius

Cancer anthonyi

Loxorhynchus grandis

Habitat type

0

2

4

6

8

10

12

14 a. 2000

Shallow soft bottom

Mound under platform

0

2

4

6

8

10

12

14

Deep mound

Shallow mound

Deep soft bottom

b. 2001

Mean CPUE

0 0 n/a

n/a n/a

Cancer antennarius

Cancer anthonyi

Loxorhynchus grandis

Habitat type
Final Study Report – Page and Dugan 

16

In contrast, the CPUE of Cancer anthonyi was more variable among habitats. In 2000, there 

was a trend of higher CPUE on deep shell mound-only (6.4 ± 1.7 crabs trap-1) compared with 

shallow soft bottom (3.4 ± 0.9 crabs trap-1) and shallow mound-only sites (1.4 ± 0.5 crabs trap-1: 

Fig. 6a). However, differences in mean values were not significant due to high variability in 

CPUE among sampling dates. In 2001, we observed a trend of higher CPUE on soft bottom 

(8.9 ± 2.7 crabs trap-1), compared with mounds under platforms (0 crabs trap-1), and deep (1.1 ± 

0.5 crabs trap-1) and shallow (0.9 ± 0.3 crabs trap-1) shell mound-only sites (Fig. 6b). Again, 

these differences in abundance were not significant due to high variability in CPUE among 

sampling dates. 

The CPUE of the majid crab, Loxorhynchus grandis, was generally much lower than for Cancer

spp. and no patterns were evident among habitats. In 2000, mean CPUE ranged from 0.6 ± 0.2 

crabs trap-1 at deep shell mound-only to 1.6 ± 0.4 crabs trap-1 at shallow soft bottom sites (Fig. 

6a). In 2001, CPUE of L. grandis was significantly greater on shallow shell mound-only (0.7 ± 

0.1 crabs trap-1) and deep soft bottom (0.4 ± 0.2 crabs trap-1) than deep shell mound-only (0.06 

± 0.06 crabs trap-1) and mounds beneath platform sites (0 crabs trap-1; Table 4, p < 0.05, post 

hoc t-test with Bonferroni correction). 

Table 4. Results of repeated measures Two-way ANOVA evaluating the effect of habitat type and time on the 

abundance (CPUE) of two Cancer spp. and of Loxorhynchus grandis in 2000 and 2001. Data log10(x+1) 

transformed prior to analysis. 

 Habitat type Time Habitat x Time 

F P df F P df F P df 

2000 

C. antennarius 13.06 <0.05 2,3 0.43 0.78 3,1 2.42 0.32 6,2 

C. anthonyi 1.74 0.32 2,3 2.32 0.44 3,1 2.89 0.28 6,2 

L. grandis 1.00 0.46 2,3 8.81 0.24 3,1 5.52 0.16 6,2 

 

2001 

C. antennarius 26.00 <0.01 3,4 5.57 0.16 3,2 0.44 0.86 9, 5 

C. anthonyi 4.29 0.10 3,4 0.24 0.86 3,2 0.67 0.72 9, 5 

L. grandis 7.33 0.42 3,4 1.83 0.37 3,2 1.39 0.37 9, 5 

In addition to Cancer spp. and Loxorhynchus grandis, several other species of mobile 

invertebrates were present in traps during sampling in 2000 and 2001 (Table 5). The most 

abundant were the sea stars, Asterina miniata, which were caught in traps at shallow and deep 

depths on shell mounds, but not on soft bottom, and Pisaster giganteus, which were present in 

traps only at mounds under platforms. Individuals of the whelk, Kelletia kelletii, were most 

abundant in traps at shallow mound-only sites (Table 5). 

Comparison of current and historical estimates of macroinvertebrate abundance at 

shallow shell mound sites 

The species composition and ranges of densities of macroinvertebrates on the mound under 

Platform Hilda prior to platform removal were comparable to those found by us on the mound 

under Platform Gina (Table 5). Species present in photographs and sampled in this study 
Role of Food Subsidies and Habitat Structure 

17 

included the seastars Pisaster brevispinus, P. giganteus, P. ochraceus, and Asterina miniata, sea 

urchin, Strongylocentrotus franciscanus, and sea cucumbers, Parastichopus parvimensis and P. 

californicus and crab, Cancer antennarius. Species in the photographs at Hilda prior to 

platform removal that were not found on the mound under Platform Gina included the predatory 

sea stars Dermasterias imbricata and Pycnopodia helianthoides. 

Figure 7. Mean CPUE of invertebrates other than crabs in traps deployed in a) 2000 and b) 2001. n = 2 sites per 

habitat type (4 sampling dates per year). 

Table 5. Species and densities of benthic macroinvertebrates estimated from photographs taken in 1976 on the 

shell mound under Platform Hilda. Mean values ± 1 SE, n=5 photographs. 

Species Common name Mean (No. m-2) Range (No. m-2) 

Cancer antennarius Crab 0.5 ± 0.3 0 – 1.1 

Asterina miniata Seastar 4.5 ± 0.8 2.7 – 7.1 

Dermasterias imbricata Seastar 0.1 ± 0.1 0 – 0.5 

Pisaster giganteus Seastar 0.1 ± 0.1 0 – 0.5 

Pisaster ochraceus Seastar 0.2 ± 0.1 0 – 0.5 

Pisaster brevispinus Seastar 0.6 ± 0.3 0 – 1.6 

Pycnopodia helianthoides Seastar 0.3 ± 0.2 0 – 1.1 

Strongylocentrotus franciscanus Sea urchin 0.1 ± 0.1 0 – 0.5 

Parastichopus californicus Sea cucumber 0.9 ± 0.4 0 – 2.2 

Parastichopus parvimensis Sea cucumber 4.3 ± 1.7 1 – 10.4 

0 00

b. 2001

Habitat type

Mound under platform

Deep mound

Shallow mound

Deep soft bottom

Shallow soft bottom

M

e

an C

P

U

E

0

2

4

6

8

10

12

a. 2000

0

2

4

6

8

10

12

Kelletia delletii 

Asterina miniata 

Pisaster ochraceus 

P. giganteus 

0000

0000

n/a

00 00 0000

0 00 00 0000

b. 2001

Habitat type

Mound under platform

Deep mound

Shallow mound

Deep soft bottom

Shallow soft bottom

M

e

an C

P

U

E

0

2

4

6

8

10

12

a. 2000

0

2

4

6

8

10

12

Kelletia delletii 

Asterina miniata 

Pisaster ochraceus 

P. giganteus 

0000

0000

n/a

00 00 0000

00 0000
Final Study Report – Page and Dugan 

18

DISCUSSION 

Structure of shell mound communities 

Our results suggest that the structure of shell mound communities is strongly influenced by the 

presence of the platform structure, and the food subsidies provided by the clumps of mussels 

and associated organisms that continually slough from the platform to the seafloor (Fig. 3a). 

Predatory and omnivorous seastars (Pisaster spp., Asterina miniata), in particular, were much 

less abundant and of a smaller size at the shell mound-only sites. Mussels, the major 

component of this faunal litterfall (Wolfson et al. 1979, Page et al. 1999), are the primary prey 

of Pisaster spp. in rocky intertidal and subtidal habitats, where these seastars are the dominant 

predators (Landenberger 1969, Paine 1974). 

Pisaster spp. prey upon fallen mussels on the shell mounds under platforms (Simpson 1977, 

Wolfson et al. 1979, Bomkamp pers. obs.). The low density (Fig. 3) and small size (one 

individual of 10 cm arm radius) of Pisaster spp. at the mound-only sites suggests that these 

predators were food limited there. In addition, individuals of Pisaster were present in historic 

photographs of the mound-only sites prior to platform removal (Table 5). These seastars 

(particularly P. giganteus and P. ochraceus, Morris et al. 1980) have a strong preference for 

hard substrate habitat and are rarely found on soft bottom. Given that seastars would have to 

cross an expanse of >3 km of soft bottom to reach inshore hard bottom habitat, we hypothesize 

that nearly complete mortality of seastars may have occurred on the shallow mounds-only sites 

following platform removal in 1996. 

Individuals of the omnivorous seastar, Asterina miniata, were also more abundant and larger at 

the mounds under platform sites than at mound-only sites (Fig. 3a, Table 5). On the mound 

under Platform Gina, these seastars were observed feeding on barnacles fallen from the 

platform and aggregated around fallen mussel clumps, but at mound-only sites they were never 

observed with large food items (Page, pers. obs.). A. miniata prefer hard substrate habitat 

(Morris et al. 1980) and appear unlikely to cross the wide expanse of soft bottom to neighboring 

shell mounds or inshore to natural reefs. Our observation of the significantly lower density and 

smaller body size of A. miniata at mound-only compared to mound under platform sites 

suggests that the individual size and population density of these seastars respond to the presence 

of greater food availability under existing platforms. 

The gastropod, Cypraea spadicea, a carnivorous scavenger, was present in significantly higher 

densities on the mound under platform than mound-only sites (Fig. 3), also suggesting a 

response to the overlying platform structure. Although there was no difference in the size 

structure of C. spadicea populations among sites, tissue dry weight standardized for shell length 

was significantly greater for individuals from mounds under platform than mound-only sites 

(Fig. 5), suggesting better condition (e.g., more lipid storage, Mann 1978) and/or gonadal 

development of these individuals in the presence of faunal litterfall provided by platform 

structures. 

The large gastropod mollusk, Megathura crenulata, was absent from the shallow mound-only 

sites. It is not known whether the presence of the platform structure influences the distribution 

and abundance of M. crenulata because there is no evidence for the presence of this species at 
Role of Food Subsidies and Habitat Structure 

19 

mound-only sites prior to platform removal in the historical photographs we analyzed. 

However, on the mound under Platform Gina, individuals of this species were often associated 

with aggregations of seastars feeding on freshly deposited litterfall (Page, pers. obs.). 

Two species of gastropods, the carnivorous predators/scavengers, Conus californicus and 

Kelletia kelletii, were found in low densities (~0.01 to 0.1 individuals m-2) during transect 

sampling only at the mound-only sites. The diet of C. californicus and K kelletii is diverse 

(Kohn 1966, Rosenthal 1971), so the absence of these predatory species in samples from the 

shell mound under Platform Gina is puzzling. K. kelletii has been reported to be a major 

component of the prey of P. giganteus (Rosenthal 1971). Thus, one possible explanation for 

the absence of these gastropods on the mound under Gina is predation pressure by the abundant 

Pisaster. However, this possibility remains to be tested. 

In contrast to the pattern for predatory echinoderms, the relative abundance (as CPUE) of 

commercially important crabs, Cancer antennarius and C. anthonyi, did not differ among shell 

mound sites (Fig. 7). These crabs are predators and carnivorous scavengers (Morris et al. 1980) 

and likely consume components of faunal litterfall when available, but have greater mobility 

than other invertebrate taxa of shell mounds, and can forage over a larger area both on and off 

the mounds. For example, Page et al. (1999) reported that local fishermen captured a specimen 

of C. anthonyi, tagged at Platform Holly, 8 km from the platform. 

Populations of the macroinvertebrate detritivore, Parastichopus parvimensis, appeared to be 

little affected by the removal of the platform structure. This sea cucumber is a non-selective 

deposit feeder, ingesting organic matter and infaunal organisms within soft sediments (Yingst 

1974, Rogers-Bennett & Ono 2001). There were no differences in the density of P. parvimensis

among mound-only and mound under platform sites (Fig. 3a). Differences in population 

structure of this sea cucumber among sites (Fig. 4) were likely due to factors other than the food 

subsidy provided by the platform as the mean body length of animals from the mound under 

platform site was intermediate between those of the shell mound-only sites. 

 

Effects of the allochthonous food subsidy and platform structure on mobile epibenthic 

animals

We hypothesize that the food subsidy provided by the dislodged organisms (clumps of mussels 

and associated organisms) that fall from platform structures to the shell mounds exerts bottom 

up control upon populations of predatory and omnivorous sea stars. Bottom up effects may 

influence overall community structure by shifting the relative abundance of organisms at 

different trophic levels, and/or by altering the densities of particular strongly interacting species 

such as Pisaster spp. (e.g., Menge 1992, Bustamante et al. 1995, Sanford & Menge 2001). 

Other studies have reported unusually high densities of predatory echinoderms beneath oil 

platforms (Wolfson et al. 1979) and also suggested that food subsidies from platforms increase 

the density of some species immediately beneath and adjacent to the structure (Bascom et al. 

1976, Wolfson et al. 1979, Davis et al. 1982, Page et al. 1999). 

The platform structure could also affect the abundance and size structure of benthic organisms 

through the provision of recruitment habitat. For example, individuals of the crab, Cancer 
Final Study Report – Page and Dugan 

20

antennarius were hypothesized to move (or fall) from Platform Holly to the shell mound 

beneath the structure based on observations of juvenile crabs on the structure and the higher 

abundance of adult crabs on the shell mound than the structure (Page et al. 1999). However, 

some species sampled in this study on shell mounds (Asterina miniata, Cypraea spadicea, 

Parastichopus spp.) have not been sampled or observed on the structure of 7 existing platforms 

(including Gina) in the Santa Barbara Channel (Page et al., unpubl. data), suggesting that these 

species recruit preferentially to the hard benthos provided by the shell mound. 

Effects of benthic substrate composition 

There was no overlap in species composition of mobile macroinvertebrates between benthic 

communities on the shell mound under Platform Gina and the adjacent soft bottom locations 

(Fig. 3). Similarly, qualitative descriptions of the soft bottom near shell mound-only locations 

prior to platform removal mention the presence of sea pens, tube worms and anemones, all soft 

bottom species (Simpson 1977, de Wit 2001). Thus, our results also suggest that the presence 

or absence of hard substrate was an important factor in determining the distribution of shell 

mound associated taxa. The distribution of the two most abundant crab species in our study 

corresponded with known substrate preferences. Cancer antennarius, which prefer hard 

substrata (Winn 1985), occurred in significantly higher abundance at shell mound than soft 

bottom sites (Fig. 6). 

In contrast, C. anthonyi, which prefer sandy or muddy habitat or the interface between hard and 

soft substrata (Carroll and Winn 1989), showed a trend of higher abundance at the soft bottom 

sites (Fig. 6). Despite the potential for suitable habitat on the edge of the shell mound, C. 

anthonyi was not captured at mound under platform sites. This pattern of distribution of C. 

anthonyi differs from that reported in Page et al. (1999) where female C. anthonyi were found 

in high abundance on the shell mound under Platform Holly (~35 km west of these study sites) 

at the same time of year as in this study. However, depth may be important in accounting for 

this difference, as the bottom at Holly is ~66 m, compared with the deepest site (50 m) in the 

present study (Platform Houchin). Both Reilly (1987) and Page et al. (1999) suggested that C. 

anthonyi migrates seasonally between depths. In summary, our results suggest that highly 

mobile species, such as crabs, may respond more to the habitat structure provided by the shell 

mounds, and are less dependent on the food subsidy provided faunal litterfall. 

Temporal changes following platform removal 

The present study provided a unique opportunity to compare similar benthic habitats in the 

presence and absence of a platform structure. The relative effect of the platform on the benthic 

community appeared to vary among mound species in relation to trophic level, degree of 

mobility, and substrate preference. At the time of the current study, 5 to 6 years after platform 

removal, the mound-only sites appear to have lost most predatory and omnivorous sea stars, 

based on photographs of the shell mound under Platform Hazel and sampling data from 

Platform Gina. Those sea stars remaining on the shell-mound only sites were smaller than those 

on the mound under Platform Gina. Mobile crabs (e.g.., Cancer antennarius) and the sea 

cucumber, Parastichopus parvimensis, which prefer hard substrate, were seemingly less 
Role of Food Subsidies and Habitat Structure 

21 

affected, but given the estimated sedimentation rate at mound-only sites (Hazel and Hilda, 0.9 

to 1.4 mm year-1; Heidi and Hope, 1.5 to 2.0 mm year-1, de Wit 2001), these sites may no longer 

provide suitable habitat in a few years. Thus, while the presence of the platform structure may 

enhance secondary productivity in the benthic community, this effect may disappear rapidly (<5 

yr in this case) after the platform is removed. Although not an experimental manipulation, the 

“natural experiment” of platform removal investigated here strongly suggests that the “artificial 

reef” communities under offshore oil platforms may not be resilient to platform removal. 
Final Study Report – Page and Dugan 

22

ACKNOWLEDGEMENTS 

We thank J. Bram, C. Culver, E. Hessell, B. Mardian, C. Reger, A. Willis, and B. Wolcott for 

assistance in the field. B. Evans kindly provided shell mound photographs. J. Childress, R. 

Schmitt, B. Kinlan and A. Bull provided helpful comments on the manuscript. This research 

was made possible by a grant to H.M.P. and J.E.D. from the Minerals Management Service, US 

Department of the Interior, under MMS agreement No. 1435-01-00-CA-31063 and the Coastal 

Marine Institute, University of California, Santa Barbara. The views and conclusions contained 

in this document are those of the authors and should not be interpreted as necessarily 

representing the official policies either expressed or implied of the U.S. government. 
Role of Food Subsidies and Habitat Structure 

23 

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27 

The Department of the Interior Mission

As the Nation's principal conservation agency, the Department of the Interior has responsibility for most 

of our nationally owned public lands and natural resources. This includes fostering sound use of our 

land and water resources; protecting our fish, wildlife, and biological diversity; preserving the 

environmental and cultural values of our national parks and historical places; and providing for the 

enjoyment of life through outdoor recreation. The Department assesses our energy and mineral 

resources and works to ensure that their development is in the best interests of all our people by 

encouraging stewardship and citizen participation in their care. The Department also has a major 

responsibility for American Indian reservation communities and for people who live in island territories 

under U.S. administration. 

The Minerals Management Service Mission 

As a bureau of the Department of the Interior, the Minerals Management Service's (MMS) primary 

responsibilities are to manage the mineral resources located on the Nation's Outer Continental Shelf 

(OCS), collect revenue from the Federal OCS and onshore Federal and Indian lands, and distribute 

those revenues. 

Moreover, in working to meet its responsibilities, the Offshore Minerals Management Program

administers the OCS competitive leasing program and oversees the safe and environmentally sound 

exploration and production of our Nation's offshore natural gas, oil and other mineral resources. The 

MMS Royalty Management Program meets its responsibilities by ensuring the efficient, timely and 

accurate collection and disbursement of revenue from mineral leasing and production due to Indian 

tribes and allottees, States and the U.S. Treasury. 

The MMS strives to fulfill its responsibilities through the general guiding principles of: (1) being 

responsive to the public's concerns and interests by maintaining a dialogue with all potentially affected 

parties and (2) carrying out its programs with an emphasis on working to enhance the quality of life for 

all Americans by lending MMS assistance and expertise to economic development and environmental 

protection.