HABITAT VALUE OF SHELL MOUNDS TO ECOLOGICALLY AND COMMERCIALLY IMPORTANT BENTHIC SPECIES

Principal Investigators: Mark Page (UCSB), Jenifer Dugan (UCSB) and

    James Childress (UCSB)

Summary of Research

Shell mounds form over time under offshore oil platforms as encrusting invertebrates, chiefly mussels, barnacles, and scallops, fall from the platform support surfaces and accumulate on the substratum.  These mounds provide habitat for a diverse invertebrate community that depends on organic input in the form of faunal litterfall from the overlying structure for nourishment.  When platforms are decommissioned and removed, the shell mounds remain, but faunal litterfall is no longer available as a food source for the shell mound community.  The fate of shell mounds following platform decommissioning and removal is controversial, because their habitat value is unknown. To assess habitat value of these mounds relative to shell mounds with existing platforms and to natural reefs, we compared the distribution and abundance of commercially important crab species (Cancer antennarius, C. anthonyi, C. productus, and Loxorhynchus grandis) and other invertebrate and fish taxa.  In addition we compared the size structure and growth rates of these organisms among habitats.  

Figure 1.  Illustration of the fall of mussels and other organisms from offshore oil platforms to the sea floor and the formation and topography of shell mounds before and after platform removal. 

Using shell mounds under existing platforms “Hogan,”  “Houchin” (Pacific Operators Offshore), and “Gina” (Nuevo Energy Company), and shell mounds at the sites of four decommissioned platforms “Hazel”, “Hilda”, “Hope”, and “Heidi” we:  (1) quantified the abundance and distribution of ecologically and commercially important benthic organisms on the shell mounds, (2) determined the population size structure of the most abundant taxa at each site, (3) used the growth rates of organisms to evaluate the habitat value of shell mounds.  The results of our research will potentially contribute to decisions regarding shell mound fate following platform decommissioning.

We investigated the distribution and abundance of benthic invertebrates using two techniques.  For commercially important crabs, baited commercial crab traps were deployed at each soft bottom, reef, shell mound, and platform location (excluding platform Gina).  Traps were lowered to the bottom at each sampling location and retrieved after a 24-hour soak time.  Captured crabs were counted, sexed, and carapace length (for majid crabs) or carapace width (for cancrid crabs) was measured.  Sampling was repeated once a month for 4 months beginning in September.  Our second technique used band transects to estimate the abundance of invertebrate taxa other than crabs on the two shallow shell mounds (Hazel and Hilda), on soft bottom adjacent to Hazel, Hilda, and Gina, and on the shell mound at platform Gina. Divers attached transect lines to a central point (the buoy chain at the shallow shell mounds, the research vessel’s anchor line on the soft bottom sites and a conductor pipe on platform Gina), and extend the lines out in a radial fashion; the result was a “wheel spoke” sampling regime.  The divers then swam the length of the transects and collected selected benthic macroinvertebrate taxa in a one meter swath.  In order to correct for potential over sampling of the area closest to the central point inherent in this sampling design, the transects were divided into 4 segments.  For analysis, data was weighted with regard to distance from the central point, e.g. the segments closest to the central point were weighted less than those farther away. 

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