and Results/Conclusions

Decommissioning oil production platforms is a complex process that can present considerable challenges. The fate of platform-associated hard-bottom structures (shell mounds) remaining following the decommissioning of four California offshore platforms was evaluated through the use of Habitat Equivalency Analysis (HEA). Developed by the National Oceanic and Atmospheric Administration (NOAA), HEA quantifies and compares the net environmental effects on affected habitats, including short-term and long-term effects of project alternatives and compensation measures.  Four alternatives evaluated for the shell mounds were ranked according to their relative net environmental impact on fish habitat value. Alternatives were: leave in place with enhancement of a nearby salt marsh, enhancing shell mounds with artificial reef structures, covering with a sand cap, and removal by dredging.

Shell mounds are hard-bottom structures composed of mollusk shells that fell from the platforms to the seabed covering the cuttings and fluids discharged during drilling operations. Previous studies have demonstrated that shell mounds provide a complex hard bottom habitat that supports a variety of fish and invertebrate species typical of a natural hard bottom community. To determine whether the shell mounds continue to provide high value hard-bottom habitat, field studies were conducted to characterize the ecological utilization by fish and invertebrates of the shell mounds and the surrounding soft bottom areas of the Santa Barbara Channel.  A multi-season fish trapping study using standard commercial fish traps is currently underway and the results of the first three deployments are presented and discussed.

The assemblage of fish and invertebrates observed in the fish traps from the mounds include several species of rockfish, ling cod, rock crabs, whelks and bat stars that are typical of natural hard bottom communities in the Santa Barbara Channel.  Results to date indicate that the shell mounds have more fish and invertebrates and a more diverse benthic community than the natural soft-bottom areas.  These results support previous studies that indicated the fish habitat value of the shell mounds is greater than that of the surrounding soft bottom habitat and supports the HEA results that leaving the mounds in place with enhancement of a nearby salt marsh is the environmentally superior alternative.