Trophic links and condition of a temperate reef fish: comparisons among offshore oil platforms and natural reef habitats: final technical summary final study report

Abstract

This study was conducted in the Santa Barbara Channel at two offshore oil and natural gas platforms (Holly, Houchin) and two inshore natural rocky reef outcrops (Naples,Mohawk).From Study Results: “Amphipod crustaceans comprised the highest proportion of the small invertebrate assemblage at all sites. There were significant differences in the assemblages of small invertebrates among sites. Variation in proportions of gammarid and caprellid amphipods were the two most important contributors to the dissimilarity in all pairwise comparisons between sites. Differences were more apparent among sites when the amphipod assemblages were compared at a finer scale of taxonomic resolution. Amphipod assemblages at the platforms differed markedly from natural reefs and were characterized by a high proportion (>50%) of exotic species. Amphipod assemblages from the two platforms were more similar to each other than to the two natural reefs. The exotic caprellid amphipod, C. mutica, and the exotic gammarid, Erichthonius brasiliensis, contributed most to the dissimilarity of amphipod assemblages between platforms and natural reefs.

Densities of gammarid amphipods were approximately 100x higher at Platform Holly compared with Mohawk Reef, but similar between Platform Houchin and Naples Reef. Densities of caprellid amphipods were also often 10 to 100x higher and less variable over time at Platform Holly compared with the other sites. Densities of C. mutica on the two platforms were much higher, several hundred individuals per 400 cm2, than densities of native caprellid species either on the platforms or natural reefs.

Diet of painted greenling was similar across platform and natural reef sites with amphipod crustaceans comprising from 89 to 98% by number of the stomach contents. There were significant differences in several pairwise comparisons between sites in the relative proportion of prey types in fish stomachs in September and April. However, a consistent pattern related tohabitat (platform vs. natural reef) was not evident. From 29 to 38% of the caprellid amphipods in the stomachs of fish were identifiable to species.

The exotic Caprella mutica comprised from 31 to 52% of these identified individuals at Platforms Holly and Houchin. As expected, C. mutica was not found in the stomachs of painted greenling from the natural reefs. In September, painted greenling of a given size weighed significantly more at length at Platform Holly compared with fish from Platform Houchin and Naples and Mohawk Reefs. In April, painted greenling weighed more at length at the two platforms and Naples Reef compared to Mohawk Reef. Fulton's condition factor (K) reflected the significant differences in body length and weight relationships observed among sites. In September, values of K were higher at Platform Holly compared with the other sites. In April, values of K were lower at Mohawk Reef compared with the other sites.

There was a significant positive relationship between K and amphipod biomass for both the September and April samples. We calculated amphipod prey availability per capita by dividing the biomass values of amphipods by the density of painted greenling at each site for each sampling period. There was also were significant positive relationships between K and per capita amphipod biomass for both September and April.”