Research examining fish assemblages associated with offshore petroleum platforms in the Santa Barbara Channel (SBC) have indicated an ecological importance of these structures; however, the platforms further south on the warm-temperate San Pedro Shelf (SPS) remain largely undescribed. Midwater fish surveys (depth < 31 m) were conducted at six of the seven petroleum platforms located on the SPS over a two-year period. All platforms shared a core species group of warm-temperate nearshore reef species including: California sheephead (Semicossyphus pulcher), blacksmith (Chromis punctipinnis), garibaldi (Hypsypops rubicundus), opaleye (Girella nigricans), and kelp bass (Paralabrax clathratus). Disparities between inshore (< 4.8 km from shore) and offshore (> 14 km from shore) platform communities occurred from a greater influence of pelagic baitfishes and observations of rockfish species ( Sebastes) offshore, while sea basses (Serranidae) and surfperch (Embiotocidae) species were predominant inshore. Minimal recruitment was observed at both the inshore and offshore platform communities, with the exception of rockfishes and blacksmith. Offshore fish community structure showed significant seasonal differences due to increases in pelagic baitfishes (i.e., jack mackerel ( Trachurus symmetricus) and Pacific sardine (Sardinops sagax )) and blacksmith recruits during warmer periods. This seasonal influence was mostly confined to the upper 20 m of the structure. Water depth and ambient water temperature had significant relationships with total fish density, total fish biomass, and species composition. Meanwhile, inshore platform communities exhibited seasonal fluctuations largely due to aggregations of barred sand bass (Paralabrax nebulifer). Inshore platform fish community structure (total fish density, species diversity, and species richness) showed significant relationships with water temperature, but no relationships with water depth.

Overall, the SPS platforms midwater fish assemblages were different from those described in the SBC, due to greater rockfish densities (> 25 times) and richness (> eight species) in the SBC. However, the SPS platform communities had increased densities of warm-temperate nearshore reef species as well as schooling pelagic species. Compared to adjacent natural and artificial habitats on the SPS, the reduction of the sand/reef ecotone habitat and vertical orientation of the platforms influenced differences in species composition. At the inshore platforms, increased densities of some recreationally targeted species, including kelp bass and barred sand bass, suggest the SPS platforms may be beneficial either through a reduction of harvesting pressure or greater productivity. Therefore, unlike the SBC platforms which were shown to be important habitat for juvenile rockfishes, the SPS platforms midwater sections appear important for adult nearshore reef fishes. Furthermore, evidence of reproduction by some species confirms these platform communities are producing larvae. However, partially removing the SPS offshore platforms (to a depth of 25 m) would eliminate critical habitat for several fish species and result in a potential loss of 95% of the total fish density and 77% of the total fish biomass, thus reducing the productivity advantages of these structures.