The role of bottom crossbeam complexity in influencing the fish assemblages at California oil and gas platforms.

Abstract

Between 1995 and 2002, we surveyed fish assemblages at seven oil platforms off southern and central California using the manned research submersible Delta. At each platform, there is a large horizontal beam situated at or near the sea floor. In some instances, shells and sedi­ment have buried this beam and in other instances it is partially or com­pletely exposed. We found that fish species responded in various ways to the amount of exposure of the beam. A few species, such as blackeye goby (Rhinogobiops nicholsii), greenstriped rockfish (Sebastes elongatus), and pink seaperch (Zalembius rosaceus) tended to avoid the beam. However, many species that typically associate with natural rocky outcrops, such as bocac­cio (S. paucispinis), cowcod (S. levis), copper (S. caurinus), greenblotched

(S. rosenblatti), pinkrose (S. simula­tor) and vermilion (S. miniatus) rock­fishes, were found most often where the beam was exposed. In particu­lar, a group of species (e.g., bocaccio, cowcod, blue (Sebastes mystinus), and vermilion rockfishes) called here the “sheltering habitat” guild, lived pri­marily where the beam was exposed and formed a crevice. This work dem­onstrates that the presence of shelter­ing sites is important in determining the species composition of man-made reefs and, likely, natural reefs. This research also indicates that adding structures that form sheltering sites in and around decommissioned plat­forms will likely lead to higher densi­ties of many species typical of hard and complex structure.