Comparison of fish communities associated with natural hard- bottom  areas and offshore oil and gas structures

Abstract

From the text: “The primary purposes of this study were : (1) to collect quantitative data for comparison of reef fish populations associated with natural hard bottom areas and offshore oil and gas structures and (2) to develop fish population sampling methods which can be applied in deep areas that exclude or limit direct observations. During Phase I, 25 sites were surveyed, described, and classified into shallow water (inshore of the 35-m isobath) and deep water (offshore of the 35-m isobath) hard bottom features and platforms. The shallow water hard bottom sites consisted of relatively small, low-relief, outcrop features normally covered by thick growths of ascidians, bryozoans, and hydroids. Atlantic spadefish, gray triggerfish, red snapper, sheepshead, and tomtate were present. The deep water hard bottom sites consisted of large high-relief features rising above a nepheloid layer into clear water. The tops of these features supported warm water species of corals, crustaceans, fishes, and sponges. The principal faunal characteristic distinguishing both deep water hard bottom and platform sites from shallow water sites was the occurrence of large numbers of tropical reef fishes at the deep water sites. Angelfishes, butterflyfishes, creole-fish, and creole wrasse were common members of the deep water areas while common shallow water species such as Atlantic spadefish and sheepshead were absent. During Phase III, the standing stocks of reef fishes were quantitatively assessed and compared at one hard bottom area (Sonnier Bank) and at four platforms . The distributions and abundances of fishes were highly variable.

Based on the types of fish species present and their relative abundances, the ichthyocommunity of Sonnier Bank was distinct from those of the platforms. At Sonnier Bank, virtually no fishes were in the water column at the surface and mid-water stations. Most species were directly associated with the hard bottom habitat. The bank was a diversified habitat area, ranging from a high-relief (31 m) rock outcrop at the peak to flat mud bottom at the outer stations. Fish populations varied from platform to platform . Offshore structures were responsible for concentrating fish populations vertically in the water column. There appeared to be a direct relationship between the abundance of small sedentary reef fishes and schooling species that remained near the structure and the amount of available platform habitat. Some hard bottom fishes utilized the relief offered by the platform as a ladder to expand their vertical migratory range. Quantitative comparisons of fish assemblages associated with hard bottom areas and offshore platforms were complicated by natural variability in the spatial and temporal distribution patterns of fishes. Certain species were closely associated with particular areas of platforms and occurred in patches, groups, or clusters. Other species exhibited a pronounced spatial relationship with depth and water clarity. If a slight water current was flowing past a platform, virtually the entire fish community was positioned up-current from the platform. The abundance and distribution of fishes at Jackaman's Hole changed radically over a period of one month and diurnal variation was noted in the behavior of several species. To evaluate the standing stock of fishes associated with hard bottom of platform areas using remote sampling equipment, such examples of spatial and temporal variations must be considered in the sampling methods.”