Study of the effect of oil and gas activities on reef fish populations in the Gulf of Mexico OCS area

Abstract

The study area was the northern Gulf of Mexico outer continental shelf (OCS) between 90° and 94°W longitude and the 18 and 200-m isobaths From the Executive Summary: “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.

Extensive OCS oil and gas activities have taken place near the natural reefs. 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. Naturally occurring gas seeps were frequently observed. Hard substrate was 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. Shallow water platforms were covered by an epibiotal assemblage numerically dominated by barnacles, bryozoans, hydroids, and encrusting sponges. Deep water platforms supported larger numbers of bivalves and octocorals than did shallow water platforms. Both shallow and deep water platforms supported large populations of Atlantic spadefish, blue runner, greater amberjack, red snapper, and sheepshead.

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 the Atlantic spadefish and sheepshead were absent.

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. The largest number of fishes was found at the platform in Vermilion Block 161, while the most species were encountered in Vermilion Block 164.

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 or platform areas using remote sampling equipment, such examples of spatial and temporal variations must be considered in the sampling methods. Numerous low-relief, shallow water (<35 m), hard bottom areas were identified during this study. These areas support substantial reef fish populations and are important to local commercial and sports fishing interests. None of these low-relief fishing banks are currently protected under oil and gas biological lease stipulations.

One previously unidentified high-relief feature (East Cameron Area, Block 293, Hard Bottom, 29 Fathom Place) was described during this study. This rock outcrop feature, rising from a depth of 57 m to within 30 m of the surface, supports extensive invertebrate and fish communities which are typical of those associated with previously described topographic highs. This feature is not presently shown on any bathymetric charts, nor is it included on the BLM, OCS topographic features list.

Based upon the results of this study, the following recommendations were made.

1) Further study of the newly identified, shallow water features and deep water, hard bottom feature should be considered to determine if they should be included under biological lease stipulations.

2) Further quantitative evaluations of the natural spatial and temporal variations in fish assemblages associated with hard bottom and platform areas are required before accurate impact assessments are possible.

3) Movements of tagged fishes before and after platform placement should be studied to determine if placement and presence of an offshore structure reduces reef fish populations on neighboring hard bottom areas.

4) Future remote censusing studies of deep water fish populations along the outer continental shelf or continental slope should consider selection of vehicle, camera, and observation technique. The manufacturer, the vehicle, the operator (pilot), and the manufacturer's support and maintenance capabilities must all be evaluated in the vehicle selection. Camera selection depends on both the objectives of the study and the physical conditions likely to be encountered at the study site. The selection of operating technique will depend upon the objectives of the survey, the environmental conditions anticipated at the study area (e.g., currents, relief, size of structure, and water clarity). Techniques range from those yielding only a qualitative species list to those yielding specific standing stock or biomass estimates.

Once these initial decisions have been made, an extensive period of shallow water (within scuba range) field testing should be undertaken to compare the remotely collected data to that of diver observations. When the investigators are satisfied that their remotely collected data either correspond to that of direct observation, or can be made to correspond through the use of specifically developed correction factors, the program may enter the deep water phase which would involve only remotely collected data. To evaluate ROV remote sensing methods for deep water fish community analysis, tests should be conducted to compare ROV and manned-submersible results.