Effects of depth, location, and habitat type on relative abundance and species composition of fishes associated with petroleum platforms and Sonnier Bank in the northern Gulf of Mexico

Abstract

From the Executive Summary: “Petroleum platforms differ from traditional artificial reefs in that their vertical profile extends throughout the water column and provides habitat from the photic zone to the substrate. Historically, a major obstacle in deriving scientific data about the effect of deep water artificial reefs (and platforms) on fishes and other nekton was the difficulty in quantifying abundance and species composition associated with these reef communities. We have successfully used two complementary techniques, hydroacoustics and point count visual surveys, to estimate the abundance (biomass) and species composition of fishes at several of these structures. In these studies dual-beam hydroacoustics provided estimates of acoustic biomass, density, size distribution, and the near field area of influence. A remotely operated underwater vehicle (ROV) was used to determine species composition.

We have found from 10,000 to 30,000 fishes associated with individual platforms; the lowest numbers were found at the largest and deepest structures, leading us to question the value of deep water structures as artificial reefs. We have also reported that density of fishes around platforms was ten times greater than open water and from two to three times greater than fish density associated with the upper portion of the West Flower Garden Bank off eastern Texas; species composition also differed between these two habitat types.

The purpose of this project was to extend our research to address the effect that depth and latitude have on the fish community associated with platforms and to compare the fish community at a natural hard bottom (Sonnier Bank) to that of a nearby standing platform. The test of these hypotheses is critical both to future Minerals Management Service policy and to platform dependent artificial reef programs. We proposed to describe the fish communities at several platforms along a north-south transect twice a year for each of three years, sampling along a new transect each year. Transects were located east of the Mississippi River in year one, in the central Gulf of Mexico off the Louisiana coast in year two, and the western Gulf of Mexico off the coast of Texas in year three. A contract modification allowed us to add an additional survey of the Sonnier Bank features to provide for comparison of the fish communityat platforms to that of nearby natural substrate.

Across the three regions (east, central and west), we observed a significant difference in fish biomass. Fish biomass per unit area was highest in the central region and lowest along both the eastern and western regions. Fish biomass at platforms in the central region were also dominated planktivores that reside high in the water column. We speculate that increased nutrients from the Mississippi river may allow for increased primary and secondary productivity in the central region. Species richness was higher in the central and western GOM compared to the eastern GOM.

Average acoustic fish size also varied significantly across regions and was again highest in the central region and lowest in the western region (Figure 12a, 13). Targets along the western region averaged 30% smaller than those of the central or eastern regions. Targets along the eastern region were also slightly smaller than target size along the central region. A comparison of the high-relief pinnacles at Sonnier Bank and the standing platform VR279 again demonstrated that based on our acoustic fish biomass estimates, fish biomass, abundance and size were all substantially greater around the standing platform, particularly in the upper water column. Fish biomass over the low-relief feature at Sonnier Bank showed a greater fish biomass compared to the pinnacles

This research continues to support the premise that standing oil and gas platforms make useful artificial reefs because they support fish densities that can be from 10 to 1000 times greater than the densities found over adjacent sand and mud bottom habitats. Also, fish densities at standing platforms almost always exceed those found both at artificial reefs (both partially removed and toppled) and at natural habitats such as Sonnier Bank. However, the fish species associated with artificial reefs (including standing platforms) differ from those found on natural habitats. Future research efforts might be directed toward determining the reasons for this difference.”