Association of sea turtles with petroleum platforms in the north-central Gulf of Mexico

Abstract

In 1988 there were over 4,500 petroleum platforms in the north-central Gulf of Mexico. Once a platform is no longer used for petroleum production, federal regulations require that the platform be removed. For platfform removals, explosives are commonly used to sever pilings that anchor the platform to the bottom. The use of explosives has the potential. to kill or injure marine animals, including sea turtles, in the vicinity of the platform. The five species of sea turtles which occur in the Gulf of Mexico are listed as either threatened or endangered under the provisions of the Endangered Species Act of 1973. The five species are as follows: loggerhead (Caretta caretta), leatherback (Dermochelys coriacea), green sea turtle (Chelonia mydas), Kemp's ridley (Lepidochelys kempi) and hawksbill (Eretmochelys imbricata).

Anecdotal evidence indicated that at least some individual sea turtles, primarily loggerheads, were commonly found in the vicinity of specific platforms. However, the general relationship of the sea turtle population to petroleum platforms was unknown. From June 1988 through June 1990, we used aerial suzveys to estimate sea turtle abundance and to study the spatial relationship between sea turtles sighted near the surface of the water and petroleum platforms. We surtteyed seven study areas which sampled the range of water depths (3–200 m) in the oil and gas fields offshore of Louisiana. For each study area we used three types of statistical procedures (Hanill and Wright's method, Kendall's rank correlation and chi-square analysis) to test the null hypothesis: Surfaced sea turtles were randomly located with respect to platform locations. We used line transect methods to estimate sea turtle density for each study area.

During the study, we sighted 316 chelonid sea turtles of which we estimated 92% were loggerheads. Most of the sea turtles (780) were sighted just northeast of the Mississippi River delta in two study areas offshore of Breton and Chandeleur Islands, Louisiana. Sea turtles were present throughout the year but fewer sea turtles were sighted during the coldest months (January and February). East of the river, sea turtle densities were seasonally variable, ranging from 0.92 sea turtles/100 km2 in winter to 4.83 sea turtles/100 km2 in spring. Because of the small number of sea turtles sighted in the five study areas west of the river, seasonal density estimates were not made. However, the annual densities in these areas ranged from 0.50 sea turtles/100 km2 in-13–48 m water depths to 0.11 sea turtles/100 km2 in 60–120 m water depths. Rather than leaving the north-central Gulf of Mexico in winter, we believe some sea turtles may have brumated or moved to slightly deeper water during cold weather- periods. We saw mud trails coming off some loggerheads. These mud trails indicate that they had been brumating by partially burying:in bottom sediments. West of the river, sightings of sea  turtles in deeper water areas increased slightly in winter. However, this was not observed east of the river. East of the river, all three statistical tests indicated that, except during winter, offshore of Chandeleur and Breton Islands, sea turtles were positively associated with platform locations (i.e., generally closer to platforms than expected). In winter, sea turtles were randomly located with respect to platform locations. In the study areas west of the river, sea turtles were randomly located with respect to platforms locations. Before the explosive removal of a platform can proceed, current mitigation measures require that sea turtles can be sighted within 1,000 m of the platform. mast of the river, based on the density of sea turtles (corrected for subsurface turtles) and the observed distance distribution of sea turtles to platforms, we estimated the probability of one or more chelonid sea turtles being within 1,000 m of any platform selected at random was great, generally more than 60%. West of the river, depending on the study area, we estimated that this probability ranged from 2–70.

We identified 18 petroleum platforms which may have had one or more positively associated chelonid sea turtles at some time during the study. To understand why sea turtles were associated with these 18 platforms, we compared them to other platforms using nine platform characterization variables. Overall, the platforms with associated sea turtles tended to be smaller unmanned platforms that were closer to shore than the other platforms. Offshore of Breton and Chandeleur Islands, we found chelonid sea turtles preferred more shallow water (generally <20 m) over sandy bottom sediments. West of the river, we did not detect a sea turtle preference for bottom sediments but most were in waters less than 50 m deep.