Proof of concept for platform recruited fish, phase 1: do platforms provide habitat for subadult red snapper?

Abstract

There are currently over 3,500 oil and gas structures in the northern Gulf of Mexico (Gulf). Platform operations and their prior drilling operations produce trace amounts of lead, barium, vanadium, and lanthanum residues that are leached into the surrounding waters and are deposited on the sea floor. These residues have isotopic ratios different from those typical of the Gulf seafloor and can be used as “markers” if they become incorporated into hard-parts, such as
otoliths, of fishes associated with oil and gas platforms. These markers are harmless because they occur at extremely low concentrations and are bound to the inedible otolith, or ear bone, of the fish. From 2002 to 2004, 115 red snapper were collected from oil and gas platforms and artificial reefs off Louisiana and Alabama. Otoliths were removed and analyzed from 98 of those collected using Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). The objective of this study was to determine if a trace element isotope ratio fingerprint could be detected and described as unique to red snapper inhabiting the platforms. Stepwise and canonical discriminant function analyses were used to compare red snapper otolith fingerprints from on and off platforms, and from east and west of the Mississippi River. Classification accuracies based on the probability of an individual fish being correctly classified into the habitat from which it was sampled were over 90% for each of the two main comparisons. When comparing the elemental composition of red snapper otoliths from Louisiana oil and gas structures and Louisiana artificial reefs, the classification accuracy was 93.75%. When comparing the elemental composition of red snapper otoliths from Louisiana artificial reefs and Alabama artificial reefs, the classification accuracy was 91.06%. Vanadium 51, Lead 206, Lead 207, and Lead 208 all appear to be linked with oil and gas structures or their prior drilling operations, as the concentrations of these four elements or isotopes were significantly higher in otoliths
sampled from platforms in Louisiana than in otoliths sampled from artificial reefs in either Louisiana or Alabama. These platform isotopic fingerprint results indicate that it may be possible to sub-sample from otoliths of adult fish in Phase II of our study to determine agespecific habitat affinity, and to determine if the new recruits now expanding into the eastern Gulf as the red snapper population rebuilds were associated with oil and gas platforms during some
portion of their early life. We reason that if oil and gas platforms provide high quality habitat and refuge from shrimp trawls for juveniles, then high numbers of adult recruits should be derived from the pool of individuals who utilize said habitat, particularly off Louisiana and other areas where natural habitat is scarce. By focusing on recruitment to adult populations in Phase II, our quantitative approach will provide a more direct assessment of the relative contribution of
different juvenile reef fish habitats than is possible via traditional habitat suitability approaches. Therefore it may be possible in future studies to determine if oil and gas platforms contribute disproportionately to the survival of juvenile and adult red snapper in other areas of the Gulf. If this contribution proves to be true, then platforms could be considered essential fish habitat and, therefore, a potentially viable management tool for stock rebuilding.