Vulnerability of Larval Fish Populations to Oil Well Blowouts in the Northern Gulf of Mexico

On April 20th, 2010, a fire broke out on the Deepwater Horizon (DWH) offshore oil drilling rig on the Macondo prospect located off the coast of the Gulf of Mexico (GOM). This fire and resulting explosions resulted in the sinking of the Deepwater Horizon rig and the largest marine oil spill in history. An estimated 4.9 million (+/- 10%) barrels were released into the Gulf of Mexico over the ensuing 87 days. Many economically important fish species spawn in northern Gulf of Mexico waters; the spawning seasons and pelagic larval phase of many of these species occur within the temporal extent of the DWH oil spill (April through July). Fish eggs and larvae in waters exposed to Macondo oil likely experienced lethal and sublethal physiological effects, leading to potential losses in year class strength depending on the proportion of a population’s larval production encountering oil. Differing spatial distributions of larvae due to different spawning locations and seasons could predict that some species were disproportionately affected by the DWH oil spill. This study aims at quantifying the impact of the spill across numerous taxa, by estimating the proportion of species-, genus-, or family-specific fish larval abundances located within the spatial/temporal domain of the DWH spill until the Macondo well was capped. Estimates and related uncertainties were based on empirical ichthyoplankton data collected over 27 years and on observed and modeled distribution of surface oil slicks and concentrations. In addition, two hypothetical oil spills were simulated on the Western Florida Slope and on the Western interior of the Gulf of Mexico to assess the impact of oil spills to the ichthyoplankton from different offshore locations that would have occurred during the DWH spill period. Ichthyoplankton data collected during annual plankton surveys (and other resource surveys) as part of the Southeast Area Monitoring and Assessment Program – Gulf of Mexico (SEAMAP) during years 1982-2009 were used to describe the composition and distribution of ichthyoplankton in the northern Gulf of Mexico. The SEAMAP larval fish data along with oil surface distributions (both actual and simulated) were used to estimate the proportions of larvae of 115 selected fish taxa that were potentially exposed to DWH oil. Bootstrapping methods were applied to the SEAMAP data to quantify the variability. Proportions of larval fish potentially exposed to oil were calculated for four oil spill scenarios: (1) the DWH spill, (2) an September-December oil spill with the same spatial footprint as the DWH spill but occurring later in the year, (3) a west Florida Slope spill occurring during the months of April-July and centered at 27˚N, 85˚W, and (4) a western GOM spill occurring during the spring and centered at 27˚N, 93.5˚W. Spill scenarios (3) and (4) were modeled using the Connectivity Modeling System (CMS). The CMS is a Lagrangian model which predicts oil droplet distribution and degradation based on oil properties and ocean currents. Significant differences in the proportion of larvae potentially exposed were found in the DWH spill and the three simulations. The proportion of fish larvae exposed to the DWH spill varied between 0% (many species) and 26.8% (Cynoscion nebulosus). The proportion of fish larvae exposed to simulated spills varied between 0% and 78.9% (Bonapartia pedaliota in Western GOM spill). Both the west Florida Slope oil spill and the western GOM spills had a disproportionally greater impact on a larger number of taxa than the DWH spill, even after correcting for their larger spatial extent. For the DWH spill (Scenario 1), the potentially most impacted taxa were Cynoscion nebulosus, Engraulidae, Rachycentron canadum, and Etropus spp. If the DWH spill had occurred in the fall (Scenario 2), the most potentially impacted taxa would have been Leiostomus xanthurus, Elopidae, and Pomatomus saltatrix. For a west Florida slope exposure (Scenario 3), the potentially most impacted taxa would be Holocentridae, Acanthocybium solandri, Coryphaena spp., and Pomacanthidae. For a western GOM spill (Scenario 4), the most potentially impacted taxa would be Bonapartia pedaliota, Thunnus thynnus, Caranx spp., and Holocentridae.. The historical SEAMAP data set combined with the CMS modeling tool provides a powerful planning tool to understand the potential impacts of oil spills in the northern GOM and the relative sensitivity of locations in the Northern GOM to oil spill effects