Overlap between Atlantic bluefin tuna spawning grounds and observed Deepwater Horizon surface oil in the northern Gulf of Mexico

Abstract

The 2010 Deepwater Horizon oil spill impacted the northern Gulf of Mexico (GOM) during the spring spawning season of Atlantic bluefin tuna (BFT). Overlap between BFT spawning habitat and surface oil in the northern GOM was examined using satellite-derived estimates of oil coverage, and spawning habitat models. Results suggested that although eggs and larvae were likely impacted by oil-contaminated waters in the eastern GOM, high abundances of larvae were located elsewhere, especially in the western GOM. Overall, less than 10% of BFT spawning habitat was predicted to have been covered by surface oil, and less than 12% of larval BFT were predicted to have been located within contaminated waters in the northern GOM, on a weekly basis. Our results provide preliminary but important initial estimates of the effects of the spill on larval BFT mortality, as concern continues over the appropriate management responses to impacts of the spill.

Introduction

On April 20th, 2010, the Deepwater Horizon oil rig exploded while drilling on the Macondo prospect in the Gulf of Mexico (GOM). During this incident, the wellhead was damaged, and oil began leaking into the water column. Initial efforts to seal the well were unsuccessful until achieved in September 2010, by which time an estimated 4.9 million barrels of oil were discharged into the ocean environment (Camilli et al., 2010, Crone and Tolstoy, 2010).

The GOM marine and coastal environment is biologically diverse, and supports a variety of commercial and recreational fisheries. Many species use the GOM for spawning or nursery habitat, and there has been considerable concern regarding the impact of the spilled oil, petroleum byproducts and dispersants on marine life. Particular attention has been focused on the Atlantic bluefin tuna (BFT), a large, highly migratory species that spawns in the GOM during spring (Richards, 1976, Richards, 2010, Scott et al., 1993). Despite the pan-Atlantic range of BFT, the GOM is the only confirmed, major spawning location for the western Atlantic stock (Richards, 1976, Scott et al., 1993). Spawning occurs predominantly from April to June, with a peak in May (Muhling et al., 2010). Due to their high value as a food fish, BFT have been heavily exploited historically, and they are currently considered to be undergoing overfishing (McAllister and Caruthers, 2007). In addition, recent attention was given to the possibility of listing BFT under the CITES convention, which would prohibit international trade in the species. To accurately assess whether this action is justified, and to evaluate the effects of the oil spill on stock recruitment, information on the likely effects of the oil spill on the early life history stages of BFT in the GOM is required.

Both crude oil and weathered oil byproducts are highly toxic to fish eggs and larvae (Incardona et al., 2004). Oil contamination may cause increased mortality of eggs and larvae even at low concentrations (Carls, 1987, McGurk and Brown, 1996), and the addition of dispersants is likely to increase this effect (Couillard et al., 2005). Exposure to oil and oil byproducts also leads to a range of sub-lethal effects on fish eggs and larvae, including premature hatching (Carls et al., 1999), morphological malformations (Hose et al., 1996, Norcross et al., 1996) and genetic damage (Norcross et al., 1996). Low levels of dissolved oil hydrocarbons may also slow larval growth rates, and affect swimming and feeding behaviors (Tilseth et al., 1984). Mortality rates on malformed, premature or slow-growing larvae are likely to be extremely high (Carls et al., 1999, Rice et al., 1993).

In this study, the overlap between predicted BFT spawning habitat and surface oil in the northern GOM was estimated using satellite data for April and May, 2010. Spawning habitat was defined using a habitat model trained with archival data collected from previous spring ichthyoplankton cruises, and surface oil extent was estimated using a suite of satellite products. In addition, a preliminary product examining potentially contaminated water was developed. Overlap between spawning habitat and both surface oil, and potentially contaminated water, was quantified on a weekly basis between 19th April and 30th May, 2010.