Toxicity of weathered Deepwater Horizon oil to bay anchovy (Anchoa mitchilli) embryos

The BP-contracted Deepwater Horizon Macondo well blowout occurred on 20 April 2010 and lasted nearly three months. The well released millions of barrels of crude oil into the northern Gulf of Mexico, causing extensive impacts on pelagic, benthic, and estuarine fish species. The bay anchovy (Anchoa mitchilli) is an important zooplanktivore in the Gulf, serving as an ecological link between lower trophic levels and pelagic predatory fish species. Bay anchovy spawn from May through November in shallow inshore and estuarine waters throughout the Gulf. Because their buoyant embryos are a dominant part of the inshore ichthyoplankton throughout the summer, it is likely bay anchovy embryos encountered oil in coastal estuaries during the summer and fall of 2010. Bay anchovy embryos were exposed to a range of concentrations of two field-collected Deepwater Horizon oils as high-energy and low-energy water accommodated fractions (HEWAFs and LEWAFs, respectively) for 48 h. The median lethal concentrations (LC50) were lower in exposures with the more weathered oil (HEWAF, 1.48 µg/L TPAH50; LEWAF, 1.58 µg/L TPAH50) compared to the less weathered oil (HEWAF, 3.87 µg/L TPAH50; LEWAF, 4.28 µg/L TPAH50). To measure delayed mortality and life stage sensitivity between embryos and larvae, an additional 24 h acute HEWAF exposure using the more weathered oil was run followed by a 24 h grow-out period. Here the LC50 was 9.71 µg/L TPAH50 after the grow-out phase, suggesting a toxic effect of oil at the embryonic or hatching stage. We also found that exposures prepared with the more weathered Slick B oil produced lower LC50 values compared to the exposures prepared with Slick A oil. Our results demonstrate that even relatively acute environmental exposure times can have a detrimental effect on bay anchovy embryos.