The Deepwater Horizon oil spill of 2010 occurred during the critical spawning season of the blue crab, Callinectes sapidus, a commercially and ecologically important species in the Gulf of Mexico. The eggs of the blue crab hatch into small planktonic larvae that spend weeks feeding in offshore waters. When they complete their larval phase, they move into shallower waters, settle to the bottom, and begin developing into juvenile crabs. Laboratory studies have shown that gulf oil and the dispersant used to accelerate the breakdown of the oil are highly toxic to blue crab larvae. It was therefore anticipated that the oil spill would result in massive mortality of crab larvae leading to a sharp reduction in the abundance of crabs the following year. However, instead of a decrease, the number of crabs in 2011 actually increased. There are multiple possible explanations for why blue crabs did not decline following the oil spill: relatively few larvae may have encountered oil or dispersant at toxic levels, larvae from other sources may have compensated for those lost as a result of the oil spill, or after being broken down by bacteria the oil entered the food web and provided more food for larvae and increased their survival. Another complicating factor is that the biggest accidental marine oil spill on record prompted one of the largest fishery closures on record. Release from both directed crab fishing and bycatch in shrimp trawls (which may exceed directed fishing) could have had such a large positive effect on blue crabs that it overwhelmed the negative effects of the oil spill. Although there is little direct evidence to support the latter explanation, it would explain why not only crabs, but many species of fish also increased in numbers after the oil spill. It also makes it difficult to say whether or not the oil spill did cause an increase in mortality for crab larvae, or what would have happened if the fisheries had not been closed. It is important to find answers to these questions if we are to understand and learn from what happened during the oil spill.
Before the oil spill, the two investigators on this proposal were already studying blue crabs in the northern Gulf of Mexico. They were collecting specimens and generating data on larval dispersal, rates of larval settlement, and population genetics. They continued this work through the oil spill of 2010 and afterwards, providing them with a unique opportunity to study the effects of the oil spill on blue crab larvae and address the questions raised in this proposal. They will conduct laboratory experiments to assess how crab larvae are affected by oil and dispersant at concentrations found during the spill. These experiments will allow them to identify the chemical signatures of oil exposure, determine the physiological effects of exposure under environmental conditions that match the Gulf of Mexico, and identify genes that are regulated in response to oil or dispersant exposure. They will test post-larvae that were collected from different sites along the coast during the oil spill to see if they have the chemical signatures of exposure. These results will be correlated with oceanographic models based on data from 2010 to reconstruct the movements of larvae through the Gulf of Mexico and determine whether the larvae that settled along the coast were exposed to oil while they were developing offshore. They will use population genetic methods to determine if larvae from more distant sources made up for larvae lost in the Gulf of Mexico.
This proposal addresses theme 3 of RFP-II in two ways. First, it proposes to investigate the environmental effects of the petroleum/dispersant system on organisms, in this case blue crabs. Second, by looking at the factors that may have prevented or compensated the expected catastrophic impact on blue crabs in the northern Gulf of Mexico, it addresses the science of ecosystem recovery. Results from this research will inform management and policy decisions regarding blue crab fisheries. Members of both the Neigel and Taylor labs will continue to present their findings to the Louisiana Blue Crab Task Force, which represents stakeholders, management agencies and academics. We will also continue to involve students through outreach programs for K-12 students (UNO-CERF) and minority students (SACNAS, LAMP) and through informal interactions such as mentoring students who enter science fairs.