Fate and Potential Effects of the Deepwater Horizon Oil Spill on Commercial Penaeid Shrimp

Defense Date

6-2014

Document Type

Capstone

Degree Name

M.S. Marine Biology

Second Degree Name

M.S. Coastal Zone Management

First Advisor

Curt Rose

Second Advisor

Richard E. Spieler

Abstract

The major objective of this Capstone is to evaluate the potential and probable effects of the Deepwater Horizon oil spill and related remedial activities on brown (Farfantepenaeus aztecus) and white (Litopenaeus setiferus) shrimp in the northern Gulf of Mexico. This spill began on April 20, 2010, and continued for 87 days until the well was capped on July 15, 2010. Approximately 4.9 million barrels of oil were spilled, which is considered to be the largest accidental release of oil to the marine environment. The oil spill occurred at a water depth of approximately 1,500 m.

Brown and white shrimp support a valuable commercial fishery in the northern Gulf of Mexico, with a total value of $301 million in 2009 (the year before the oil spill). However, the spilled oil covered, on a cumulative basis, as much as 176,100 km2 (68,000 square miles) of water, causing the closure of as much as 35% of federal fishing grounds during June and early July.

The spawning grounds of the two species of shrimp are inshore of the origin of the oil spill, typically occurring at water depths of 18 – 137 m for brown shrimp and 7 – 55 m for white shrimp. However, the oil slick moved through these spawning areas and sometimes penetrated the estuaries, which serve as important nursery areas for postlarval. juvenile, and subadult shrimp.

In addition, the toxicity of the major chemical (Corexit 9500A) used to disperse oil in the water column is evaluated. It was concluded by the U. S Environmental Protection Agency (EPA) that Corexit 9500A tested alone was less toxic than oil tested alone, that when tested alone was less toxic than dispersant-oil mixtures, and that dispersant-oil mixtures were generally no more toxic than oil alone.

Several remedial activities were conducted by the State of Louisiana in an attempt to mitigate the effects of the Deepwater Horizon oil spill. The State opened gates along the levees of the Mississippi River to allow river water to flow into coastal marches, with the objective of forming a hydrodynamic barrier to the shoreward movement of oil. Low salinity of the river water has the potential to harm postlarval and juvenile brown shrimp, which are less tolerant of low salinity than white shrimp. However, the basic seasonal pattern of salinity in potentially impacted waters of Louisiana during most years before and after the oil spill cannot be distinguished from the pattern during the year of the spill.

The State also proposed construction of up to 163 km of sandy berms offshore of its barrier islands to block inshore movement of oil. However, it ultimately constructed only 26 km of berms because, in part, of environmental concerns expressed by the scientific community.

An evaluation of commercial shrimp landings (catches) in the northern Gulf of Mexico was performed as part of this Capstone. Total landings (all states combined) of brown shrimp tended to decrease during the years of 1990 – 2012, as reflected by a statistically significant coefficient of determination (R2) of 0.1912; whereas total catches of white shrimp increased over these years, with a highly significant R2 of 0.5369.

Total landings of brown shrimp during 2010 (the year of the oil spill) were 73 million pounds, which was the lowest annual amount reported during the 1990–2012 time period. This catch was also uncharacteristically low in comparison to the lower limit of the 95% confidence interval (CI) of 114.1 to 133.5 million pounds for the mean annual catch before the oil spill (1990 – 2009) of 123.8 million pounds. For white shrimp, total landings in 2010 were 92 million pounds, which were not characteristically low in the context of the lower limit of the 95% CI of 76.8 to 96.4 million pounds for the mean annual catch before the oil spill of 86.6 million pounds. In addition, the decrease in total catch of white shrimp in 2010 (-21.1%) was similar to the decrease that occurred in a previous year (-24.3% in 2007).

Commercial harvests of brown and white shrimp were also evaluated on a state-specific basis for the years just prior to and after the oil spill (2006 – 2012). This evaluation identified Texas as the state characterized by the least decrease in annual landings of brown shrimp from 2009 to 2010. In the case of white shrimp, although Louisiana and bordering states to the east (Mississippi and Alabama) experienced a decreased annual harvest from 2009 to 2010, Texas and Florida exhibited an increase in catch.

The material reviewed and developed in this Capstone leads to several conclusions regarding the effects of the Deepwater Horizon oil spill on penaeid shrimp. First, it is probable that eggs and larvae were the most vulnerable life stages of shrimp since these life stages inhabit offshore waters, which experienced the most continuous and widespread amounts of oil and its components. Other life stages of shrimp (late postlarvae, juveniles, subadults, and adults) typically inhabit the substrate of estuarine or marine waters, in which exposure to these materials would likely be intermittent.

Second, it appears that any hazard to early life stages of penaeid shrimp was primarily caused by toxicity of oil and its components, as contrasted to detergents (primarily Corexit 9500A). The effects of remedial responses to the oil spill by the State of Louisiana – release of fresh water from inland sources and construction of coastal barriers – are judged to be de minimis.

Finally, evaluation of commercial landings of penaeid shrimp in the northern Gulf of Mexico has the potential to further explore possible effects of the oil spill on shrimp, when interpreted in the context of various simplifying assumptions. This evaluation suggests that brown shrimp were more affected by the oil spill than white shrimp, and that this effect was more pronounced in Louisiana (and nearby states of Mississippi and Alabama) than Texas. Total recovery of the brown shrimp fishery was evident in 2011.

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