(This blog is part of a series of interviews with scientists who are championing marine research in the Gulf of Mexico.)
Shrimp are not just an integral part of the Gulf Coast’s culture and cuisine, but they are also a pillar of its economy. The impacts of the BP Deepwater Horizon oil disaster to this iconic animal are a great concern. Drs. Kim de Mutsert and Joris L. van der Ham of George Mason University study the oil’s effects on white and brown shrimp in the Gulf of Mexico. De Mutsert specializes in applied fish ecology, estuarine ecology and ecosystem modeling, including the effects of coastal restoration scenarios on fish, shrimp and oysters in Louisiana. Van der Ham, formerly a postdoctoral researcher at Louisiana State University, is an invertebrate zoologist who has investigated the effects of the BP disaster on inshore shrimp populations. We interviewed them about their research and what more needs to be done.
Ocean Conservancy: The two of you have researched the effects of the BP Deepwater Horizon oil disaster on white and brown shrimp in the Gulf of Mexico. Could you briefly summarize your findings for us?
Drs. Kim de Mutsert and Joris L. van der Ham: We compared the abundance and size of brown and white shrimp collected in estuaries that were heavily impacted by the spill with shrimp that were collected in estuaries that were minimally impacted by the spill. We found that both species of shrimp were more abundant in heavily impacted estuaries than in minimally impacted estuaries during the season following the spill. Size, however, did not differ between estuaries. This counterintuitive result could mean that the shrimp grew slower, which delayed the exodus of shrimp offshore to spawn. It could also simply be an effect of fishing closures, which were implemented after the oil spill in the more heavily impacted sites. Our study did not give us conclusive evidence on which of the two mechanisms was responsible.
OC: The oil disaster began around the start of white shrimp spawning season, so the impact would have been noticed quickly. Was there an effect on other species throughout the Gulf ecosystem via the food web? Additionally, were there noticeable changes in the shrimp fisheries in the Gulf?
DM & VdH: The increase in shrimp abundance will certainly have had an effect on the other species via the food web. If, however, this effect had any significant community-wide consequences remains unclear. Inshore shrimp fisheries were affected by the spill because of the fishing closures that were implemented immediately after the spill. Later, the media reported low catch rates in 2011, but we have not found any evidence of dramatically decreased abundances.
OC: Shrimp return to the deep-water in the Gulf – where they can be eaten by reef fish, such as snapper and grouper – to spawn. Could the oil found in shrimp affect those fish?
DM & VdH: There were many reports in 2011 and 2012 of shrimp with black branchial cavities (where their gills are located and through which water is circulated for respiration). If this is indeed oil, then that represents a possible source of toxicity for the fish that fed on those shrimp. It is not clear, however, how much this contributes to the compromised health of individual snapper and grouper compared to direct exposure to spilled oil offshore. In general though, vertebrate species are able to metabolize contaminants such as PAHs (polycyclic aromatic hydrocarbons) they ingest through food relatively quickly, so the direct exposure to oil (through the gills) is likely to play a larger role in higher trophic (feeding) level species [such as reef fish].
OC: Tell us why problems in the growth rate or distribution of shrimp should be of concern.
DM & VdH: It is important for shrimp to reach a large body size. Especially for females, the large body size ensures that the individual can store enough energetic reserves to produce a large number of offspring. Shrimp will start migrating offshore to mate and spawn when they reach a minimum threshold size. If their growth rate is not sufficiently fast, shrimp will delay their migration, and as a result a population may produce less offspring.
OC: Can you give us a sense of the extent to which the Gulf’s food web, in which shrimp play such an integral part, has been affected by oil?
DM & VdH: This is still hard to say, since organisms that have longer turnover times (lifespan) could suffer sublethal effects as a result of exposure to oil or reduced food availability that could lead to reductions in population size in the future. Examples of such sublethal effects are an impaired immune system or reduced fecundity [ability to reproduce].
OC: Was there a historical baseline established for the health of Gulf shrimp populations prior to the BP Deepwater Horizon oil disaster?
DM & VdH: Yes. The Louisiana Department of Wildlife and Fisheries (LDWF) has been conducting inshore monitoring studies that have included shrimp since the 1980s. In addition, the National Oceanic and Atmospheric Administration keeps track of fisheries landings data, including the Louisiana shrimp fisheries. Even though neither of these are direct measures of the health of shrimp populations, they do monitor the abundance of shrimp at inshore locations.
OC: What research is still needed? Where are the gaps in our knowledge?
DM & VdH: The sublethal effects of the oil spill on long-lived species are much harder to determine. The current state of the pollutants and the condition of higher trophic level organisms and how the oil affects their reproductive success could use continuous research. We also would have liked to carry out a study concurrent to our field study that focused on the physiological effects of oil and PAHs in shrimp, and the levels of these compounds in shrimp in the field. This would have helped us tease out cause-and-effect relationships that could provide invaluable information for future disasters. Unfortunately, we were unable to secure funding for those components.
OC: What recommendations can we make to our policymakers to restore and protect Gulf shrimp and their habitat?
DM & VdH: Keep track of shrimp abundance through monitoring projects such as those carried out by LDWF to determine every year when the start of the shrimp season should be and what the size of the stock is. A focus on the reduction of wetland loss is warranted in coastal restoration projects, as these estuaries are crucial to shrimp and other organisms that use these habitats. Because of the fast turnover time (lifespan) of shrimp, the population responds quickly to our actions. It is for example not unlikely that the fishing closures in response to the oil spill resulted in such a boost in shrimp abundance that negative effects of the spill were quickly resolved. Actions such as these can be applied in cases in which shrimp abundances decline for reasons that are either unknown or hard to solve.More from This Blog Series:
- The New Gulf of Mexico Disaster Imperative: Scientific Baselines and Long-term Monitoring
- Interview: Dr. John Incardona on Oil’s Heartbreaking Impact on Fish and What it Means for Gulf Restoration
- Interview: Dr. Blair Witherington on Oil’s Impact on Turtles in the Gulf of Mexico
- Interview: Dr. Bill Montevecchi on Oil and Dispersant Effects on Birds Wintering in the Gulf of Mexico
- Interview: Dr. Eric Hoffmayer on Tracking Whale Sharks in the Gulf of Mexico
- Interview: Dr. Paul Montagna on Deep-sea Impacts of the BP Oil Disaster
- Interview with Marine Mammal Expert Dr. Ruth H. Carmichael on the Stranding of Dolphins, Manatees and Whales
- Stay tuned for more from this series…