"Like most marine fishes, red snapper have a pelagic larval stage, during which they spend three to four weeks in the plankton exposed to many sources of mortality, including a gauntlet of predators, variable environmental conditions, patchy prey resources, and during the Deepwater Horizon oil spill, potential exposure to toxicity from oil and dispersants," Frank Hernandez Jr of the University of Southern Mississippi, US, told environmentalresearchweb. "Because relatively small changes in natural mortality rates encountered during larval stages can result in large changes in subsequent juvenile recruitment, we were interested in any additional impacts related to the Deepwater Horizon oil spill."
ERL." href="http://images.iop.org/objects/erw/news/12/1/36/pic1.jpg">Ichthyoplankton sampling stations
Red snapper are one of the most economically important reef fish in the Gulf of Mexico. They are fished commercially and for sport; the species spawns during the summer in the northern part of the gulf.
Data from a long-term plankton survey enabled Hernandez and colleagues to assess the period before the spill (2007–2009), what happened during the 2010 spill, and the after-effects in 2011 and 2013.
"By combining the plankton survey data with environmental data (e.g. freshwater discharge, wind speed and direction, and temperature) from local observing systems, we were able to examine larval abundance and body condition (a proxy for growth) related to both natural variability and the Deepwater Horizon oil spill," said Hernandez.
Red snapper larvae were in worse condition during and after the oil spill than they were beforehand, the results showed. Their abundance remained the same.
The team also found that, in general, the more freshwater discharging from Mobile Bay, the worse the condition of the red snapper larvae. "The mechanism for this relationship is unclear," said Hernandez. "It may be that high freshwater discharge onto the continental shelf may result in sub-optimal physiological conditions for larval red snapper, or perhaps a change in prey dynamics. This is something we hope to examine in a future study."
Even when the team removed the effect of environmental variables such as freshwater discharge, the larvae were still in worse condition during and after the spill. "While we cannot definitely say that the poor larval condition is directly related to the Deepwater Horizon oil spill, our findings do suggest that some combination of conditions related to this event negatively impacted larval red snapper condition," said Hernandez, who believes this suggests "we are missing a piece of this puzzle".
Human activities like the large-scale closure of fisheries during the oil spill may also have influenced larval condition.
Much previous research in this area has been in the lab, examining toxicological effects on egg and larval development. A few studies have used plankton samples from the field to estimate the overlap in time and space between the oil-spill footprint and the distribution of fish early life stages. "We believe ours is the first study to use field-collected larval fishes to examine Deepwater Horizon oil spill impacts on parameters that affect larval survival (condition, growth)," said Hernandez.
Hernandez believes the study highlights two important points – that a major challenge for the scientific community is teasing apart any impacts related to the oil spill versus the background noise in the ecosystem, and the importance of having baseline data.
"We were fortunate to have data from a long-term plankton survey, which provided valuable information on larval fish abundance and distribution, as well as the larval fish specimens to measure and analyze for condition metrics," he said. "These datasets are relatively rare in the Gulf of Mexico, as few agencies are willing to fund and sustain monitoring programmes. It is near impossible to estimate environmental impacts, direct restoration efforts, and attempt to make the ecosystem 'whole' again if there are no prior baseline conditions for guidance."
Now the researchers would like to combine their red snapper findings with their results for spanish mackerel and king mackerel larvae, as well as looking at other possible causes for the decreased red snapper condition, such as prey availability and larval diets."
"Also, we are interested in seeing if the observed negative relationship between larval red snapper condition and river discharge holds up on a larger, Gulf-wide scale, therefore we hope to conduct a similar analysis using data from a larger Gulf of Mexico plankton survey," said Hernandez, who would also like to integrate the larval red snapper findings with those of researchers looking at Deepwater Horizon impacts on juvenile and adult fish. "A synthesis that includes all life history stages will provide a more powerful analysis of potential Deepwater Horizon oil spill impacts on the red snapper population and fishery as a whole," he said.
Hernandez and colleagues reported their findings in Environmental Research Letters (ERL).
Related links
- Condition of larval red snapper (Lutjanus campechanus) relative to environmental variability and the Deepwater Horizon oil spill F J Hernandez Jr et al 2016 Environ. Res. Lett. 11 094019
- ERL
- Frank Hernandez
- ERL Focus on the Deepwater Horizon Oil Spill