Researchers used population models to investigate how reduced survival and fertility after environmental disturbances, such as an oil spill, might affect sperm whale populations. Model simulations indicated that the magnitude of a disturbance had a stronger impact on recovery from lethal and sublethal effects than its duration. When comparing lethal and sublethal effects of the same duration, reduced survival influenced a population’s probability of recovery more than reduced fertility, even when reproduction stopped completely for the full duration of a disturbance. The researchers published their findings in Ecotoxicology: Analysis of lethal and sublethal impacts of environmental disasters on sperm whales using stochastic modeling.
Sperm whales in the Gulf of Mexico are distinct from those in the Atlantic Ocean because females and juveniles stay in the Gulf, they are smaller in size and have fewer numbers, and they have genetic differences. Because the Gulf of Mexico sperm whale population is small, closed, and slow-growing, disturbances such as the Deepwater Horizon incident may affect population viability. It is unknown to what extent and how long these sperm whales were exposed to spill toxicants, but acoustic studies conducted in 2007 – 2010 confirm that the whales were present in impacted areas during the incident.
Given the lack of data on vital rates and toxicant effects on these whales, researchers developed a mathematical model based on previously determined survival probabilities (Chiquet, et.al., 2013) to examine scenarios of sperm whale populations’ recovery from disturbances. The team also incorporated demographic stochasticity (population growth variation at the individual level) into the model using the simulation process described in Caswell, 2001.
Lead study author Azmy S. Ackleh stated, “Using mathematical modeling to study the recovery time of a population after a disaster is a rather new concept. This alternative approach of utilizing mathematical models can help us understand what might happen to population trends should disturbances of different types and magnitudes occur.”
Model simulations indicated that, when fertility is reduced by 70%, the population faced the danger of not being able to recover to pre-disturbance levels within 50 years when reduced fertility lasted for 26 or more years. However, if fertility is reduced for 10 years, the population always showed recovery within 100 years even when no reproduction occurred during that time, which corresponds to patterns for other long-lived species. This is in sharp contrast to how reduced survivability affected recovery: when survival was reduced by 6% for 10 years, the probability for population recovery in 100 years was 19%. When the disturbance lasted for 10 years, the survival reduction had to be less than 11% for the population to recover within 150 years. If survival was reduced by more than 11%, the probability of full recovery within 150 years was nearly zero. If a disturbance lasted for a long period of time, a small change in the reduction of survival rates due to this disturbance resulted in a large change in population dynamics.
This study assumed that no other disaster or detrimental event to a population occurred within the time of recovery. Doing so brings up questions about population sustainability if a second disaster happens while recovering from a previous event. The authors recommend that future studies address this scenario.
“The methodology developed in this study is general enough and can be applied to many other species, not just sperm whales,” said Ackleh. “Our approach could be used to shed light on how variety of populations were impacted by the Deepwater Horizon incident.”
Data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at doi:10.7266/N7ZK5DQ7 and doi:10.7266/N7TT4P1C.
This research was made possible in part by a grant from the Gulf of Mexico Research Initiative (GoMRI) to the Littoral Acoustic Demonstration Center – Gulf Ecological Monitoring and Modeling (LADC-GEMM) consortium. Other funding sources included ERC Advanced Grant 322989.
The Gulf of Mexico Research Initiative (GoMRI) is a 10-year independent research program established to study the effect, and the potential associated impact, of hydrocarbon releases on the environment and public health, as well as to develop improved spill mitigation, oil detection, characterization and remediation technologies. An independent and academic 20-member Research Board makes the funding and research direction decisions to ensure the intellectual quality, effectiveness and academic independence of the GoMRI research. All research data, findings and publications will be made publicly available. The program was established through a $500 million financial commitment from BP. For more information, visit http://gulfresearchinitiative.org/.
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