Original Story by Nilde Maggie Dannreuther and Stephanie Ellis
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Researchers conducted swim tests on Gulf of Mexico Cobia fish to investigate potential impacts from oil exposure. Compared to control fish, oil-exposed fish had a 36% lower stroke rate (volume of blood pumped per heartbeat) and an 18% higher resting heart rate. The increased heart rate may have helped to maintain cardiac output and compensate for decreased stroke volume. As exercise increased, oil-exposed Cobia experienced lower stroke volumes, likely from weaker heart contractions, compared to controls. As oil-exposed fish approached maximum swim speed, their heart rates did not increase as much as controls (36% vs 61% increase, respectively) and they experienced 12% lower oxygen consumption and metabolic rate than control fish. The maximum swim speed of oil-exposed cobia was 22% slower than control fish, which could affect their ability to capture food and escape predation.
The researchers published their findings in Comparative Biochemistry and Physiology Part C: Toxicology & Pharmacology: Cardio-respiratory function during exercise in the cobia, Rachycentron canadum: The impact of crude oil exposure.
The cardiorespiratory system supplies oxygen to meet the energetic demands of metabolically active tissues. For teleost fishes, the increased cardiac output (volume of blood per minute ejected by the heart) and associated oxygen delivery supports their swim performance. Cardiac output is the product of stroke volume and heart rate, but the role of these functions in driving cardiac output may vary between fish species.
Previous experiments suggested that crude oil exposure may reduce exercise performance in fish, probably a result of compromised cardiovascular function (Aquatic Toxicology Nelson et al., 2016; Environmental Science & Technology Xu et al., 2016; and several National Resource Damage Assessment or NRDA studies). However, in vivo experiments are required to confirm if cardiac impairments account for reduced maximal metabolic rate and swim performance. This study’s team tested the previous experiments’ hypothesis by simultaneously measuring cardiovascular performance, oxygen consumption, and swim performance in vivo in a pelagic teleost, cobia (Rachycentron canadum).
Using methods described in an earlier NRDA study, researchers conducted 24-hour exposure trials with cobia in seawater (control) and seawater containing high energy water-accommodated fractions (HEWAF) of oil at 20% dilution (preliminary studies using 10% HEWAF of oil and average polycyclic aromatic hydrocarbons or PAHs of 5.24 ± 4.5 ?g/L had no impact on measured parameters). The crude oil was highly-weathered slick oil collected at the Deepwater Horizon site in July 2010. Initial mean PAH concentration of the 20% HEWAF was 38.29 ± 4.84 ?g·l?1. After the 24 h period, PAHs were depleted by 90%, decreasing mean PAH concentration to 3.70 ± 1.24 ?g·l?1.
Oil-exposed fish relied more on stroke volume than heart rate to elevate cardiac output, while control fish relied more on heart rate. Homeostatic mechanisms such as increased heart rate may have offset the decreased stroke volume in oil-exposed fish; however, further research is needed to understand if and how these mechanisms may sustain cardiovascular function following oil exposure.
“Our findings suggest that the cobia cardiovascular system is affected by sublethal crude oil exposure but can still make adjustments to maintain the convective transport of oxygen,” said study author Dane Crossley. “However, cardiovascular performance is curtailed during swimming, which will impact the fish’s ability to capture food, escape from predators, and endure routine migration patterns. Given the possibility of future uncontrolled release of oil into the natural environments, the effects on animal populations must be identified to successfully mitigate the overall impact.”
Data are publicly available through the Gulf of Mexico Research Initiative Information and Data Cooperative (GRIIDC) at doi:10.7266/N7N877W8.
By Nilde Maggie Dannreuther and Stephanie Ellis. Contact email@example.com with questions or comments.
This research was made possible in part by a grant from the Gulf of Mexico Research Initiative (GoMRI) to the Relationship of Effects of Cardiac Outcomes in Fish for Validation of Ecological Risk (RECOVER)consortium.
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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