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Blood nitrogen uptake and distribution during diving in bottlenose dolphins

Key Questions

  • Are "the bends" (free nitrogen bubbles) involved in the deaths of beaked whales following acoustic exposure, and are they triggered equally by E&P industry sources and by naval sonar?
  • Are small cetaceans at risk for decompression sickness ("the bends") related to specific dive patterns?

Summary

This project was designed to investigate how bottlenose dolphins(Tursiops truncatus) avoid decompression sickness by measuring heart rate, lung volume, and blood nitrogen levels during and after diving.

Recent findings in beaked whales stranded after naval exercises have suggested that tissue nitrogen supersaturation, bubble formation, and a subsequent decompression sickness-like syndrome are the cause of beaked whale strandings associated with the use of naval sonar. In order to test this idea, a better understanding of cetacean diving physiology and, in particular, nitrogen uptake and distribution was required. Nitrogen levels and other health indicators were measured pre- and post-dive in a dolphin to investigate whether bottlenose dolphins would experience decompression sickness under certain dive patterns.

Results showed that the bottlenose dolphin heart rate declined from pre- and post-dive surface rates of 100-150 beats per min (bpm) to 20-40 bpm during dives. Analyses of blood samples revealed that blood nitrogen levels after dives were essentially indistinguishable from a dolphin at rest. This lack of significantly elevated nitrogen levels after a dive failed to support the hypothesis that nitrogen super-saturation is a mechanism of nitrogen bubble formation in cetaceans after acoustic exposure. This work was supported jointly by the Programme, Office of Naval Research, and NOAA Fisheries.

Objectives and methods

  • The project utilised a trained US Navy dolphin.
  • The dolphin was trained to wear a sensor harness during dives and present fluke for blood pre and post-dive.
  • Specific goals were to 1) document heart rate responses during diving activity, 2) evaluate a chest impedance meter as an index of lung volume during dives, and 3) measure blood N2 levels during and after dives.

Importance

This project demonstrates the Programme's engagement in resolving one of the most-pressing issues in the effects of underwater anthropogenic noise on animals. Because of the results of this project, the E&P industry need not include decompression sickness in its risk assessments until and unless strong new evidence emerges in support of the nitrogen super-saturation hypothesis.

Institutions/PIs

The Regents of University of California, Scripps (Paul Ponganis).

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