Deadly acute Decompression Sickness in Risso's dolphins

Area of Science:

• Marine Biology
• Veterinary Pathology
• Diving Physiology

Background:

• Air-breathing diving vertebrates were presumed immune to decompression sickness (DCS).
• Recent findings link DCS in marine mammals (beaked whales, sea turtles) to anthropogenic activities like sonar and bycatch.
• This study investigates DCS in cetaceans stranded in the Canary Islands.

Purpose of the Study:

• To determine the cause of death in two Risso's dolphins exhibiting signs of severe acute decompression sickness.
• To differentiate between anthropogenic causes and natural events leading to DCS in marine mammals.

Main Methods:

• Necropsy of 493 cetaceans stranded in the Canary Islands (2000-2015).
• Pathological examination and gas analysis of tissues from two Risso's dolphins.
• Exclusion of common causes of mortality including disease, trauma, and human interactions (ship collision, sonar, fisheries).

Main Results:

• Two Risso's dolphins (Grampus griseus) showed pathological evidence of severe acute decompression sickness.
• Gas analysis confirmed nitrogen bubbles consistent with DCS.
• Other lethal causes, including anthropogenic factors, were systematically ruled out.

Conclusions:

• Struggling with prey, specifically squid, during a deep hunting dive is identified as the most probable cause of DCS in these Risso's dolphins.
• This finding suggests that physiological stress during deep dives, rather than solely external factors, can precipitate DCS in marine mammals.