PhD Thesis defense by Daniel García Párraga at the VISAVET Centre of the Complutense University of Madrid

November 17^th, 2021

Decompression sickness (DCS), clinically diagnosed by reversal of symptoms after recompression treatment, has never been reported in breath hold divers, despite the predictable occurrence in their tissues when nitrogen stresses high enough to lead to bubble formation and lesions based on studies in terrestrial experimental models. In marine mammals, lesions compatible with DCS were described in post mortem studies, in cases where altered diving behavior is assumed because of exposure to high-powered underwater acoustic sources (e.g., active military sonar) and in cases of animals accidentally caught at depth in fisheries. In sea turtles, despite the abundant literature on diving physiology and work on the disruption caused to animals by accidental capture, this is the first report of gas embolism (GE) and DCS in this group. This Thesis is the most comprehensive compilation to date of the disease in any diving vertebrate: it provides evidence of its existence including associated symptoms and lesions, describes diagnostic and therapeutic options, and provides preliminary data on the impact of the disease in the clinical setting of a rehabilitation center as well as under field conditions on board fishing vessels. The study compiles for the first-time information on this new clinic-pathological entity based on more than 300 affected loggerhead turtles (Caretta caretta) as well as isolated specimens of green turtle (Chelonia mydas), olive ridley turtle (Lepidochelys olivacea) and leatherback turtle (Dermochelys coriacea). The work is based on the study of a total of 497 cases recovered from fishing trawls and gillnets in the Valencian Community, as well as in trawlers in Brazil and Italy, studying the presence of the disease and its impact on the animals both on arrival at the rehabilitation center and in its early phase while on board. The severity of the embolism, as well as the characterization of the associated lesions in vital organs have been assessed in the different studies by conventional radiography, computed axial tomography, magnetic resonance imaging, ultrasound, and post mortem examination. In the latter, gas embolism has also been confirmed and characterized in a manner very similar to that described in cetaceans suspected of decompression-associated GE. The incidence of GE was 57% on arrival at the rehabilitation center and 100% immediately after bottom trawl capture. We provide evidence that environmental factors such as water temperature, depth and net setting times correlate with the mortality of turtles caught accidentally. Likewise, the type of fishery (trawl/gillnet), the presence of seawater aspiration, the severity of the embolism, or the level of consciousness in the first neurological examination after recovery from nets on board, also condition survival. On the other hand, the severity of the embolism suffered in animals received at the rehabilitation center was associated with the type of fishery, the depth of the gear, the presence of seawater aspiration and the size of the turtle (CCL and weight). At the rehabilitation center, the overall mortality (including those admitted dead) associated with embolism cases (28.91%) was almost 8 times higher than mortality in the non-embolism group (3.76%). However, the survival rate of affected animals that arrived alive and could be treated by specific hyperbaric oxygen therapy (HBOT) exceeded 90%. In surveys on board fishing vessels, total 30-day mortality ranged from 44% to 63%. Survival of animals in good condition that could be tagged and released back (excluding those that died on board) was 63.88%, which would imply 1 in 3 direct reintroductions.

The discovery of DCS in sea turtles opens a new era for research in diving physiology not only in turtles but also in other diving vertebrates, raising new hypotheses or models potentially applicable to other groups of pulmonates including humans and other diving marine vertebrates. In addition, it calls for reviewing and rethinking new strategies to mitigate the mortality associated in the short, medium, and long term with bycatch in order to mitigate the impact of fisheries on turtle populations, favoring their conservation.

The author preparing a turtle days after its recovery from the decompression accident for its exploration in Nuclear Magnetic Resonance	Monitored with ECG and monophasic Doppler and Obtaining an X-ray film in DV projection
Plain X-ray in DV projection of loggerhead turtles upon arrival at the RC. © García-Párraga	X-rays of sea turtle specimens with GE. © García-Párraga
Dorsal view of the 3D representation of the air-gas-filled spaces of a turtle with moderate systemic GE before (A) and after (B) HBOT. © García-Párraga	Tomographic findings in an individual with slight aspiration of seawater and presence of gas associated with the digestive tract, respectively. © García-Párraga
3D volumetric reconstructions of turtles with severe GE. © García-Párraga	MRI images associated with the presence of kidney lesions in an animal affected by severe GA and treated with HBOT. © García-Párraga
Dorsal view of 4 3D volumetric reconstructions showing the air-filled cavities inside the animals, corresponding to the 4 degrees of GE involvement. © García-Párraga	3D volumetric reconstructions showing the air-filled cavities inside the turtles, corresponding to a case without GE, moderate GE and severe GE. © García-Párraga