Physical model of the development of external signs of barotrauma in Pacific rockfish

Abstract

Four species of Sebastes (Pacific rockfish) showed evidence of a wide array of internal injuries from capture-induced barotrauma, including liver and swimbladder damage, organ displace- ment related to esophageal eversion, and hemorrhage in the pericardium and abdominal cavity. However, clear evidence of swimbladder rupture was not observed in all fish with external signs of barotrauma. Injection of air through the body wall into the swimbladders of rockfish carcasses gen- erated all of the common external signs of barotrauma documented in wild-captured fish, suggesting that the physical effects of swimbladder gas expansion can create these gross external signs without embolism from dissolved blood gases. Dissections of injected black rockfish S. melanops carcasses showed that, typically, injected air escaped the swimbladder without obvious rupture, moving in an anterio-dorsal direction, generating bulges and air bubbles that were externally visible through the branchiostegal membrane. Injected air also collected dorsally to the esophagus, posterior to the pha- ryngeal teeth, causing the esophagus to roll outwards into the buccal cavity (esophageal eversion). Injected air also frequently traveled further forward, collecting medially to the eyeball, leading to exophthalmia, and then moved distally along the fascia, invading the corneal stroma from the edges, resulting in corneal emphysemas. Air injected into the swimbladders of quillback rockfish S. maliger carcasses generated similar eye effects, but also escaped through ruptures in the branchiostegal membrane and did not generate esophageal eversion, which is also infrequent in wild-caught speci- mens. These results demonstrate that the major external signs of barotrauma in Pacific rockfish can develop as result of escaping swimbladder gases following an internal 'path of least resistance'.