A pdf of the following paper is available from mmoore 'at' whoi.edu Vet Pathol 46:536--547 (2009) DOI: 10.1354/vp.08-VP-0065-M-FL Gas Bubbles in Seals, Dolphins, and Porpoises Entangled and Drowned at Depth in Gillnets M. J. MOORE, A. L. BOGOMOLNI, S. E. DENNISON, G. EARLY, M.M. GARNER, B. A. HAYWARD, B. J. LENTELL, AND D. S. ROTSTEIN Biology Department, Woods Hole Oceanographic Institution, Woods Hole, MA 025433 (MJM, ALB, GE); Department of Surgical Sciences (Radiology), School of Veterinary Medicine, 2015 Linden Drive, Madison, WI 53706 (SED); Northwest ZooPath, 654 W. Main, Monroe, WA 98272 (MMG); NOAA Fisheries, Woods Hole, MA 02543 (BAH, BJL); College of Veterinary Medicine & NOAA Cooperative Center for Marine Animal Health, University of Tennessee, 2407 River Drive, Knoxville, TN 37996 (DSR) Abstract. Gas bubbles were found in 15 of 23 gillnet-drowned bycaught harp (Pagophilus groenlandicus), harbor (Phoca vitulina) and gray (Halichoerus grypus) seals, common (Delphinus delphis) and white-sided (Lagenorhyncus acutus) dolphins, and harbor porpoises (Phocaena phocaena) but in only 1 of 41 stranded marine mammals. Cases with minimal scavenging and bloating were chilled as practical and necropsied within 24 to 72 hours of collection. Bubbles were commonly visible grossly and histologically in bycaught cases. Affected tissues included lung, liver, heart, brain, skeletal muscle, gonad, lymph nodes, blood, intestine, pancreas, spleen, and eye. Computed tomography performed on 4 animals also identified gas bubbles in various tissues. Mean6SD net lead line depths (m) were 92644 and ascent rates (ms21) 0.3 6 0.2 for affected animals and 76 6 33 and 0.2 6 0.1, respectively, for unaffected animals. The relatively good carcass condition of these cases, comparable to 2 stranded cases that showed no gas formation on computed tomography (even after 3 days of refrigeration in one case), along with the histologic absence of bacteria and autolytic changes, indicate that peri- or postmortem phase change of supersaturated blood and tissues is most likely. Studies have suggested that under some circumstances, diving mammals are routinely supersaturated and that these mammals presumably manage gas exchange and decompression anatomically and behaviorally. This study provides a unique illustration of such supersaturated tissues. We suggest that greater attention be paid to the radiology and pathology of bycatch mortality as a possible model to better understand gas bubble disease in marine mammals.