Progression and severity of gas bubble trauma in juvenile salmonids

We conducted laboratory experiments to assess the progression and to quanti fy the severity of signs of gas bubble trauma (GBT) in juvenile chinook sal mon Oncorhynchus tshawytscha and steelhead Oncorhynchus mykiss exposed to d ifferent levels of total dissolved gas (TDG), and we attempted to relate th ese signs to the likelihood of mortality. When fish were exposed to 110% TD G for up to 22 d, no fish died, and there were few signs of GBT in the late ral line or gills. Bubbles in the fins, however, were relatively common, an d they progressively worsened over the experimental period. When fish were exposed to 120% TDG for up to 140 h, chinook salmon had an LT20 (time neces sary to kill 20%, of the fish) ranging from 40 to 120 h, whereas steelhead had LT(20)s ranging from 20 to 35 h. In steelhead, bubbles in the lateral l ine, fins, and gills displayed poor trends of worsening over time, showed s ubstantial interindividual variability, and were poorly related to mortalit y. In chinook salmon, only bubbles in the lateral line showed a distinct wo rsening over time, and the severity of bubbles in the lateral line was high ly correlated with mortality. When fish were exposed to 130% TDG for up to 11 h, LT(20)s for chinook salmon ranged from 3 to 6 h, whereas those for st eelhead ranged from 5 to 7 h. In chinook salmon, bubbles in the lateral lin e and fins, but not those in the gills, showed distinct trends of worsening over time. In steelhead, bubbles in the lateral line displayed the most si gnificant trend of progressive severity. In both species at 130% TDG, the s everity of all GBT signs was highly correlated with mortality. The progress ive nature of GBT and the methods we developed to examine fish for GBT may be useful for monitoring programs that aim to assess the severity of dissol ved gas supersaturation exposures experienced by fish in the wild. However, the efficacy of such programs seems substantially hindered by problems ass ociated with (1) the variable persistence of GBT signs; (2) the inconsisten t relation of GBT signs to mortality; (3) the insufficient knowledge of the relation between exposure history and GBT sign development for fish in the wild; and (3) an extreme amount of interindividual variation in terms of s usceptibility to GBT.