Progression and Severity of Gas Bubble Trauma in Juvenile Salmonids

Abstract

Abstract We conducted laboratory experiments to assess the progression and to quantify the severity of signs of gas bubble trauma ({GBT)} in juvenile chinook salmon Oncorhynchus tshawytscha and steelhead Oncorhynchus mykiss exposed to different levels of total dissolved gas ({TDG)}, and we attempted to relate these signs to the likelihood of mortality. When fish were exposed to 110% {TDG} for up to 22 d, no fish died, and there were few signs of {GBT} in the lateral line or gills. Bubbles in the fins, however, were relatively common, and 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 necessary to kill 20% of the fish) ranging from 40 to 120 h, whereas steelhead had {LT20s} ranging from 20 to 35 h. In steelhead, bubbles in the lateral line, fins, and gills displayed poor trends of worsening over time, showed substantial interindividual variability, and were poorly related to mortality. In chinook salmon, only bubbles in the lateral line showed a distinct worsening over time, and the severity of bubbles in the lateral line was highly correlated with mortality. When fish were exposed to 130% {TDG} for up to 11 h, {LT20s} for chinook salmon ranged from 3 to 6 h, whereas those for steelhead ranged from 5 to 7 h. In chinook salmon, bubbles in the lateral line 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 significant trend of progressive severity. In both species at 130% {TDG}, the severity of all {GBT} signs was highly correlated with mortality. The progressive 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 dissolved gas supersaturation exposures experienced by fish in the wild. However, the efficacy of such programs seems substantially hindered by problems associated with (1) the variable persistence of {GBT} signs; (2) the inconsistent 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 (4) an extreme amount of interindividual variation in terms of susceptibility to {GBT.}