Rate of disappearance of gas bubble trauma signs in juvenile salmonids

To assess the rate of disappearance of gas bubble trauma (GBT) signs in juv enile salmonids, we exposed spring chinook salmon Oncorhynchus tshawytscha and steelhead O. mykiss to water containing high levels of dissolved gas su persaturation (DGS) for a time period sufficient to induce signs of GBT, re duced the DGS to minimal levels, and then sampled fish through time to docu ment changes in severity of GBT. Because of the tendency of GBT signs to di ssipate at different rates, we conducted trials focusing on emboli (bubbles ) in the gill filaments and lateral line and separate trials that focused o n bubbles in the external surfaces (fins, eyes, and opercula). Bubbles in g ill filaments dissipated almost completely within 2 h after transfer of fis h to water of nearly normal DGS (104%), whereas bubbles in the lateral line dissipated to negligible levels within 5 h. Bubbles on external surfaces w ere more persistent through time than they were in gill filaments and the l ateral line. Although typically dissipating to low levels within 48 h, exte rnal bubbles sometimes remained for 4 d. Assuming a direct relation exists between easily observable signs and direct mortality, our results suggest t hat fish can recover quickly from the potentially lethal effects of DGS onc e they move from water with high DGS to water of almost normal gas saturati on. These results should be of fundamental importance to fishery managers i nterpreting the results of monitoring for the severity and prevalence of GB T in juvenile salmonids in the Columbia River system and perhaps elsewhere.