A meta-analysis of gas bubble trauma in fish

Total dissolved gas (TDG) supersaturation generated by dams is known to cause gas bubble trauma (GBT) and mortality in fish, but despite many studies on the topic, there have been no recent attempts to systematically review the data. We conducted a systematic review and meta-analysis to determine how different levels of TDG supersaturation in laboratory experiments impact mortality and GBT outcomes of freshwater fishes. We also examined all TDG laboratory studies on freshwater fish to identify research gaps in the GBT literature. Factors that improved the linear mixed-effects models and Cox proportional hazards models of the relationship between TDG supersaturation and time to 50% mortality, time to 10% mortality, time to the appearance of bubbles in the gills and time to external GBT symptoms include depth, temperature, oxygen-to-nitrogen ratios, species, body mass, the interaction between TDG and depth and author group for one or more of the models of the relationship between TDG and GBT outcomes. Of the 99 GBT studies we found in our search, 74% quantified mortality outcomes, with limited assessment of quantitative behavioural, histological and performance outcomes. Moreover, the majority of studies were conducted on salmonids. We therefore recommend additional studies on non-salmonid species to enhance our understanding of the mechanisms of GBT and community effects with use of more diverse sublethal outcomes. We also recommend random subject allocation to treatments, complete reporting, consistent methods between treatments and the use of control groups (which were often lacking) for more rigorous experimental designs.