Gas bubble disease is noninfectious, and it’s caused by environmental or physical trauma resulting from a change in pressure in spring or groundwater, per the National Library of Medicine. Officials with the California Department of Fish and Wildlife believe up to 830,000 salmon fry — the young that have just consumed their egg sac and emerged from the gravel where they are born — succumbed to the disease as they migrated through the 16-foot-wide Iron Gate Dam tunnel, where they may have encountered a “severe pressure change.” The tunnel and the dam itself are considered “old infrastructure” and are among the three others targeted for removal later this year.