Images of a bocaccio with severe barotrauma at the surface and in a basket being sent to the bottom for recompression. Image from NOAA SWFSC
Here is a great PSA put out by NOAA Southwest Fisheries Science Center, all about the effects of barotrauma on rockfish and how to help recompress rockfish once you’ve caught them. We use the inverted basket method to send our fish back down after we catch them for diet analysis.
The bulging eyes and stomach forced out through the mouth are the result of increased pressure on the internal organs from the swim bladder.
Boyle’s gas law says that as the pressure on a unit of gas decreases, the volume of that gas increases proportionally. That is, when the pressure is cut in half, the volume doubles.
Rockfish live in deep water, typically 20m or deeper, and often several hundred meters deep. Standing at sea level, you have all the weight of the gaseous atmosphere pressing down on you. When you dive into the water, the weight of the water is added to that, at a rate of the equivalent of one atmosphere every 10m you descend. The pressure at 20m is 3atm. If a rockfish is brought from 20m to the surface, the pressure on it decreases to 1/3 the original pressure, which means the volume of its swim bladder increases 3-fold.
Unlike salmon and other surface-oriented fishes, the swim bladder of rockfish is not connected to the esophagus. Salmon add gas to their swim bladder by swallowing gulps of air from the surface. This limits them to staying near the surface. Rockfish use a capillary system like that surrounding our lungs to deliver gas to the swim bladder. The benefit of this system is that rockfish are not tied to the surface, and can colonize deeper habitats. However, it also prevents them from being able to quickly vent gas as the surrounding pressure decreases.
Since rockfish can’t burp, when they are brought to the surface the swim bladder swells and causes the stomach and eyes to bulge out.