Barotrauma: not just an issue for saltwater fishes
This article was written by Jamie Madden, who recently earned her master’s degree from Carleton University in Ottawa, co-advised by Dr. Steven Cooke and Dr. Andy Danylchuk. Her research focuses on recreational fishing and best practices for retention and barotrauma in freshwater fish.
Have you ever brought a fish up from depth and noticed its eyes bulging, its stomach coming out of its mouth, or it being especially bloated? These are the tell-tale symptoms of barotrauma, and something we commonly think of happening to marine species like red snapper as they are hauled up from depth. However, did you know that the same physics can cause barotrauma in freshwater fishes? Did you also know that sometimes fish may be impacted by barotrauma but not have any of the dramatic tell-tale signs? Knowing about what causes barotrauma, how to detect it, and how to alleviate the impacts are especially important when releasing fish.
From Brown et al. 2009 (https://doi.org/10.1577/T08-122.1)
Barotrauma primarily affects fish that don’t have a connection between their swim bladder and esophagus – these fish are termed physoclistous. Compared with physostomous fish that have a connection (like trout or catfish) and can “burp” out air to empty their swim bladder, physoclistous fish rely on diffusion and resorption of gasses to adjust their buoyancy, which is a much slower process. When physoclistous fish are caught in relatively deep water and pulled to the surface, the rapid decrease in pressure causes all the air in their body to expand, most notably the air in their swim bladder. In turn, the expansion causes internal displacement, which can be seen through the classic symptoms ofbulging eyes and protruding stomachs.
A fish with barotrauma.
Barotrauma research and media has focused heavily on marine species (see here or here), including research into ways to mitigate the impacts. Typical mitigation methods include many variations of descending devices, usually containing a weight that attaches to the fish and a secondary line. These devices can be as simple as a weighted hook, or as complicated as a clamp which releases at a preset depth. Many saltwater anglers even keep a simple weighted milk crate on board to lower fish back down to depths. Alternatively, the practice of using a hollow needle to puncture the swim bladder and release air (called venting or fizzing) is a popular, though controversial practice.
Much less research has been done on freshwater systems, species, and mitigation despite that any physoclistous fish that resides in even somewhat deep water can be affected by barotrauma, and widely targeted tournament fish like walleye and smallmouth bass are no exception. Further complicating things is the fact that barotrauma vulnerability can change in a species depending on the time of year, with changing seasons and temperatures prompting fish to move into deeper habitat. Like marine species, freshwater fish are equally subject to bloating and organ protrusion, but may also experience barotrauma with less obvious external cues. Most reliably, the expansion of the swim bladder makes them buoyant, causing them to float on the surface of the water and struggle to descend back to depth after release. Even this symptom, however, may not be obvious. Fish may seemingly swim off into the deep once released, only to pop back up to the surface when they fail to overcome their positive buoyancy a while later.
With little research done on freshwater mitigation techniques, we designed a study to shed light on barotrauma incidences in freshwater and determine how anglers can best deal with these situations. Our study tested the above listed mitigation techniques on walleye with barotrauma in Northern Ontario.
The SeaQualizer descending device in action. From https://seaqualizer.com/technology/
Venting of a fish suffering from barotrauma.
What did we do?
Monitored barotrauma incidence in walleye caught from different depths and after holding them in livewells for 30 minutes
Tested the effectiveness of four different barotrauma mitigation techniques for returning walleye with barotrauma to depth.
Mitigation methods included Fishsaverpro weighed hook, SeaQualizer pressure release clamp, weighted milk crate, venting.
Used biologgers to monitor fish depth and behaviour for 10 minutes after release to see if they recovered from the barotrauma.
What did we find?
Fish exhibited barotrauma symptoms in water as shallow as 7.5m / 25ft.
Time spent at surface exacerbated barotrauma symptoms (holding fish in a livewell = more and worse symptoms)
The SeaQualizer pressure-release clamp was not effective in freshwater, releasing fish much higher than the set depth and failing to alleviate their buoyancy. The simple weighted hook and weighted crate technique equally had 13% failure rates.
Venting reliably allowed fish to return to depth.
The orientation of fish once returned to depth was influenced by mitigation technique: fish were found upside down 92% of the time after being descended in the crate, 62% after the weighted hook, 33% of the time after the SeaQualizer, 20% of the time after venting.
What does all this mean?
Barotrauma happens in freshwater fish. It should be avoided by anglers by 1) limiting deep water fishing when not harvesting, and 2) being prepared with mitigation methods in situations where it can occur. Research is ongoing regarding best practices on venting vs descending, and our study found advantages of venting both in reliability and condition of the fish (orientation) after release. That said, venting should only be attempted by informed anglers. If unsure, descend fish using a simple weighted hook.