When fish are caught and retrieved quickly from deep water, injury may result from depressurization. Depressurization can result in over-inflation of the gas bladder, inability to submerge when released, gas embolisms, internal and/or external haemorrhaging and death.
Freshwater fish have one of two basic types of swim bladders. Fish, such as carp, esocids, trout and salmon have a duct which connects the swim bladder to the alimentary canal. These fish can expel gas and make buoyancy adjustments more quickly than fish such as, bass, walleye, perch and most panfish which lack a connecting duct and rely on diffusion to deflate their swim bladder. Thus, while susceptibility to depressurization varies among fish species, it has the potential to be a significant source of mortality. ¹
Barotrauma is the injury caused when fish at depth are quickly brought to the surface by anglers.
This research, in Canada, recorded horrendous suffering to fish in a lake in Ontario during a fishing tournament:
"Barotrauma results from a process called decompression, where fish are brought from depth to the surface quickly, leading to rapid changes in ambient pressure."
In a review of “fizzing”, Kerr ² suggested that the practice should be discouraged, as significant damage can result from the procedure, and that fishing deep (5-6 m) should be restricted if fish are intended to be released.
To prevent decompression, catch-and-release angling for species in deep water should be avoided. ¹
HORRENDOUS INJURIES CAUSED BY PRESSURE INSIDE FISH
"The decline in ambient pressure can have profound physiological and physical consequences... Beyond problems with swim bladder distention (which, in some species, includes stomach or anal eversion or swim bladder bursting), ... internal (peritoneum, kidneys, dorsal aorta)... and external (fins, gums, body surface) hemorrhaging; ocular pressure; formation of gas bubbles within the circulatory system, gills, heart, and brain... and general tissue damage"
"At a fall competitive angling event on Rainy Lake in northwestern Ontario, we evaluated the incidence of barotrauma among tournament-caught smallmouth bass."
HIGH PERCENTAGE TRAUMATISED
"Overall, 76% of fish had at least one sign of barotrauma (either hemorrhaging or swim bladder distention)"
"32% of fish had two or more indicators and were thus deemed to have severe barotrauma."
"Of our examined fish, 64% showed signs of hemorrhaging and 42% showed signs of extreme bloating."
LEFT TO SUFFER
"When telemetered fish were released at a common site, we determined that fish with negligible signs of barotrauma evacuated the release site more rapidly than fish with severe barotrauma did."
"Some fish with barotrauma floundered at the surface when released, and one of these fish was subsequently hit and killed by a boat."
"At the end of the monitoring period, 20% of fish with severe barotrauma had died; two additional individuals (20%) that were still at the release site were moribund"
"Stress indices were higher in fish with barotrauma and tended to be highest among fish with barotrauma that died after release.
"Outside of a laboratory environment, a significant proportion of fish with severe barotrauma may die after release."
"When released, fish that are unable to return to depth immediately because of the added buoyancy could face predation; ... solar radiation or thermal stress; involuntary transport to shore or undesirable habitats via waves, currents, tides, or wind; injury from impact with boats; or additional physiological disturbances as they struggle to return to depth."
"All fish with extreme bloating also had problems maintaining equilibrium and were floating on the water surface during observations. In addition, when placed in the live release boats, many of these same fish floated upside down but continued to ventilate their gills."
Red Snapper Catastrophic Decompression
Fish brought to the surface from deeper water can quickly suffer "catastrophic decompression". This is when the swim bladder, used for buoyancy, massively expands because of the lower pressure at the surface.
This research found that Red Snapper endured: damage to the swim bladder; bulging eyes; external bleeding; stomach turning inside out; intestinal protrusions; stretching and tearing of membranes; damage to the liver, spleen, and heart; loss of equilibrium; and death.
Fish hooked from the deep often cannot return after they are released, as their swim bladders will have become too buoyant. They struggle about at the surface, find themselves at higher water temperatures, and are vulnerable to being eaten by predators.
Even less severe injuries can mean long-term harm, and threaten survival.
Although fish taken from the deepest water are the most vulnerable, fish plucked from relatively shallow depths are also susceptible - most, or perhaps all, fish with swim bladders can suffer harm.
² Kerr, S.J. 2001. A review of “fizzing”- a technique for swim bladder deflation. Ontario Ministry of Natural Resources, Fisheries Section, Peterborough, Ontario.