Which barotrauma mitigation method is best for red snapper and red grouper caught in deep water? Recompression outperforms venting, according to Gulf of Mexico study

Written by Kristen Kusek, Former Communications Director for USF CMS

The best way for recreational fishers to release red snapper and red grouper caught in the deeper waters of the Gulf of Mexico -- and physically traumatized from the pressure change experienced during the ascent -- is to use a descending device. This method ensures the greatest likelihood of survival when a fish is let go because it’s out of season or too small, according to a study reported in Fisheries Research.

“My hope is that people give descender devices a try because the science is clear,” said Chris Stallings, an associate professor at the USF College of Marine Science and the study’s lead author.

A descending device is a weighted tool attached to a fish’s mouth that helps a fish return to the depth at which it was caught (and then safely releases it). “Our study counters the prevailing belief that these devices merely turn the discarded fish into shark bait,” Stallings said.

The way fishers have traditionally helped a fish return to depth is by venting its swim bladder – piercing it with a sharp, hollow needle that immediately releases the trapped air so the fish can swim on its own back to depth.

In this study, fish that were recompressed using a descending device to at least 60 feet were twice as likely to survive than those that were vented.

Diving into the details

Barotrauma, the same challenge SCUBA divers face if they ascend too quickly, is rooted in a classic gas law you may have learned in 7th grade: the pressure and volume of a gas have an inverse relationship. As pressure decreases, as it does when a fish is reeled into shallower water, the volume of the gas in its swim bladder increases—making it akin to a balloon that’s about to pop. Swim bladders are gas-filled sacs that help fish like red snapper and red grouper control their buoyancy. Not all fish have them.

Barotrauma affects fish with swim bladders pulled up from a depth of about 60 feet or deeper. Symptoms include bulging eyes, bloated bellies and distended intestines. Some fish never recover from the trauma and can die from the injuries or from predation.  (Visit Barotrauma | FWC (myfwc.com)  for excellent info.)

For this study, the team partnered with the charter boat fishing industry to conduct 14 research trips to two parts of the eastern Gulf of Mexico in 2014 and 2016: the Panhandle and the West Florida Shelf due west of Tampa Bay. They caught and tagged 1030 snapper and 190 grouper and compared which method performed best: venting or recompression using a descender device called a SeaQualizer, which costs about $60.

They also used a fish “elevator” – often a milk crate but in this study, a cage with outward swinging doors on the bottom and an attached GoPro that allowed them to observe behavior of fish descended in it.  Fishers who later caught the tagged fish reported them so the team could track the data.

“The difference was striking,” Stallings said. Survival – measured by tag return rates - for fish weighted with the SeaQualizer and released at a depth of at least 60 feet, where their swim bladders recompressed naturally, was two to two and a half times higher than for vented fish. Survival was similar for vented fish and those recompressed to depths shallower than 60 feet. In short, deeper was better, although it wasn’t necessary to return the fish to a depth deeper than 60 feet, Stallings said.

Red snapper and red grouper are two of the most highly targeted and prized reef fishes in the Gulf of Mexico. Scientists estimate that discards account for more than 75 percent of the recreational catch of red snapper and red grouper—making it a significant problem in the Gulf, where fishing is a year-round sport and hobby.

“Considering how many red snapper and red grouper are discarded, this could mean a huge difference in the total number of fish that survive if people adopt recompression methods,” he said.

A tribute to a missed colleague and friend

This study is the first of its kind to be done in the eastern Gulf of Mexico. Other barotrauma studies have shown similar results for these and other species and areas, such as the west coast of the US where descending devices are more commonly used.

The field work for this study was led by coauthor Oscar “Butch” Ayala, who tragically passed away two days after the study was published. Ayala, previously Stallings’ graduate student, was an assistant research scientist at the Florida Fish and Wildlife Research Institute (FWRI) for more than 15 years. He was instrumental in developing FWRI’s at-sea observing program that improves critical stock assessment data.

“We miss his energy, kindness and mad fishing skills so very much – and his work on this study adds tremendous value for fishery managers and anglers in the Gulf,” Stallings said.

“But we’ve got more to do. We need to consider mitigation methods not only on a species-by-species basis, but also through the lens of regional differences,” Stallings said, such as how deep fishers are fishing, the methods they use, what predators might feed on the released fish and more.

Butch’s legacy continues.