Fish Get the Bends Too—and Millions Are Dying From It

Every day, many deep-dwelling fish that are pulled to the surface on fishing lines suffer an explosive fate: Their internal organs rupture from the rapid pressure changes during the ascent.

The condition—similar to the bends, which scuba divers can experience when ascending too quickly—means that many fish do not survive being caught and then released.  Fishermen often discard nontarget species and fish that are illegal to keep because they are too small, out of season, or exceed a catch limit.   

The problem is so severe and widespread that it is hurting the overall health of some fish populations. Of anglers’ six most-caught reef fish species in the eastern Gulf of Mexico, about 11 percent of fish that are thrown back die after release, according to data gathered from stock assessments and statistics maintained by the National Oceanic and Atmospheric Administration’s Fisheries Service.  That amounted to 3.1 million fish from 2012 to 2016. In the South Atlantic, more than 7.6 million black sea bass—about  13 percent of the fish released by anglers—died after release from 2012 to 2016, according to the most recent stock assessment and NOAA Fisheries statistics.*

But many fish could survive catch-and-release if they were quickly returned to the depths at which they live, which often can relieve built-up internal pressure before it kills them.  Fortunately, there are tools to help fishermen do just that, and leaders who set fishing rules in the Gulf of Mexico and along the U.S. Southeast coast from North Carolina to Florida are taking steps to increase use of these descending devices. 

Saving fish could mean more robust fish populations. And that can lead to increased catch limits and more days that anglers may be allowed to target some species.  The South Atlantic Fishery Management Council is considering requiring the devices on board fishing boats that catch snapper and grouper species and is expected to host public meetings on the subject this spring.  The Pew Charitable Trusts is encouraging the council to include a research and monitoring plan to measure compliance and the effectiveness of the new policy.

The Gulf of Mexico Fishery Management Council “strongly encourages” anglers to use descending devices and, when needed, venting tools, which penetrate the abdomen of a fish to release expanded air, relieving internal pressure.    

A top-of-the-line reusable descender can cost about $50 and is easy to use, so anglers are unlikely to cause unintentional harm to the released fish. The weighted device clips to the fish’s jaw and detaches itself, based on pressure, when a fish reaches a certain depth. This means anglers don’t have to worry about releasing the fish at the wrong depth.

Several studies have demonstrated the effectiveness of descending devices.  Laboratory experiments on red snapper conducted by scientists at the Harte Research Institute for Gulf of Mexico Studies at Texas A&M University, Corpus Christi showed that rapidly recompressed fish had 100 percent survival at about 100 feet in depth and 83 percent survival at about 200 feet. Another Harte Research Institute study examining red snapper showed that fish released at the surface without any help were three times more likely to die than rapidly descended fish helped by devices. 

Mortality of discarded fish is one of the biggest challenges facing many fish populations. In the South Atlantic, a recent stock assessment  found that 28.5 percent of recreationally caught red snapper die after release, which amounts to more than 460,000 fish in 2017. For commercially caught red snapper, the number is 38 percent, and some other species’ mortality rates are even higher. Another stock assessment reported that nearly all snowy grouper die after they are thrown back, mainly because they are hauled up from very deep water.  

This wasted catch is prompting action by fishery managers. In the fall, the NOAA’s Bycatch Reduction Engineering Program awarded $81,000 to the South Carolina Wildlife Federation to work with the South Atlantic council in educating fishermen about the use of descending devices, fish handling techniques, and best practices for releasing fish under a range of conditions and locations. The program will give 500 descending devices to anglers, provide an online tutorial plus 12 education workshops, and conduct follow-up interviews with device users.

And early this year, gulf fishery leaders will host a workshop of scientists, fishery managers, fishermen, fishing industry representatives, and conservation groups to further develop research to monitor the use and effectiveness of descending devices and venting tools.

Descending devices show strong promise in offering anglers a chance to help the fish populations they rely upon.

* The estimated numbers of fish caught and released are from the NOAA Fisheries Marine Recreational Information Program database. Pew used those numbers, along with discard mortality rates from the most recent Southeast Assessment, Data and Review reports, to estimate the number of fish that died after release. The six most-caught species in the eastern Gulf of Mexico are gag, red grouper, red snapper, gray triggerfish, greater amberjack, and vermilion snapper.

Holly Binns directs The Pew Charitable Trusts’ efforts to protect ocean life in the Gulf of Mexico, the U.S. South Atlantic Ocean, and the U.S. Caribbean, and Leda Cunningham manages The Pew Charitable Trusts’ work to protect ocean life in the South Atlantic Ocean.