Do sharks survive after the hook?
Fitbit-like sensors are the best tools for monitoring whether sharks survive catch-and-release fishing — essential data for fisheries management — according to a peer-reviewed study published June 23 by scientists from Mote Marine Laboratory in Sarasota, Florida.
The study, published online in the scientific journal Fisheries Research, is the first to show that motion-sensing accelerometer tags detect whether a shark has survived and how it recovers from capture stress with much greater certainty than other prevailing technologies. Usually scientists fit sharks with satellite or acoustic transmitters to infer whether a shark has survived using indirect signals like location or depth. In contrast, accelerometers measure sharks’ fine-scale movements directly and with high resolution, including every tail beat, body tilt, ascent and descent.
“In recreational and commercial fisheries, it is largely unknown how many sharks survive the stress of capture and release,” said Dr. Nick Whitney, the Mote staff scientist who led the study. “Commercial fisheries may document their bycatch, but they have no way of knowing whether the animals they release actually survive. Recreational fishing is even trickier to study, without the trained observers found on commercial vessels. At the same time, saltwater recreational fishing is worth more than $6 billion to Florida’s economy, and sharks are ecologically important top predators. Catch-and-release fishing is a way for anglers to enjoy their sport while minimizing impacts on shark populations, and we need to know how well it works.”
Whitney, manager of Mote’s Behavioral Ecology and Physiology Program, has pioneered the use of accelerometers to study everything from nurse sharks’ mating behavior to great white sharks’ swimming patterns and the energy expended by sharks in a controlled lab setting. However, no one had demonstrated the value of accelerometers for studying post-release survival and recovery in sharks, until now.
The new study was funded by a grant from the National Oceanic and Atmospheric Administration’s National Marine Fisheries Service (NMFS), the federal agency responsible for stewardship of U.S. ocean resources and habitat.
“The work Dr. Whitney has done to date with accelerometers to examine post-release mortality has been extremely impressive and has provided, without question, the most robust species-specific estimates available of this elusive parameter,” said Dr. William B. Driggers III, NMFS Research Fishery Biologist. “His success in applying this technology with blacktip, tiger and sandbar sharks demonstrates the value of accelerometers to answer a historically unyielding question of the utmost importance to stock assessments.”
For the new study, Whitney and his team attached accelerometer tags to the dorsal fins of 20 blacktip sharks caught in January 2011 through April 2013 with the help of experienced anglers in and around Charlotte Harbor and Cape Canaveral, Florida. Blacktips are a common Florida catch, and recreational anglers can harvest two per vessel (one per person) per day, while the rest must be released. Florida’s waters also host multiple other shark species with bag limits and more than two dozen prohibited from harvest.
The researchers released the tagged sharks, and the tags were programmed to monitor the sharks’ fine-scale movements for one to seven days. Afterward, the tags detached and surfaced using a special float package designed by Whitney. The floating tags transmitted radio signals, allowing the researchers to find and recover them all.
By analyzing the tags’ detailed data, Mote scientists found 19 different metrics, or ways to measure, behaviors indicating the sharks’ survival and recovery. If a shark died, the signals were clear:
“With an accelerometer, you see every movement the animal makes and also have fine-scale information on how deep it’s swimming,” Whitney said. “You see very clearly if it’s going up and down, or if it’s coming to rest on the bottom, which would be abnormal for a blacktip shark. You can see if its tail has stopped beating and its movements have stopped entirely.”
On average, the blacktip sharks took approximately 11 hours to recover from capture stress. During that time, the numerical data show that the sharks: swam in ascending and descending patterns, which grew shallower with recovery; began to descend more slowly and ascend more quickly and actively with recovery; beat their tails more forcefully and quickly after release, then more slowly and gently with recovery; and showed more bursts of high activity with recovery.
By describing these recovery behaviors with hard numbers, the paper demonstrates how accelerometers significantly enhance the “toolbox” of shark research. In contrast, the popular satellite transmitter tags can monitor a shark’s geographic location, depth and surrounding light levels, but the depth data are less detailed and no fine-scale data on body movements are provided. Satellite transmitters can be deployed for much longer than accelerometers, and they are the go-to devices for tracking long-distance animal migrations. However, long-term monitoring is less important for catch-and-release studies; sharks in this study tended to recover or perish within hours after release. In addition, catch-and-release studies can include a larger sample of sharks using accelerometers, which cost approximately one-seventh as much as satellite transmitters and can be reused multiple times.
In Whitney’s study, 91 percent of the blacktip sharks survived. Other species may be less hearty, with physiological stress varying notably among species, according to studies using other methods and preliminary accelerometer work by Whitney and colleagues. While the current paper focuses on demonstrating the use of accelerometry to study post-release recovery and survival, future papers will focus on the detailed results with these blacktip sharks and other species of recreational and commercial interest.
“With this study, we’re laying the foundation for many other projects where we can use this tagging technique to study the effects of catch-and-release fishing in new detail,” Whitney said.