Twenty years of data collected on the Southeast Fisheries Science Center's annual Bottom Longline Survey reveal a trend of increasing depredation in the Gulf of America, as well as indications of the species responsible.
Documentation of depredation of an Atlantic sharpnose shark by a sandbar shark caught on the same hook during longline sampling. Credit: NOAA Fisheries
Depredation, the removal of or damage to hooked fish or gear by sharks or other marine predators, is a growing concern among fishing communities. Depredation results in increased mortality for managed fish stocks, economic loss, and degraded fishing experiences.
NOAA Fisheries biologists Dr. William "Trey" Driggers and Dr. Walter Ingram recently analyzed data from the Southeast Fisheries Science Center's Bottom Longline Survey. Their goal: to determine trends in depredation rates over the past two decades. This is one of the first fishery-independent analyses of shark depredation—most research has been conducted in the fishery. The data confirm what anglers are seeing on the water: increasing trends in depredation by species like sandbar, blacktip, bull, and tiger sharks.
History of the Bottom Longline Survey
The annual Bottom Longline Survey, which started in 1995, samples U.S. waters of the Atlantic Ocean, including the Gulf of America, from Brownsville, Texas, to Cape Hatteras, North Carolina. Scientists use these data to monitor trends in abundance of federally managed sharks and other fish species like red snapper. It provides fishery-independent indices of abundance for numerous federal stock assessments in the Southeast region. When the survey protocol was standardized in 2001, Dr. Driggers recognized the importance of collecting data on depredation events. He added it as a standard data collection metric for the survey.
Scientists and crew deploy longline gear on NOAA Ship Oregon II. Credit: NOAA Fisheries
Since 2005, the survey has collected data on:
- Whether depredation occurred
- Predator species preying on the hooked catch, when possible
- Species of the depredated catch
- Variables like location, depth, temperature, and salinity
This has resulted in a standardized record of depredation events for all survey longline sets for more than two decades.
"When I first started at NOAA, I wondered what we might expect to see as populations of large sharks recovered from historically depleted levels. Increased interactions with fisheries—like depredation—seemed like a logical consequence," recalled Dr. Driggers. "We didn't know exactly what the future would look like, but we recognized the value of beginning to collect the data to describe it. Twenty years later, this standardized record allows us to examine how depredation has changed, which species are responsible, and what may be driving those changes."
The analysis included data from 2,846 survey longline sets in the Gulf of America from 2005 through 2025, with 27,001 captured fish assessed for depredation. Instances of damaged gear (e.g., leader with no hook) were not included as a depredation event for analytical purposes. That damage could not be verified as a predatory act and could have resulted from operational actions (e.g., gear snag).
Shark Depredation Rates Tripled Since 2005
Of the 1,508 verified depredation events, we directly identified the predator species 547 times (i.e., the fish preying on the hooked catch). For these events, more than 99 percent of the predators were sharks. The exceptions were two great barracudas and one greater amberjack. Responsible sharks included 13 species. The most common predators, in decreasing order, for the verified depredation events, were:
- Sandbar shark
- Blacktip shark
- Bull shark
- Tiger shark
The estimated depredation rates fluctuated from year to year likely due to changes in predator abundance and limited sampling. The overall trend has tripled since 2005.
This plot displays the standardized index of depredation across 20 years of data collection on the bottom longline survey. The standardized index is an annual measure of depredation generated by combining predictions from statistical models that account for differences in environmental conditions and sampling locations, allowing meaningful and direct comparisons among years. Credit: NOAA Fisheries/Driggers and Ingram 2026
These results are consistent with the trends reported by anglers and other researchers. Examining these data from a fishery-independent perspective provides another clue to help us understand what's driving these trends.
A Closer Look at Depredation on Red Snapper
The depredation rate of red snapper in the Gulf of America also exhibited a variable but pronounced increasing pattern. Standardized depredation rate was generally low early in the time series and reached its highest levels during the most recent years.
A red snapper caught during longline sampling with evidence of depredation. Credit: NOAA Fisheries
Paving the Way to Understand Shark Depredation
While many research projects have examined depredation rates in both recreational and commercial fisheries, this is the first long-term quantitative assessment of depredation from a fishery-independent survey. By using fishery-independent data, we can evaluate patterns over time without the influence of changes in fishing effort, targeting, reporting behavior, or management actions that can affect fishery-dependent observations. This gives us a robust capability to address key shark depredation questions while directly supporting NOAA's Strategy for Understanding Shark Depredation.