Scientists tag fish to learn more about their biology, ecology, and movements. In its most basic form, tagging allows scientists to gather information about the distance between where a fish is originally tagged and then recaptured. Over time, tagging techniques have evolved from tying ribbon around the tail of migrating fish to modern satellite tagging. LDWF currently uses conventional tagging, telemetry (or electronic tagging), and natural tagging techniques.
Biologists are able to determine daily, seasonal, and annual trends and patterns on fish using conventional tagging studies.
First used in U.S. waters in 1873, conventional tags are the most basic tag type. A conventional tag is marked with an identification number as well as contact information for reporting the fish when it’s caught. The tag is physically attached to a fish in a way that is visible outside of the fish. Conventional tag studies rely on anglers recapturing tagged fish and calling in information about the fish.
Conventional tagging programs depend on recapturing fish to obtain information so large numbers of fish typically must to be tagged to ensure moderate recapture rates. This requires a lot of effort and money—as a solution, cooperative tagging programs have formed, bringing together anglers, conservation and academic organizations, and biologists to tag fish and collect and analyze data.
One of LWDF’s earliest conventional tagging projects was in the 1940s working with fishermen to band tarpon, using a method similar to bird banding, in an effort to end the debates about catch and release survivability. Since then, LDWF has used conventional tagging as part of several programs focusing mostly on important recreational species like red drum, speckled trout, red snapper, and yellowfin tuna. In the future, LDWF plans to use this method to improve our understanding of the southern flounder stock, leading to better management of this fishery.
From the Greek roots of “far off” and “measure”, telemetry means measuring from a distance. In conservation biology, telemetry is used to track tagged animals. Telemetry tags can provide biologists with more information about a fish’s movements than conventional tagging because they record multiple times throughout the life of the tag vs. the single recapture of a conventional tag. LDWF uses several types of telemetry tags: acoustic, satellite, and internal archival.
Acoustic tagging is based on the principles of SONAR (sound navigation and ranging), allowing scientists to repeatedly locate and track tagged fish as they travel around an array of deployed receivers. Biologists surgically implant a battery-powered acoustic tag into the body of a fish. When activated, each tag emits a unique acoustic signal at short, random intervals for up to two years. Acoustic signals can travel a long distance underwater (up to half a mile away) and can be detected by listening stations or receivers. The receivers collect and store the detected signals, which biologists routinely download via Bluetooth technology. Detected signals include date and time, unique fish identification, and even depth and temperature. Data from acoustic tags provide a detailed look at a fish’s movement patterns, habitat, and behavior related to seasonal and environmental changes within the area of the receivers.
LDWF and partners have conducted acoustic tag studies on numerous species throughout the state:
- A study with Louisiana State University found that speckled trout in Calcasieu Lake preferred deep channels during cold fronts and storm events. In addition, data showed that female trout avoided areas with low salinity and did not leave the lake as often as males.
- A study with University of New Orleans found that red drum preferred the northern area of Bayou St. John and were able to survive in this waterbody.
- An ongoing, large-scale, multispecies study in Lake Pontchartrain has preliminarily indicated that:
- About a quarter of tagged speckled trout leave the lake with some returning in later seasons. Data also show that trout shift toward the eastern, saltier portion of the lake when salinity drops.
- Bull sharks leave the lake when the temperature drops below 68°F in the fall or winter and return when the temperature rises above that threshold in the spring.
LDWF’s receivers in Lake Pontchartrain have picked up signals from fish tagged outside of the lake by other institutions, such as Gulf sturgeon tagged by the U.S. Army Corps of Engineers, U.S. Fish and Wildlife Service, and the University of Southern Mississippi’s Gulf Coast Research Lab. Likewise, these institutions have detected LDWF and partner-tagged fish moving near their receivers. The institutions exchange data so researchers receive valuable information about their tagged fish from locations that they otherwise would not have known.
Satellite tags are the most sophisticated type of fish tag that LDWF uses. These tags measure and record temperature, depth, and location throughout the life of tag. LDWF uses two types of satellite tags: pop-off satellite tags (PSATs) and smart position only tags (SPOTs).
PSATs are attached to a fish and pop off at predetermined date and time. While attached to the fish, PSATs record depth, temperature, and location. Once the tag pops off, it floats to the surface and all of the valuable information in the tag is downloaded via satellites. Because these tags transmit from the surface, they can be recovered with a hand-held antenna.
In cooperation with researchers from NOAA, the University of Mississippi’s Gulf Coast Research Lab, and Texas A&M University at Galveston, LDWF biologists are using PSATs to study both the movements and post-release survival of blacktip sharks along the Gulf coast. Blacktip sharks are one of the most common coastal sharks in Louisiana, and one of the most popular recreational and commercial catches, too. Information from this study will help manage this recreationally and commercially important fishery.
SPOTs can either be attached to a fish’s fin or towed behind a tagged fish and work best on species that spend a significant time at the surface. When at the surface, these tags transmit signals that can be detected by the satellite-based Argos Tracking System. SPOTs can generate multiple positions per day for a single fish. Most live-tracking applications are based on SPOTs.
LDWF biologists are using SPOTs to study scalloped hammerhead and tiger sharks to better understand their habitat use and distribution in the northern Gulf. In addition, biologists are assessing the risk of the scalloped hammerhead being incidentally caught in other fisheries, as catch rates have declined and they have been considered for listing under the Endangered Species Act. Biologists capture these sharks by rod and reel or bottom longline and handle them alongside the vessel or in a modified cradle. They gather length and weight measurements and determine the shark’s gender. They then mount a SPOT to the shark’s dorsal fin and release the shark. When the shark later surfaces, the tag transmits a signal through the Argos satellite system, allowing biologists to actively track the shark with accuracy between 250 to 1,500 meters (about 820 to 5,000 feet). Since 2012, biologists have tagged and released 31 scalloped hammerhead sharks (25 males, 5 females, 1 unknown) and 22 tiger sharks (14 males and 8 females) in the northern Gulf.
LDWF uses both PSATs and SPOTs to study whale sharks’ movement and preferred habitat in the Gulf region. The largest fish in the sea, whale sharks are known to occur in feeding aggregations from 10 to more than 100 individuals where there is an abundant food source. The natural bank habitats south of Louisiana are important feeding habitat in the northern Gulf for these aggregations. Researchers have attached tags to 37 whale sharks (24 PSATs and 13 SPOTs). Using data from these tags, researchers have found that whale sharks travel at much deeper depths than previously reported. In fact, the first series of tags used in this study were actually crushed by the pressure at the depths that these sharks were traveling when crossing the Gulf. Researchers changed the tag housing to account for this, and now there are several records of whale sharks at depths of over 2,000 meters in the Gulf, nearly 20 football fields deep!
Internal Archival Tags
Like satellite tags, internal archival (IA) tags record temperature, depth, and location but have a much longer lifespan—more than five years. IAs are surgically implanted in fish, which means a fish must be recaptured and kept to retrieve the IA tag. As a result, these tags are typically only used on species that have a reasonable chance of being recovered. Researchers often offer generous rewards for IA tags because they cannot retrieve information from these tags unless they’re returned.
LDWF uses a combination of PSATs and IA tags to study the biology, ecology, and movement of yellowfin tuna, one of the most important commercial and recreational fish in Louisiana and the Gulf. LDWF biologists developed a new method for attaching PSATs on yellowfin tuna because these fish travel great depths to feed vertically in the water column every day. They use IA tags because these tags can last more than five years and the recapture rate for yellowfin tuna is fairly high. To date, LDWF has released over 163 IA-tagged yellowfin tuna (28 PSATs) with 31 recaptures. Some of the fish were at large for more than three years.
Fishermen should keep their eyes out for tagged yellowfin tuna for at least the next five to ten years. You can see the light stalk of the internal archival tag protruding from the abdomen of the fish. If you capture a tagged yellowfin tuna, record the exact GPS coordinates, date and time of capture, and an accurate length measurement. Carefully remove the tag from the fish; the light stalk of the tag should remain intact with the body of the tag. LDWF offers a reward for tags returned in good condition. To report information and return the tag, call 855.728.8247 or email firstname.lastname@example.org.
Natural tags are just what they sound like—a natural way of identifying an animal and determining its movement without using an attached artificial tag. Biologists can use natural coloration, markings, genetics, or chemical markers (seawater chemistry) to monitor individuals or even groups of fishes.
For example, whale sharks have unique spot patterns that don’t change over time. These patterns create natural tags similar to a human fingerprint. The same pattern-matching algorithm that NASA uses to map stars in the night sky is used to map the spots of whale sharks and identify individuals. There is a photo identification library called Wildbook for whale sharks which houses sighting data from around the world from both marine biologists and citizen scientists. Photos include spot patterns as well as scars or other diagnostic features used to distinguish between individual animals; cutting edge software allows rapid identification using pattern recognition and other tools.
LDWF has participated in the Northern Gulf of Mexico Whale Shark Research Program by supplying photographs of whale sharks’ natural tags. During the study, LDWF submitted more than 200 photos to Wildbook; of these, 64 new sharks were identified. One of these sharks (known as H-021 in the database) has been tracked for 14 years. It was first documented in Belize in 2000, seen again in 2005 and 2006 in the Caribbean Sea between Honduras and Belize, and later tracked in 2010 in Mexican waters near the Yucatan Peninsula where it was seen repeatedly for several years. LDWF biologists last spotted the shark in July 2014 on Ewing Bank in the north-central Gulf and were able to attach a PSAT to it. The tag stayed on the shark for 47 days as the animal moved southwest to the Bay of Campeche at a speed of about 22 kilometers (nearly 14 miles) per day.
LDWF has also been working with scientists at Texas A&M University at Galveston to develop natural tags for yellowfin tuna based on their otoliths, or ear bones. The chemical composition of otoliths from different areas of the ocean reflect differences in the seawater chemistry in these areas. A fish’s otoliths retain a chemical record of the different areas the fish inhabited during its lifetime. In this study, biologists have been collecting baby yellowfin tuna to document the baseline chemical record of the Gulf of Mexico and then applying that record to adults caught by recreational fishermen throughout the Atlantic Ocean, including the Gulf off Louisiana. Comparing the two records allows biologists to make an educated guess about where the adults may have spent the early part of their lives. To date, the study has shown that the majority of fish caught in the north-central Gulf were spawned off the African coast, but a significant amount were also spawned in the Gulf itself. Results from this ongoing study could have a profound impact on how yellowfin tuna are managed in the future as it identifies which nursery grounds contribute the most to the Gulf fishery.