Scientists tag fish to learn more about their biology, ecology, and movements. Tagging a fish allows a scientist to gather information where the fish is originally tagged and where it is later recaptured (if reported/recorded). Scientists use this information to answer questions about a fish’s migration patterns, habitat preferences, diet, mortality, and more. There are a variety of tag types available. Depending on variables such as species, fish behavior, and study area, some tag types may be better suited to use in a study than others. LDWF has used conventional tagging, acoustic telemetry, satellite tagging, and natural tagging techniques in its 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 attached to a fish externally and is visible on the 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 be tagged to ensure moderate recapture rates. This requires a lot of effort and money—as a solution, cooperative tagging programs have been 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. LDWF will also investigate using this method as part of a suite of research techniques to improve our understanding of southern flounder and better inform management of this fishery.
From the Greek roots of “remote” and “measure”, telemetry means measuring from a distance. Telemetry tags record or send data multiple times throughout the life of the tag. LDWF has used several types of telemetry tags: acoustic, satellite, and internal archival. Using telemetry tags for fish research allows scientists to collect a large amount of data after the fish is released. In the case of acoustic and satellite tags, the fish does not have to be recaptured in order to collect the data.
Biologists use acoustic tagging to collect data on fish movements from a distance. Two key pieces of equipment are needed for acoustic tagging—a transmitter and a receiver. Biologists surgically implant a battery-powered acoustic transmitter into the body of a fish. When activated, each transmitter emits a unique acoustic signal at short, random intervals for up to three years. Acoustic signals can travel a long distance underwater (up to half a mile away in Lake Pontchartrain) and are detected by receivers throughout waterbodies and in habitats of interest. The receivers collect and store the detected transmitter signals; biologists visit the receivers to download the signals via Bluetooth technology. These 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 has partnered with other state agencies, universities, and the U.S. Fish and Wildlife Service on acoustic tag studies on speckled trout, red drum, Gulf sturgeon, and juvenile bull sharks 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.
- A large-scale, multispecies study in Lake Pontchartrain 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.
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 used 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 be used to inform management of 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 have used SPOTs to study scalloped hammerhead and tiger sharks to better understand their habitat use and distribution in the northern Gulf. In addition, biologists assessed 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 captured these sharks by rod and reel or bottom longline and handled them alongside the vessel or in a modified cradle. They gathered length and weight measurements and determined the shark’s gender. They then mounted a SPOT to the shark’s dorsal fin and released the shark. When the shark later surfaced, the tag transmitted 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 has used 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 feed in aggregations of 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 has used 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 used 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 or so 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 also worked 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 collected baby yellowfin tuna to document the baseline chemical record of the Gulf of Mexico and then applied that record to adults caught by recreational fishermen throughout the Atlantic Ocean, including the Gulf off Louisiana. Comparing the two records allowed 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 study could impact how yellowfin tuna are managed in the future as it provides information on which nursery grounds contribute to the Gulf fishery. (Note that LDWF does not manage yellowfin tuna, but these research results could inform both federal and international management of this species.)