Although a catch-and-release fishery in many regions, tarpon populations face many challenges including habitat loss, recreational fishing pressure and directed commercial and subsistence harvests by longlines and gillnets in Mexico, Cuba and the Caribbean. Because tarpon are such long-lived creatures, these threats can have rapid and significant effects on populations, and depleted stocks can take an extremely long time to recover, if ever.
Despite the importance of the recreational tarpon fishery, we have insufficient knowledge of spawning locations, overwintering areas, migration routes and fisheries characteristics for effective conservation. Given the historical decline in tarpon abundance and numerous threats to present day populations, we urgently need more information that can be directly and immediately applied to domestic and international conservation efforts.
By using tarpon and other species to gather temperature and depth data about the 26°C isotherm, we could essentially fill in the gaps in thermal structure models used to estimate OHC.
Tracking tarpon with satellite transmitters over the past decade has provided unprecedented insight into the mysteries of migrating tarpon. The tags gather location and CTD (conductivity, temperature, and depth) data using cigar-sized devices, known as PAT tags, attached to the backs of tarpon. These tags record data for a few months and then separate from the fish and float to the surface. At this point, our team retrieves the tag and transfers the data for further analysis.
While there is still much to learn, it has been revealed that tarpon in the Gulf of Mexico and the southeastern U.S. are interconnected. Recently, scientists in other fields have discovered the value of this same animal-derived data from our research. Nowhere is this more visible than among those who study fish and their interactions with the ocean and weather.
Each day, hurricane scientists like our UM colleague Professor Nick Shay plot the depth and location of the 26°C isotherm—a zone of warm water equal to 78.8°F, better known as ocean heat content (OHC). In recent studies for intense hurricanes, OHC data have been shown to be extremely useful since hurricanes are known to intensify after passing over pools of warm ocean water that often extend deep into the water column. Thus, it is the depth of the 26°C isotherm that sets the value of OHC. Typically OHC is measured by deploying profilers into the path of storms from research aircraft or by satellite altimetry. As it so happens, we are using satellite tags to track tarpon, the legendary game fish that generates $6 billion for the U.S. sport fishing industry. Based on data from the tags, we discovered that tarpon adhere to the 26°C isotherm during their annual migration through the Gulf of Mexico and the Caribbean Sea. The beauty of tarpon is that they provide data over the coastal regimes where satellite measurements (and OHC estimates) may not be as reliable compared to those over the deep ocean. In addition, data acquired from tarpon may also help better define frontal boundaries, the same boundaries where tarpon like to feed and reproduce, and on which the tarpon migrations are timed. By using tarpon and other species of fish to gather temperature and depth data about the 26°C isotherm, we could essentially fill in the gaps in thermal structure models used to estimate OHC. The fish in other words could help scientists build more accurate ocean models used in hurricane predictions. Clearly, such data over sustained periods of time may open up new research opportunities to improve the thermodynamical models used in estimating OHC.
More recently, we have been experimenting with SPOT tags, short for Smart Position Or Temperature transmitting, that can track a fish’s position in real-time with greater accuracy than PAT tags. The device’s GPS sensor is incredibly accurate, a fact which was reinforced one day this past summer. While monitoring the signal from a 150-pound tarpon tagged in Apalachicola in early June, we saw that fish move 350 miles south to Charlotte Harbor by late June, exactly opposite of what we expected. It then moved northwest some 200 miles and then north back to the coast. Along the way we noticed that the SPOT tag was traveling along Route 98. It finally came to rest in Pensacola, Florida. Someone evidently found the tag on the beach and brought it home. We dispatched two lab technicians to recover it, and they pinpointed the signal to a residential address.
Another goal has been to use satellite and acoustic telemetry tagging technologies to reveal inshore habitat and forage base utilization patterns. This sort of data provides evaluation of the proportion of time that tarpon spend in the interior mangrove creeks and coastal bay habitats of Everglades National Park, relative to time spent in proximal offshore areas, data that will advise optimal water management strategies for the coastal margins, bays and estuaries. Through statistical modeling with other long-term synoptic data sets, including vessel registrations, aerial surveys of boater uses and fishery creel census, we can evaluate population connectivity of this premier game fish and determine Everglades National Park’s ecological and economic contributions to the larger regional ecosystem.
Six tarpon ranging in weight from 53- to 139-pounds were double tagged with both SPOT5-satellite and VEMCO V-9 acoustic tags and employed novel tracking methods for tarpon in the Everglades and throughout the South Florida ecosystem. The work revealed that tarpon regularly transit between saltwater and freshwater, sometimes traveling hundreds of miles up rivers. The reasons may include avoidance of predation by sharks, feeding forays and riding body parasites.
A clear connection is emerging between fish behaviors and physical ocean processes. We could have never imagined that a network of animals attached to sensors could provide higher resolution thermal structure to improve ocean models used in predictions of large-scale tropical storms. Tagged tarpon are basically living observational platforms telling us where they go and further, revealing crucial things about the ocean environment. The potential use of PAT and SPOT tags holds serious cost-effective promises for oceanographers and fisheries scientists. A single PAT tag on a tarpon could provide 50,000 conductivity, temperature and depth profiles a year. If you tag 20 fish a year, you would get a million CTD profiles. However, using fish as oceanographers has its own limitations. You can’t tell a fish where to swim, nor can you guarantee that a fish will be in the 26°C isotherm before a storm. In addition, most fish don’t cover the entire water column, which limits the amount of sampling that can occur. For this reason we have proposed tagging tiger sharks, yellowfin tuna and blue marlin for data-gathering purposes—each coastal game fish with similar habits to tarpon that enjoy the 26°C isotherm as well.
Ideally, this might include at any given time 1,000 tagged fish of various species swimming in the Gulf of Mexico, mapping the entire ocean column in near real-time. Given the rapid advancements of the technology, satellite tags the size of ballpoint pens are only a few years away, opening the field for a wider category of fish. Integrating the numerous data points with our satellite framework could significantly improve the ocean models used for forecasting hurricane intensity.
The potential impact is huge and by letting these fish talk to us even more deeply, we can sustain the fisheries and hopefully reduce human suffering caused by catastrophic storm encounters around the world.
To become involved in this new and exciting conservation adventure, you can adopt a tarpon through the Tarpon & Bonefish Research Center by picking up the cost of one or more satellite PAT tags and even join us on an expedition to observe tag deployment. Furthermore, the Bonefish & Tarpon Trust provides a dollar-for-dollar match of your donation. Visit bonefishresearch.com for tagging results and information on how you can participate.