Knee deep in PIT tags and swamp poons
Sometimes the best fishing is in uncomfortable settings. Our prey often seeks refuge in difficult to reach areas, areas fishermen eagerly expend high energy to access due to the potential for high returns. This is the sort of location I found myself in during a recent research trip to Puerto Rico: an overgrown, mosquito-infested mangrove swamp teeming with juvenile tarpon. After an overnight flight and a 4am car ride through the mountains, I spent the morning wading in knee-to-thigh deep mud. Four hours and forty jumped tarpon later, I knew this would be a fruitful collaboration.
Working with the Puerto Rican Department of Natural Resources was the best experience of my young career. After all, it’s not often that I’m asked to take part in a research trip that involves fishing 80 percent of the time. The trip was part of a project to determine if juvenile tarpon (ten to thirty inches) from an impounded lagoon are able to move to the greater estuary and contribute to the economically valuable adult fishery. In other words, is this tarpon-filled lagoon a source or sink for the population? In order to approach this question we needed to overcome two major issues.
First, accessing juvenile saltwater fish habitats can be difficult. Adults are larger and have had time to outgrow or learn to avoid potential predators and therefore can use open habitats that would otherwise carry higher risks. This optimizes their growth-to-mortality ratio, and allows them to allocate maximum energy to reproduction. In comparison, juveniles are naïve and smaller, leaving them with little chance of survival when confronted with a gape-limited predator. Thus, the main defense mechanism of many juveniles is to inhabit areas that have barriers keeping predators out until they grow large enough to avoid predation. These barriers are varied, but often include habitats that are low in dissolved oxygen or have complex structures that make navigation difficult for larger individuals, such as fishermen or researchers.
Second, documenting connectivity requires marked fish to be recaptured in different habitats. This creates massive logistical problems since a transition to the estuary may take several years, during which time many marked juveniles will die. Of the survivors, few may enter the spatially limited area a researcher is able to sample, and due to the inefficiencies of many recapture gears, few of those individuals may actually get captured.
To overcome these issues in Puerto Rico, we used a high-tech mark-recapture technology. We marked tarpon with PIT tags, which are uniquely coded microchips commonly implanted in household pets. We then set up two PIT tag antennae in a creek outside the impounded lagoon. These antennae function as underwater fish tollbooths, detecting PIT tags as marked fish swim by. Thus, we were able to focus our effort on marking tarpon inside the lagoon, while letting the antennae detect juveniles that moved towards the greater estuary.
During my week in Puerto Rico, we tagged forty juveniles, and our antennae logged over one thousand detections. Unfortunately, no juveniles from the lagoon had crossed into the creek, so we are patiently waiting for the next few months of data. Over the next year, we hope to tag several hundred more tarpon and deploy more antennae. Not only will this improve our chances of detecting juveniles that have moved immediately outside of the impounded lagoon, but it will also improve our odds of catching a juvenile from the lagoon as an adult several years from now.
As is typical in any research, our first week of work sparked more questions and research ideas than we began with. For example: what age are the ten to thirty inch tarpon we were working with; what do they feed on; where is the spawning population that is responsible for these juveniles; and how long do these juveniles remain in and around the reserve? We have general answers to some of these questions, such as the age of young tarpon, but such information is from other systems, and may not be applicable to fish in this particular setting.
It is humbling to understand how little we know about these hugely popular fish, especially with regard to their juvenile stage. Without further investments to gain knowledge on all aspects of tarpon life history, we will be unable to successfully manage the species, as it would be impossible to fully understand population trends. Hopefully, through continued involvement in, and expansion of this collaboration, we can gather new ideas on how to ensure a healthy future for the Silver King.
For more information on BTT's Juvenile Tarpon Habitat Program, click here