PIT tags and Passive Antenna Systems- PART 1: A Sunpass for Sunfish.

Hi folks!

            Whether you’re a fisherman or a scientist one of the most simple and common questions while out on the water is: Where are the fish?  Well, in order to answer that we need to understand fish movement patterns along with the how, when and why fish will be in a given area.  In this 2-part post I will discuss tracking fish movement with PIT tags & passive antenna systems and my experience applying this technology across environmental extremes to both the Everglades marsh and arctic tundra river systems. 

            Recent technological advances have seen a boom in different movement tracking devices available to fisheries scientists.  Acoustics, radio telemetry, and satellite tags have provided a wealth of previously unattainable information but have species specific size restrictions and are often too expensive for the budget of many researchers.  In these cases, PIT tags and their associated passive antenna systems provide a cost effective way for the researcher to study the movements and habitat use of fish both large and small. 

            For those of you unfamiliar with PIT technology, a PIT (Passive Integrated Transponder) tag is a small, (8-32 mm long, .2-1 gram) electronic microchip in a biocompatible glass casing that serves as a unique identifier when inserted in an animal.  It is termed passive because the tag remains dormant until in the presence of an electromagnetic field which causes the tag transmit its unique number code.  Passive antenna systems (PAS) provide a way of emitting an electromagnetic field to read & record PIT tags and can be set up to be either stationary or mobile (depending upon research needs).  Without getting too technical, PAS consist of 4 main parts: 1- An antenna (closed loop of electrical wire) to emit a detection field, 2- A reader to allocate power to the antenna, 3- A datalogger to store detections, 4- A power source (i.e. battery of some sort).   There are multiple prefabricated reader & datalogger models that can be purchased but the major advantage of PAS is the ability to adapt the antenna & power source to a variety of environments and experimental design needs.  Essentially, PIT tags and PAS function much like highway toll systems, with the PIT tag acting like your car’s window transponder and the PAS being the tollways- It’s a researcher’s way of designing a Sunpass for fish! 

            Down here in south Florida we use PIT tags and PAS in conjunction with field enclosures to study the fine scale habitat use of Everglades fishes as they respond to seasonal water levels.  Our research at FIU is focused on the freshwater marsh portion of the Everglades and conducted at the Loxahatchee Impoundment for Landscape Assessment (LILA).  LILA is an 80-acre working model of the freshwater Everglades ecosystem and includes a controlled water delivery system.  This “living laboratory” allows us to manipulate water levels for experimental purposes in a realistic environment without disturbing the natural Everglades.  Each of the four 200 m x 400 m macrocosms/impoundments at LILA contain all of the representative habitat types in the Everglades marsh- from tree islands to short and long hydroperiod vegetated zones.   See the fish video at the top of the post for a look at LILA's underwater lilly forest. 

            The six large (12 m x 4 m), replicate, field enclosures we use are set up on depth gradient containing three of these marsh habitats/vegetated zones.  We use centrarchids (sunfishes) as our focal species due to their importance as prey to both wading birds and predatory fish.  Stocking PIT tagged sunfish & installing a PAS into each enclosure with 1 antenna per habitat allows us to monitor fish habitat use across seasons by recording the proportion of detections in each habitat relative to varying water levels.  In this way we gain a better understanding of when and which habitat zones fish will be in under given conditions and how this in turn can impact Everglades food web dynamics. 

            In this particular setting we have found the “flatbed” or “pass-over” antenna orientation to work effectively while reducing the technical problems associated with a lot electronics operating in a small area.  By laying the antenna flat on the substrate the detection field is directed upward and any PIT tagged fish swimming over the antenna will be detected.   Still, as I stated before there are many other ways to cater PAS to the researchers needs as long as the right environmental and structural design factors are considered.  In my next post I will elaborate on some of these considerations and talk about the challenges and amazing experience of studying fish movement with PIT tags and PAS in the arctic rivers of the Alaskan tundra. 

Hope you enjoyed this post and stay tuned for the next one!

-Greg Hill