The study tested telemetry in a giant kelp forest of the California coast (Credit: NOAA, via Flickr/CC BY 2.0)
Scientists tracking aquatic animals have to overcome obstacles that their terrestrial counterparts don’t endure. Take it from Anna Steel, who tracked largemouth bass in an estuary fraught with Brazilian waterweed, an aquarists’ favorite that can grow into impenetrable mats in the wild. Deep, turbid water made the problem worse, so Steel, like many other scientists, turned to a telemetry and tagging system to track the fish and triangulate their position.
No two underwater environments are exactly alike, however, and the effects of different topographies and water properties on telemetry systems that position-tagged animals hadn’t been well-documented. Steel combined her observations with those of other researchers, and together they published their findings in Animal Biotelemetry.
The researchers looked at telemetry systems in a coastal giant kelp forest, a waterweed-dense freshwater tidal estuary and a river. To determine what influences positioning error and efficiency, they measured time of day, water temperature, electrical conductivity, turbidity, discharge and other parameters. At each site, the researchers focused on the movements and habitat usage of a particular species: pink abalone at the coastal site, green sturgeon at the riverine site, and largemouth bass at the estuary.
Steel, a doctoral student at the University of California, Davis Biotelemetry Lab, led the research at the estuary site, a shallow tidal lagoon near Stockton, California. Alex Hearn, a project scientist at UC Davis, took on the river study, while doctoral student Julia Coates at San Diego State University tackled the coast.
Mildred Island, the study’s estuarine site where largemouth bass were tracked.
Steel installed a VEMCO Positioning System at the estuary site, comprising several hydrophone receivers and coded transmitters. Each receiver was fitted to a mooring to keep it in place.
“After we [set up the VPS] we spent a couple weeks tagging fish, which meant we had to sit on the back of the boat with a couple of volunteer bass fishermen,” Steel said. “That was awful,” she joked, before saying, “It was a great way to start the study.” The team tagged 30 largemouth bass for the study.
Once every six weeks, the researchers visited the estuary site and measured the growth of invasive Brazilian waterweed. Every few visits, Steel and the researchers pulled up the receivers to download water quality data. The receivers were secured to the estuary bed with 90 pounds of weight plates, the same kind one might attach to either end of a barbell. Surface buoys affixed to the receivers made the devices easier to find, but the researchers learned even careful preparation can’t eliminate the workings of Murphy’s law.
“We’d gone out to pull up our receivers and, on this one day in the summer, several of our buoys went missing,” Steel said. “The volunteer and I ended up jumping into the water and swimming about 50 yards through this thick ‘creepiness’ to retrieve the receiver.”
Overwhelmingly thick waterweed wasn’t the only problem Steel and the team faced. At one point, the researchers lost data when one of the receivers reached its full storage capacity. Other receivers were rendered useless by vandalism.
“There were challenges, but I think that if you’re not running into challenges when doing research, you’re either not doing anything exciting or you’re not working hard enough,” Steel said.
Between the three study sites, the VPS exhibited the highest positioning efficiency in the estuary. The river habitat was the second most efficient site, with the coastal site being the least efficient, likely due to higher levels of bioacoustic noise. The study noted that nocturnal snapping shrimp emit sounds that receivers can mistake for transmitter pulses.
The most impactful factor of VPS performance, the study determined, was the geometry of receivers in the positioning array. Water movement, as measured by wave behavior, tidal flow or river stage, also had a profound effect on the system, as the mooring rig was slightly mobile. The study suggests that researchers planning to install a VPS or similar telemetry system should be mindful of these variables, and should expect high rates of data loss from environmental noise.
Funding for the research was provided by NOAA Species of Concern Program, the Interagency Ecological Program, U.S. Bureau of Reclamation and the National Science Foundation.
Top image: The study tested telemetry in a giant kelp forest of the California coast (Credit: NOAA, via Flickr/CC BY 2.0)