Pitch Precision: Adult Sawfish actively tracked for first time
The roar of an outboard motor. The song of a humpback whale. Regardless of the source, sound waves travel five times faster underwater than they do through the air.
Scientists from the Florida Program for Shark Research (FPSR) used acoustic technology to actively track the movements of adult smalltooth sawfish, the first marine fish to be categorized as endangered under the U.S. Endangered Species Act. Data gathered on behavior will better aid conservation efforts and provide insights into the life history of these elusive animals. Active tracking is one technique that provides high spatial resolution, so researchers are more confident about where sawfish are within a relatively small area.
In February and March of 2011, FPSR scientists conducted two 5-day trips in Florida Bay equipping sawfish with active acoustic tags in an effort to observe short-term movements and habitat use of adult sawfish.
"No one had ever done active tracking of adult sawfish before," said John Waters, manager of the National Sawfish Encounter Database (NSED). "It provided us with a better fine-scale resolution of their habitat use patterns."
Active tracking requires researchers with a portable receiver, called a hydrophone, follow the tagged animal in order to collect acoustic signals. GPS points are generated in real time and researchers record the data and follow the fish as closely as possible for as long as the signal is detected. The receiver also collects information on depth and temperature.
Traditional rod-and-reel fishing was used to capture the two adult sawfish. The animals were measured and sexed before tissue samples were obtained for genetic analysis and tags were attached to the first dorsal fins. In addition to acoustic tags, dart tags (which list an animal's ID number and who to contact if encountered) and satellite tags (to provide movement data on larger spatial and temporal scales) were used.
The signal from the first tagged sawfish was lost shortly after tagging, while the second sawfish was tracked on-and-off over a three day period.
Bethan Gillett, International Shark Attack File (ISAF) member and FPSR biologist, assisted with the tagging. "We observed that the sawfish spend more time in deeper channels during the day and then move to grass flat shallows during the night," Gillett said. "Their eyes are a really vibrant shade of green due to tapetum lucidum- the eyes have a back like a mirror that reflects light out of the eye to add more light to the overall environment."
According to Waters, public reports of sawfish encounters are frequent in Florida Bay and the upper Keys. This led researchers to propose locations for study that had the highest probability of capture.
In 2012, FPSR will be transitioning from active to passive tagging of sawfish. Passive tagging uses a series of underwater receivers that record acoustic feedback from tagged animals that swim within a certain distance of the receiver. Data is collected over a longer period, but with less fine scale spatial resolution.
"Active tracking is extremely labor intensive and requires researchers follow the animal continuously," Waters said. "Passive tracking collects similar data over longer timescales and requires less involvement, and you can track more than one animal at the same time. Passive tracking is better for generating a big picture of what habitat the species inhabits, while active tracking provided higher spatial and temporal resolution."
The reception range of both passive and active acoustic tags varies depending on underwater topography, such as hills or other landmarks, which affect sound wave movement patterns. In flat terrain, an acoustic "ping" from a tagged specimen could be picked up from a receiver as far as 500 meters-roughly a third of a mile- away.
Funding for this research was provided by a grant awarded by the National Marine Fisheries Service (NMFS).
Written by MacKenzie Burger and John Waters