Archive for hti

Catch and release, again

As explained in our recent post Fisheries Outpatient Center, we are currently conducting a study of trout (Oncorhynchus mykiss) movements in a central valley watershed. We expected that we would be able to track fish throughout the watershed, but recapturing a tagged fish was unexpected. This female O. mykiss was first caught angling on March 23, 2010. It was approximately 20 inches (505 mm) in length and appeared to be post-spawn, due to the eroded caudal fin and lean appearance. We surgically implanted an acoustic transmitter (HTI LX tag) and released the fish in the same location where she was caught. Since her release we have been tracking her location using fixed and mobile hydrophone receivers. During the last 7 months she has remained within one third of a river mile (570 meters) of the site where she was originally captured and released. To our surprise, while out angling this fall for additional O. mykiss to tag, we managed to recapture this same fish (October 20, 2010). The incision was completely healed but, interestingly, a suture remained loosely attached (sutures usually dissolve within 90 days). The fish was measured and released, again.

Video source: FISHBIO

Fisheries outpatient center


Surgically implanting acoustic transmitters into fish requires considerable equipment, planning, and preparation. The task is difficult to accomplish in a laboratory environment, and even more complicated in the field. In the outdoor environment, it is just as important to ensure surgical equipment is disinfected and the challenge is to keep everything as sterile as possible. Before surgery a fish is introduced to a bath of anesthesia, where it will lose equilibrium and responsiveness in one or two minutes. Once the fish rolls on to its side, it is removed from the anesthesia, measured, weighed, and placed upside-down in a fish cradle. The cradle helps to maintain the fish in position during surgery and a tube with diluted anesthesia administered through the mouth keeps the fish motionless throughout the procedure. A 3-4 cm incision is made ventral to the mid-line and anterior to the pelvic girdle. The acoustic transmitter is gently inserted into the abdominal cavity and 3 to 4 sutures are used to close the opening. The fish is returned to a freshwater net-pen to recover before release. Given the size of the fish and the importance of a swift procedure, the surgery requires substantial practice, patience and skill.


The fish above is one of many trout (Oncorhynchus mykiss) that are being implanted with acoustic transmitters as part of a research project to evaluate migration characteristics. Research questions include, examining factors that influence anadromous and resident O. mykiss life-history strategies, the response of individuals to environmental factors, and the extent of in-stream movements. The transmitters are HTI Model 795LX tags (Hydroacoustic Technology, Inc., Seattle, WA) that emit an underwater sound (i.e. “ping”) at a frequency of 307 kHz, which can be uniquely identified by an acoustic receiver (i.e. “hydrophone”). Each tag weighs 13 grams in air, and at a ping rate of 10 seconds pulse repetition interval (PRI) the average 795LX tag lasts 300 days. The tag “life” can be extended by increasing the PRI or decreasing the pulse width. In order to provide a unique signal for each tag, HTI programs the tags with slightly different pulse periods, an approach that differs from techniques used by other acoustic telemetry companies.


The movements of tagged fish are monitored with both mobile and fixed acoustic receivers. At fixed locations, solar powered monitoring stations record fish movement 24 hours/day, year round. A Model 295G data logger, above water, is attached by long cables to a hydrophone, strategically placed underwater, which detects and amplifies the signal. Fish movements are also evaluated intermittently using mobile tracking, in which a hydrophone is mounted to a raft as shown in the photo above. Depending on the relevant research questions, mobile tracking can provide an efficient way to increase coverage of a large study area by locating the fish even when they are out of range of the fixed stations – kind of like an at-home follow up visit.

Check the Field Notes page for a description of our solar powered tracking station in an upcoming post.

Photo source: FISHBIO

Remote acoustic tracking


One of the advantages of using acoustic tags to track fish movement is the ability to recover information from an individual fish without having to recapture it. Acoustic tags transmit an underwater sound signal or acoustic “ping” that sends identification information about the tagged fish. Hydrophone receivers can be deployed to remote locations where they “listen” for tagged fish 24 hours a day. At this particular location a single HTI hydrophone is tethered to an anchor in the middle of the river channel and a reader positioned on the bank records information gathered. A solar panel keeps a 12-volt battery charged and the detection station operational with minimal maintenance.

Photo source: FISHBIO

Mobile tracking acoustically tagged fish in the Delta

The 2009 VAMP program is a cooperative effort between a large group of state, federal, and private biologists. Fish are tracked at key fixed-station locations in the South Delta to evaluate migration routes, migration speed, and survival. Mobile tracking is used to collect additional information on migration between the fixed-station receivers, including areas of high predation rates.