In order to study how young fish such as salmon are affected by swimming through hydroelectric dams, scientists have traditionally equipped them with surgically-implanted acoustic tracking tags. Unfortunately, the implantation procedure can harm the fish, plus the weight of the device can affect their behavior. Now, however, a team at the Pacific Northwest National Laboratory (PNNL) in Washington state has developed a much lighter acoustic tag, that can be injected into fish using a needle.

Described as being "about as big as two grains of rice placed next to each other lengthwise," the tag measures 15 x 3.38 mm and weighs just 217 milligrams – that's one third the weight of previous surgically-implanted versions. Its contents include a custom-designed 3-volt battery, a circuit board, a temperature sensor, and a transducer.

That transducer produces an intermittent beeping sound. Using underwater receivers placed in and around a dam to detect those beeps, scientists are able to map the 3D location of individual fish, plus they can tell if they've been injured when traveling through the dam. PNNL has previously developed a sort of electronic fish stand-in, known as the Sensor Fish, to gather similar data.

Whereas it took at least two minutes to implant the older tags (which involved anesthetizing the fish and stitching the incision closed upon completion), the tiny new ones can be injected in just 20 seconds. Not only is this less stressful for the fish, but it also leaves them with a much smaller wound that should heal faster. Additionally, because the tagging can be done quicker, manual labor costs of fish-tracking studies should be considerably lower.

If set to beep once every three seconds (the frequency can be adjusted), each tag should be able to operate for about 120 days on one battery. The previous tags' batteries only lasted for around 23. What's more, in a study of 700 juvenile salmon injected with the tags in 2013, the survival rate of tagged fish was found to be higher than for fish carrying the older tags.

The laboratory is planning on licensing the technology to a commercial partner for production and marketing. In the meantime, led by Dr. Zhiqun "Daniel" Deng, the PNNL team is working on making even smaller injectable tags for use on young eels and lampreys.

A paper on the research was recently published in the journal Scientific Reports.

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