Acoustic passive integrated transponders for fish tagging and identification

The objective of the experiments reported here has been to develop a fish tag, which is as cheap and simple to insert as the coded wire tag and which has the remote readout capabilities of the radio frequency (rf) based passive integrated transponder (PIT) tags. The acoustic PIT tags are made using MEMS (micromachined electro-mechanical system) technology and readout is based on ultrasound waves, which propagate with relatively low losses in saline waters. The passive microchip tag responds with a combination of specific resonance signals to an interrogative ultrasound signal. The system was first tested in the laboratory with satisfactory results. During the last trials the tag was injected into the abdomen of anesthetized fish, causing no reduction in readout signal quality compared to free tags in water. The tested chips had five resonance cavities at different frequencies in the range 100–400kHz. Although there were some variations from wafer to wafer in the initial production runs, we observed precise resonance peaks at the expected frequencies and with good repeatability. The results indicate that it is possible to develop a low-cost system for underwater identification of fish suitable for a high automation level. This could open up for improvements of logistics in for instance the fish farming industry.