Ultrasonic transmitters were commercially purchased. They were cylindrical in shape, 90 mm long by 19 mm in diameter, and they weighed 29.5 g, with battery, in water. They transmitted at a frequency of 74 kilocycles/second. The number of days of active transmission was related to battery type and impulse rate. Transmitters emitting a greater number of impulses per second, with a range of 2.4 to 5.8, tended to stop transmitting sooner than those emitting fewer impulses per second. The difference in longevity between a 3 and 6 impulse per second transmitter was estimated to be 14 days. The average transmitting life of 11 transmitters was 30.5 days, with a range from 24 to 52 days. Battery capacity was a significant factor affecting longevity; transmitters with a 160 milliamperehour battery had an average longevity of only 18.5 days. None of the tags had a transmitting life equal to the 60 days advertised by the manufacturer. Within a range of 5 to 30 C, temperature was not a significant factor affecting the life span of the transmitter, although there was an indication that higher temperature slightly increased impulse rate which reduced total longevity. Impulse rate of individual transmitters is used to distinguish different fish in a body of water. The impulse rate of each transmitter increased sharply during the last few days of operation which produced greater overlap in impulse rates than prior to this change. The average deviation of impulse rate was much greater than the 10% tolerance specified by the manufacturer. Turbidity within a range of 8.7 to 41.2 ppm SiD, had little effect on transmission range. Wind, apparently due to its effect on waves, was found to add background noise which interfers with sensing impulses. Transmission range averaged 280 m and varied from 150 to 576 meters. Transmission range of a 74 k transmitter was not less when surgically implanted in the abdomen of a flathead catfish than the range of the same transmitter outside the body of the fish.
