Development of miniature pingers for tracking Atlantic salmon smolts at sea

Abstract

Miniature pingers and automated receivers were developed and tested over three years to track Atlantic salmon smolts of different size and origin as they migrated from fresh water and at sea. Pingers (8 mm diameter × 38 mm) with delayed activation were first tested and surgically implanted in large hatchery-reared smolts (31 cm). After improvements, these pingers were implanted in smaller hatchery smolts (23 cm) in a second study. They were detected by automated receivers at fixed sites and tracked at sea as far as 49 km. Range of detection was at least 400 m, and duration of improved pingers exceeded 75 d. Pingers were then reduced in size (8 mm × 26 mm) by using a custom integrated circuit, and they were implanted and tested in wild smolts (18 cm) in a third study. They were tracked over the same period and distances as the previous year. Power output was maintained and signal strength and range of detection were as good as for the larger pingers, and pinger duration was up to 86 d. In all tests, the frequency of transmitters was crystal controlled for decoding by the narrow-band automated receivers moored underwater at fixed sites. A laboratory study to examine the long-term effects of surgically implanting dummy pingers of different sizes (8 mm × 24, 28, and 32 mm) in juvenile salmon (15 cm) indicated that pinger shape needed modification to increase retention time past 5–6 mo and that pinger size should be reduced further to eliminate mortality. These studies have led to the development of miniature coded transmitters and small single-channel receivers which will make it possible to detect and track large numbers of small, wild salmon smolts over long distances and for several months at sea.