Acoustic Alarms

In an effort to reduce the by-catch rate of the threatened harbour porpoise, a project is underway to evaluate whether acoustic alarms attached to gillnets are effective in warning approaching porpoises.

The following is a summary of field testing results of this project, as stated in the Canadian Journal of Fisheries and Aquatic Sciences, "Mitigation of Harbour Porpoise (Phocoena phocoena) By-Catch in the Gillnet Fishery in the Lower Bay of Fundy" volume 56, number 1, 1999, pages 113-123.

"Demersal gillnets equipped with acoustic alarms reduced harbour porpoise (Phocoena phocoena) by-catch rates by 77% over those without alarms in The Swallowtail area of the lower Bay of Fundy during field testing in August 1996 (68% reduction) and 1997 (85% reduction). In both years combined, three harbour porpoises were caught in 249 alarmed nets versus 14 caught in 267 nonalarmed nets. The alarms spaced at 100 m apart along the net floatline produced a 0.3-s pulse at 10-12 kHz every 4 s at a level 133-145dB re 1(u)Pa at 1 m. In conditions of no rain and low wind (Sea state0-2) the alarms were presumed to be clearly audible to harbour porpoises at ranges of 0.1-0.6 km. Catch rates of Atlantic herring (Clupea harengus), Atlantic cod (Gadus morhua), and pollock (Pollachius virens) were not significantly different in alarmed nets and nonalarmed nets (except in one season when pollock were caught in lower numbers in alarmed nets). Harbour porpoise by-catch and herring movements may be linked. During years of low herring abundance, we also observed low harbour porpoise entanglement rates."

There are, however, several concerns with using acoustic alarms on gillnets. Some of these concerns include habituation, mechanical failure, noise pollution, and habitat displacement. Further study is underway to address these issues.