Application of passive acoustic reflectors to mitigate toothed whale depredation on longlines.

Indo‐Pacific longline fisheries have noted for decades that entangled fish, particularly those with swallowed hooks or with elements of metal in the entangling gear, were invariably spared from depredation attacks. To enhance the capability of fishery objects to either highlight their presence as foreign or to interfere with the clarity of returning echoes, a range of sonar reflective material logistically acceptable to the fishing industry was modeled. Small alloy spheres were shown to have target strengths equivalent to that expected of the TS of an entire tuna from a rear approach, the normal approach on hooked swimming tuna during depredation events. The performance of reflectors is governed by the ratio of the dimensions of the object to the wavelength of the toothed whale broadband echo click. Known changes in hearing and sonar focus with increasing toothed whale age must be factored into a fishery based approach. Spheres developed for the Coral Sea were trialed by Japan Fisheries Research Agency. Logistic issues were a problem while other fisheries have expanded their commercial utilisation. Spheres with greater logistic simplicity and higher target strengths have been identified for further testing and if commercially constructed could be readily incorporated into fishing operations.