The once and future ping: Challenges for the use of acoustic deterrents in fisheries
In the last 40 years, with the widespread development of gillnet and driftnet fisheries around the world, there has been an increase in the rate of incidental catch of marine mammals (Perrin et al. 1994). In the late 1970's, fishermen and aquaculturists in the pacific Northwest beacme concerned because of the damage to salmon inflicted by seals. Experimental work was undertaken to solve the problem of pinniped depredation on caught fish, and acoustic deterrents figured prominently in that work (Mate and Harvey, 1986). Researchers used sounds believed to be annoying or frightening underwater to scare the seals away from the nets. However, the early studies suggested that these acoustic deterrents became less effective over time, and may have subsequently increased the problem by alerting the seals to the presence of caught fish, creating a "dinnerbell" effect (Mate and Harvey, 1986). These studies also provided early evidence for habituation by the wild animals, and indicated that acoustic applications were incsonsitent in deterring marine mammal/fishery interactions. Nevertheless, in the 1980's as the developing aquaculture industry continued experiencing conflicts with seals, the use of acoustic deterrents became more widespread. Manufacturers increased the amplitude of the sounds to levels that appeared to be more effective in deterring seals attempting to get into the aquaculture pens. This was possible only because the fixed locations and well buoyed platforms allowed larger transducers and power sources required for loud sound production. These units, called Acoustic Harassment Devices (AHDs) have had to become increasingly loud over thelast ten years to remain effective. Current AHD sound sources levels range between 194 and 200 db re 1 micropascal @ 1 meter, with fundamental frequencies between 10 to 25 kHz (Johnston and Woodley, 1998). There is evidence that such sho7ldds can exclude cetaceans as well as pinnipeds from areas (olesiuk et al. 1996). Simultaneously in Newfoundland, Canada, cod traps (large boxes of mesh netting designed to catch cod in near-shore waters) were catching increasing numbers of humpback whales. A whale rescue team from Memorial University in St. Johns responded to the entanglement of over 500 humbacks between 1980 and 1990, and thelosses to fishermen in damaged gear and lost fishing time were extensive (Lien, 1994). Dr Jon Lien of Memorial University developed several acoustic devices, in an attempt to keep whles out of fishermen's nets. After some success, he switched to a low power electronic acoustic unit (4 kHz fundamental, with source level of 135 db @ 1 micropascal re 1 m) that was portable and reliable. Trials on cod traps in Newfoundland fishery showed a signficant reduction in whale/trap collisions using these devices (Lien, et al 1992).