Pinniped predation on fish farms is a worldwide problem and causes the industry financial losses of up to 10% of the total farm gate value (Nash et al. 2000). This has led to a need for the industry to look at non-lethal measures controlling seal damage, which include tensioning nets, deployment of a predator net or use of Acoustic Deterrent Devices ( ADDs) (Würsig and Gailey 2002). ADDs have often been considered a non-harmful method of dealing with the problem. However, the main problems with ADDs appear to be a lack of long-term efficiency and unintended effects on other marine wildlife (Jefferson and Curry 1996). A recent study has found a new method of acoustic deterrence using the acoustic startle reflex (Gotz & Janik 2011), which proved successful in deterring seals and avoiding effects on harbour porpoises over a two month period. This project tested (a) the effects of startling sounds on seal predation and marine mammal abundance around a test farm compared to adjacent control farms without ADDs over a 13 months period and (b) determined the startle threshold for bottlenose dolphins to prepare the method for use in other applications such as marine construction. The project also tested the short-term effectiveness of the startle method on two additional farms when they experienced high seal predation rates.
The use of the startle method resulted in a highly significant reduction in the number of lost fish on the long-term test site compared to the pre-deployment period (Mann-Whitney U, n=16, U=38, p=0.004, Fig 3). In fact, median losses per month were zero on the test site when the sound was played. This was a highly significant difference in predation losses compared to the control sites (Poll na Gile, Mann-Whitney U, n=22, U=103, p=0.001; Ardmaddy, n=21, U=20.5, p=0.01). Median losses were 41 fish/month on control site 1 (Poll na Gile), 39 fish/month on control site 2 (Ardmaddy), 98 fish/month on the test site before the deployment of the startle equipment and zero fish per month when the equipment was operating (Fig 3). There were only 5 consecutive events of negligible to moderate predation on the test site during the 13 months study, which consisted of 58, 14, 7, 5 and 1 fish losses. The direct comparison of monthly losses between the pre-deployment period, test period and control sites showed that the startle method was capable of reducing predation losses significantly throughout the one year deployment (Fig 3). This was also confirmed by a statistical model which showed that sound exposure was the most important explanatory factor with respect to variation in seal predation. The model also revealed that predation varied throughout the year, although different sites did not differ in their losses during different times of year. Similarly, overall predation levels did not differ across sites. Seals, porpoises and otters approached the farm throughout the entire test period. There was no significant difference in number of seals and porpoises at different distances from the fish farm throughout the test period.
Rapid response trials at two fish farms with high seal predation rates also proved highly successful. At the first farm 405 fish were killed by seals in the month before the startle equipment was deployed (Fig 8). Dive reports after deployment of the equipment showed no new, seal-related kills for 2 weeks at which time the farm was harvested. At the second farm predation also dropped to zero in the first week after deployment. However, the equipment was damaged in a storm afterwards and predation levels returned to the high levels found prior to deployment.
Our tests showed that the startle method was highly successful in limiting seal predation at an operating fish farm with no evidence for habituation during a 13 month period. Similarly, the method was highly successful at limiting predation in cases where predation pressure was high. A likely explanation for the five events of predation while the startle equipment was operating in the long term test is that the predating individuals could have had compromised hearing which could be either the result of genetic predisposition, disease, old age or previous exposure to anthropogenic noise source (such as commercially available seal scarers).
Movement data of marine mammals showed that the startle method did not influence the distribution of harbour seals, porpoises and otters. The fact that harbour seals still approached the farm quite closely when the startle equipment was operating is in contrast with previous findings when measuring approaches. However, in the previous test sound exposure was more varied and lasted for only 2 months. It is therefore likely that seals observed at the surface near the fish farm did try to avoid the sound by swimming with their heads above the water. The fact that there was virtually no predation confirms that the sound had an effect on seals. Harbour porpoises were also observed at the surface near the equipment, but kept their heads underwater confirming that the sound did not have an effect on them. The deterrence system tested in this study operated at a duty cycle of less than 1% which is between one and two orders of magnitude lower than in current commercially available deterrent devices. The fact that brief, isolated pulses were emitted at only moderate levels means that noise pollution was greatly reduced and the potential for masking of communication signals or hearing damage is low. This is in contrast to current commercially available ADDs which emit sound at high duty cycles and high source levels. We would recommend the use of this novel technology at fish farms.
The startle method would potentially also be useful to temporarily deter cetaceans from marine construction sites. One possible problem with the application for echolocating toothed whales is that these animals produce very loud echolocation pulses and therefore might have an auditory mechanism to avoid startling themselves. We therefore tested whether bottlenose dolphins would startle to pulsed sounds and what their startle threshold would be. We used two captive bottlenose dolphins to conduct tests of their reactions when listening to startle sounds. The startle was quantified through an accelerometer attached to the animal that recorded any kind of muscle flinches during playbacks of sounds. We found that both animals clearly startled to our pulses and that the startle threshold for this species lies at around 80 dB above their hearing thresholds. Since we have shown here that echolocating animals also startle, the method is likely to work also with harbour porpoises. Our results allow us now to design startle sounds specifically for dolphins and porpoises. However, further tests to see whether dolphins and porpoises sensitize in the same way as seals are still needed. If they do, the startle method can be used to deter either only seals, only dolphins and porpoises, or all of these taxa.