One–two punches to eliminate depredation by marine mammals on fish caught or raised for human consumption

Clashes have occurred between fisheries and marine mammals ever since people first began catching and raising fish. These human–wildlife conflicts have been exacerbated more recently by the legal protections afforded to marine mammals and by the increased demand by people for wild-caught and farmed fish (Gales, Hindell & Kirkwood, 2003). Losses caused by marine mammals account for 2–10% of the landed value of aquatic-farm production (Nash, Iwamoto & Mahnken, 2000), but solutions to mitigate this problem have been few and far between (Gales et al., 2003; Fertl, 2009; Schakner & Blumstein, 2013). Shooting individuals and culling populations has never been a long-term solution, and most societies no longer consider these acceptable practices (Gales et al., 2003). However, attempts to employ solutions that are more ethical have been expensive or ineffective (Gales et al., 2003), or have caused greater harm than good such as attracting other predators (the dinner bell effect; Fertl, 2009), or scaring and displacing non-targeted species (e.g. Brandt et al., 2013). Harvesters, therefore, remain poised for a non-lethal technological breakthrough, such as that proposed by G€ otz & Janik (2016), to protect their livelihoods and successfully market their fish (e.g. CERMAQ, 2012). On the surface, G€ otz and Janik have developed and tested an effective means using only adverse sounds tuned to the hearing and behaviour of phocid seals to deter these predators from killing salmon at three fish farms. They found slightly lower surfacings of harbour seals Phoca vitulina within 100 m of their loudspeakers when exposed to the sounds, and no effect of the sound exposure on harbour porpoise Phocoena phocoena and European otter Lutra lutra distributions around the sonified farms. Most impressively, they showed that the number of fish lost to seal damage when sounds were emitted was reduced by 91–97% depending on the site and test conducted. At first look, it would appear that producing speciesspecific startle responses without inflicting avoidance behaviours from other species may be the long-awaited panacea for the economic losses caused by marine mammals depredating fishing nets and feeding on pen-raised fish. But, could the solution to this conflict really be this simple? Unfortunately, percentages presented alone, without reference to the absolute numbers they are based on, can be deceptive. G€ otz & Janik (2016) did not indicate the absolute numbers, but it would appear the seals killed and partially ate an average of ~30 fish per week per cage at the short-term test sites, or about four fish per day in the absence of deterrence, and <3 fish per week during these sound experiments (see fig. 2 in G€ otz & Janik, 2016). In terms of numbers of seals that might have been responsible for the predation, there were only 10 or fewer seal surfacings per hour within 100 m of the transducer at the control sites (see fig. 4a in G€ otz & Janik, 2016). Thus, it is possible that the mortality at the studied fish farms could have been caused by a limited number of seals, maybe just a single animal given that an adult requires ~3 kg of fish per day (e.g. Howard et al., 2013) and surfaces ~12–20 times per hour (based on mean dive times of 3–5 min; e.g. Wilson et al., 2014). In terms of finding no effect of the adverse sounds on harbour porpoise, the median number of harbour porpoise surfacings per hour observed was 0 during the control period, and slightly above 0 when exposed to sound (see fig. 4c in G€ otz & Janik, 2016). This lack of longterm effect of sound on the presence of other species may, therefore, simply reflect harbour porpoise being a rare species near their study sites, or (in the absence of knowing samples sizes) that there was no statistical power to detect an effect of the sound on porpoise behaviour had there actually been one.