Is attempting to change marine mammal behaviour a generic solution to the bycatch problem? A dugong case study

Abstract

Incidental bycatch in fishing nets is a global cause of incidental mortality of marine mammals. Two classes of approaches attempt to mitigate this impact: (1) approaches that change the behaviour of the fisher (e.g. closures and gear modifications), (2) approaches that attempt to change the behaviour of the bycatch species (e.g. acoustic alarms or pingers). Even though the effectiveness of pingers has been established for very few bycatch species, pingers are now mandatory in many fisheries throughout the world. Pingers are being trialled in commercial gill net fisheries in tropical Australia to reduce the bycatch of the dugong and three species of coastal dolphins, despite an absence of robust assessments of: (1) their effectiveness in reducing bycatch, (2) the likelihood of alienating bycatch species from critical habitats. We conducted replicate experiments to test the behavioural responses of dugongs to 4 and 10 kHz pingers in an array simulating a net. Each experiment comprised three sequential 10-min treatments in which two pingers were: (1) inactive, (2) active, (3) inactive. The rate of decline of the number of dugongs within the focal arena did not change significantly while pingers were activated. Dugongs passed between the pingers (where a net would be located) irrespective of whether the alarms were active or inactive, fed throughout the experiments and did not change their orientation to investigate pinger noise, or their likelihood of vocalizing. We conclude that: (1) pingers are unlikely to alienate dugongs from critical habitats or reduce dugong mortalities in fishing nets, (2) bycatch mitigation strategies such as pingers that rely on changing animal behaviour should only be used after rigorous testing on all likely bycatch species.

Acoustic alarms elicit only subtle responses in the behaviour of tropical coastal dolphins in Queensland, Australia

“…These changes occurred once the pinger was introduced and remained after it was removed. The subtle behavioural reactions elicited in these 2 populations of Australian dolphins are qualitatively similar to the reactions of dugongs in Queensland waters to BASAtype pingers (Hodgson et al 2007).…”

“…predation risk, although doing so may increase the likelihood of a potential entanglement. However, as in dugongs (Hodgson et al 2007) and bottlenose dolphins (Cox et al 2004), this effect did not significantly change the movements of humpback and snubfin dolphins, unlike the case of harbour porpoises (Kraus et al 1997, Laake et al 1998, Kastelein et al 2000.…”

“…Following Hodgson et al (2007), we used a series of sequential experimental treatments to investigate the surface and acoustic responses of animals to a single pinger. The duration of each treatment was 10 min for humpback dolphins and 5 min for snubfin dolphins.…”

“…Regardless of the attractiveness of pingers to fishers and the interest of the Queensland Government in implementing these devices, there has been no formal assessment of their effectiveness in reducing the bycatch of humpback and snubfin dolphins. Experimental studies show that pingers do not significantly affect the behaviour of dugongs (Hodgson et al 2007), and dolphins have been caught in shark nets set for bather protection and fitted with pingers (Berg Soto 2012) as mentioned above. Thus, research independent of the fishery is also needed to evaluate the potential for acoustic deterrent devices to further reduce the bycatch of humpback and snubfin dolphins by this industry.…”

“…This approach is based on assumptions such as the ability of the pingers to (1) deter each of these 3 species of concern from fishing gear and (2) have no negative effect (such as displacing animals from key habitats) on any species in acoustic contact with these devices (Perrin et al 1994). As pointed out by Hodgson et al (2007), responsible implementation of acoustic alarms to reduce marine mammal bycatch should only be considered if pingers can be shown to reduce entanglements of at least 1 species and have no adverse effects on any other species of conservation concern (Hodgson et al 2007). There are a series of challenges when testing these requirements, which include (1) pseudo-replication (the same dolphin and its response may be inadvertently counted more than once) (Dawson & Lusseau 2005), (2) the low statistical power associated with low levels of interaction (Dawson et al 1998), (3) the possibility of dolphins habituating to the acoustic signal emitted by the devices (Dawson et al 1998) (habituation defined as 'a reduction in response over time as individuals learn that there are neither adverse nor beneficial consequences to a stimulus'; Thorpe 1963, cited in Bejder et al 2006, p. 1155 and (4) the ethical difficulty of testing the efficacy of pingers on nets with appropriate 'control nets' (Teilmann et al 2006).…”