Fidelity and movement of trawler-associated dolphins in the Pilbara Trawl Fishery The dedicated photo-identification effort of 12, 20 min samples immediately before winch-

up from one trawler over two fishing trips, spanning some 60 trawls over two weeks, identified less than 150 individual dolphins. More than a third of these dolphins (50) were photographed three to seven times and estimates of the number of dolphins associating with one of the three trawlers in the fleet varied between 170 and 280, depending on the type of model used. This represents just 7-12% of the total abundance of dolphins across the entire area of the PTF, as estimated by the simultaneous aerial survey. These data suggest that at least a proportion of the population show fidelity to trawler-associated foraging over days and weeks. These results parallel those of Jaiteh et al. (2013), who found that individual dolphins were resighted on video footage collected inside a trawl net during different days and between separate fishing trips in the PTF over days and weeks. Three distinctive individuals in the catalogue constructed from the dedicated photo- identification work were matched with opportunistically collected photographs taken from the stern of trawlers in 2008, and five individuals were genetically matched with samples collected in 2008. Each genetic re-sampling event occurred within 140 km of the initial sampling event, regardless of the time between the events. One individual was sampled just 15 km from where it was originally sampled in 2008. It can be inferred from these data

that at least some individuals also show fidelity to foraging behind trawlers over months to years. Resident communities of bottlenose dolphins (both T. truncatus and T. aduncus) are well known for developing foraging ‘traditions’ over years and even between multiple generations (e.g., Krützen et al. 2005; Sargeant et al. 2005; Daura-Jorge et al. 2012) and trawler-associated foraging by dolphins has now been reported in numerous locations around the world (e.g., Chilvers and Corkeron 2001; Kovacs and Cox 2014).

Many coastal bottlenose dolphin populations (again, both Tursiops spp.) consist largely of ‘residents’ to a particular area (Shane et al. 1986; Connor et al. 2000), although

movements in the order of hundreds to over a thousand km have been reported for some individuals (e.g., Greece - Bearzi et al. 2011; United Kingdom – Robinson et al. 2012; southern California and north-western Mexico – Hwang et al. 2014). Far less is known of pelagic populations. Again, a few individuals have been documented moving considerable distances in short periods, i.e. thousands of kilometres (Wells et al. 1999), but some offshore populations appear to include individuals with discrete home ranges and that display long-term site fidelity (e.g., Rossbach and Herzing 1999). Results from this study suggest movements in the order of tens to hundreds of kilometres only and a strong degree of fidelity to trawler-associated foraging for a proportion of the PTF-associated population.

4.3.3 Conclusions and recommendations

The abundance and density of bottlenose dolphins interacting with the PTF off north- western Australia is considerably lower than in comparable regions in the Gulf of Mexico, and similar in magnitude to some heavily degraded areas in the Mediterranean Sea. There are a number of plausible reasons for this, including being unable to account for

differences in productivity between regions, or that historical and ongoing dolphin bycatch has impacted dolphin abundance. The population, or at least the portion of the population that interacts with the PTF, is subject to bycatch levels in trawl nets similar in magnitude to those in the western North Atlantic off the USA’s east coast, where the minimum population estimate of T. truncatus over a large area exceeds 55,000 offshore individuals (Waring et al. 2014). The combined elements of this study show that the number of dolphins interacting with the Pilbara Trawl Fishery is smaller than previously believed, and that a proportion of this community displays a high degree of fidelity to trawler- associated foraging over days to years. These findings lead to the following

recommendations:

i. A prescriptive limit should be placed on the number of human caused mortalities in the dolphin population. This limit should be established according to

internationally accepted standards (e.g., Wade 1998; Thompson et al. 2000); ii. An independent observer program is again required in the PTF, at coverage levels

of 30-62%, in order to objectively estimate total bycatch with greater precision than has been achieved with self-reporting (Read 2010; Allen et al. 2014);

iii. Further estimates of abundance over a wider area are required in order to establish trends in dolphin population size and rigourously assess risks of ongoing human impacts. The use of unmanned aerial vehicles for this purpose may be more accurate, free of risk to humans and less costly than manned aerial surveys

(Hodgson et al. 2013). Furthermore, data on the surfacing intervals and dive times of common bottlenose dolphins in and around the PTF are required in order to explicitly correct for availability bias in future abundance estimates;

iv. This information should be used in combination with data on the biology of bottlenose dolphins (generation time, reproductive output) to conduct a Population

Viability Analysis (Shaffer 1990; Thompson et al. 2000) before further assumptions are made regarding the population’s conservation status;

v. Successive State and Commonwealth governments have determined “acceptable” dolphin bycatch limits and granted accreditation to the PTF in the absence of fundamental data on mortality rates and dolphin population size. The results of the current research should be used to better-inform future management of the PTF and its impacts on populations of endangered, threatened and protected species.