Operational interactions between marine mammals and commercial fisheries in Australian and South Pacific waters: characterisation and options for mitigation.
Reports of interactions between marine mammals and fisheries are on the increase globally. This is mainly because fishery effort has increased to feed the burgeoning human population and because advances in technology have allowed fisheries to exploit habitats that were until recently inaccessible. Additionally, many marine mammal populations decimated by harvesting over the past few hundred years are recovering and the growing conservation community is paying unprecedented attention to their welfare and conservation generally, with growing interest in their interactions with fisheries. Operational interactions are conspicuous and involve the close contact of marine mammals with fishing gear, either because marine mammals opportunistically or habitually target fishing activities to depredate (i.e. attempt to consume) caught fish, or because marine mammals incidentally encounter fishing gear while foraging naturally. Operational interactions often result in negative outcomes for the conservation and welfare of the marine mammals involved and for the economic viability of the fisheries involved. Marine mammals that become by-caught may receive life threatening injuries from entanglements, or may drown, thus having adverse impacts on small or recovering populations. Fisheries that are targeted by depredating marine mammals may need to replace damaged fishing gear, or may have the catch partially or completely removed, thus having adverse impacts on their economic viability. At the time this body of work commenced, little was being done to address the known or suspected occurrence of operational interactions between marine mammals and several commercial fisheries in the Oceania region. The general aim was to make significant inroads into addressing thesis, by: 1. Reviewing a major fishing method in the two regions in which there are operational interactions with marine mammals; 2. Characterising the nature and extent of depredation and by-catch where operational interactions are known to exist; and 3. Where deemed necessary in those fisheries, developing mitigation strategies and explore their efficacy. Collectively, the five research chapters in this thesis address these aims. They are stand alone case studies of marine mammal depredation and by-catch in commercial fisheries, four of which have already been published in international, peer reviewed journals. The first three research chapters focus on operational interactions involving odontocetes (i.e. toothed whales) and the second two research chapters focus on the otariids (i.e. eared seals). Chapter 2 generally defines and reviews the nature and extend of odontocete (i.e. toothed whale) depredation and by-catch in longline fisheries, which has emerged as an environmental and economic concern internationally. At least 20 odontocete species are involved across all major oceans, although depredation and by-catch rates were variable. This study also introduces fishing gear modification as a viable mitigation strategy. Chapter 3 builds on this theme in more detail by exploring depredation and by-catch, mainly by pilot whales (Globicephala spp.), false killer whales (Pseudorca crassidens) and melon headed whales (Peponocephala electra), in pelagic longline fisheries targeting tuna in Australia and Fiji. Two devices were developed to physically or psychologically deter depredating odontocetes. Unfortunately, the rarity of depredation and by-catch events did not enable the efficacy of the devices to be properly assessed, although both were found to be easily integrated into the normal fishing practice and to have little or no impact on target fish catch rates. Chapter 4 attempts to specifically address the efforts of a purse-seine fishery operating in South Australia (SA) in reducing by-catch of common dolphins (Delphinus delphis), pursuant to conditions set out under the Australian Government Environment Protection Biodiversity Conservation Act 1999 (EPBC Act). After characterising the nature and extent of the problem, it was found that a combination of a Code of Practice (CoP) using avoidance and release strategies and of gear modifications resulted in a reduction in encirclement by-catch from an estimated 377 to eight mortalities each year. Chapters 5 and 6 attempt to assess the impact of a demersal gill-net fishery on the Endangered Australian sea lion (Neophoca cinerea) in waters adjacent to SA. Specifically, Chapter 5 assesses the performance of the Great Australian Bight Marine Park (GABMP) in protecting animals of populations residing within it, pursuant to a management plan that aims to uphold the spirit of the EPBC Act. It was found that individuals tracked using satellite transmitters spent only 27.7% of their time inside the GABMP and could travel up to 9 times further than the location of the southern boundary. Additionally, it was found that by-catch occurred beyond the southern boundary and also within the GABMP during the six months each year that the fishery was allowed to operate within it, with an estimated 14 to 33 individuals killed each 17.6 month breeding cycle. Based on these findings, Improvements to the GABMP were recommended. In a similar manner to chapter 4, chapter 6 directly addresses recommendations pursuant to the EPBC Act to quantify the impact of a demersal gill-netting on all Australian sea lions across SA, by quantifying the extent of geographic overlap and the level of by-catch mortality and extent of overlap between the two. It was found that the two overlapped extensively in 68.7% of 4 km-2 cells and that by-catch was high, at 283 to 333 killed each breeding cycle. Based on these results, it was suggested that a network of permanent and temporary closures along with more extensive monitoring of fishing activities be considered. In summary, this thesis demonstrates that with sufficient political will, stakeholder support and the necessary funds, by-catch and depredation issues can be addressed and can lead to favourable outcomes for the marine mammal populations and commercial fisheries involved. Each case study presented provides many lessons, some being specific to the operational interaction, the marine mammal species or the fishery, and some being more generally applicable. Regarding the latter, more general lessons, it is acknowledged that depredation and by-catch are statistically rare events that may vary across time and space. As such, investigating and addressing them is likely to be costly, with the results still only providing a snapshot or a broad estimate that may not be representative of the overall problem. Additionally, marine mammals are intelligent and may quickly learn how to circumvent mitigation measures, despite their complexity and cost. Although marine protected areas (MPAs) such as the GABMP are implemented with the best intentions, they are often of insufficient size to provide adequate protection and may also allow a level of fishing that still has a quantifiable impact. Despite these drawbacks, all stakeholders are encouraged to adopt a spirit of collaboration and of commitment to attempting to resolve operational interactions with marine mammals where possible. Acoustic deterrence devices have many problems that are yet to be resolved, including their impractically large size and limited sound propagation and battery life. Nonetheless, their amalgamation with some of the physical deterrence technologies developed in chapter 3 may provide a more generic method of deterrence across all fisheries, thus providing hope that resolving operational interactions between marine mammals and commercial fisheries may be a viable proposition in the future.