The cryptic and transboundary nature of ghost gear in the Maldivian Archipelago

Abandoned, lost or discarded fishing gear (ALDFG), more commonly referred to as ghost gear, is a global issue that impacts many marine organisms worldwide. In the Maldivian archipelago a large number of olive ridley sea turtles (Lepidochelys olivacea) are found entangled in these nets (more commonly referred to as ghost nets) each year. However, the origin of these nets or turtles are unknown considering fishing with the use of nets is restricted to the bait fisheries within the exclusive economic zone of the Maldives. Therefore, ghost gear has a transboundary and cryptic nature, making it difficult to assess the environmental impact and origin of the gear. This thesis aimed to develop new tools and techniques which could be utilised to examine these unknowns. I revealed in a literature review (Chapter 1) that research in ghost gear entanglements amongst marine megafauna are predominantly focussed in the Atlantic and Pacific Ocean. However, Indian, Arctic and southern Oceans are far less studied. Additionally, the majority of strategies to tackle ghost gear were centred around curative measures, such as ghost gear retrieval. I advise that future solutions, best practices and research should favour preventative rather than curative methods in ghost gear management and research. Statistical classifies (Chapter 2) were built in R to predict the probability of a net entangling a turtle. It was highlighted that nets with larger mesh sizes and the absence of floats were major gear characteristics that increased the likelihood of turtle entanglement. In addition, the time of year was an important variable with a higher chance of turtle entanglements in nets found during the northeast monsoon (November – April). Unfortunately, grouping of the nets by fisheries was not possible, beyond a broad classification. This was likely a result of the wide variety of nets used in the region. However, gill and trawl nets were recognised as high-risk fisheries. Regardless of the difficulties of assigning an origin of the nets, I was able to estimate the scale of the problem. Between 3,400 and 12,200 turtles could have become entangled in ghost nets over the 51-month study period, meaning this region has the highest turtle entanglement rate in ghost nets worldwide (0.17). Nesting and sightings of free-swimming individuals are rare and therefore the majority of entangled turtles do not originate from the Maldives. To discover the source population of these entangled olive ridleys we utilised a mixed stock analyses of mtDNA from samples of turtles entangled in nets in the Maldives (n = 38) and compared them to nesting stocks from published literature (Sri Lanka, east India and northern Australia). We were able to fill in data gaps in phylogenetics by including samples from previously undescribed nesting populations, such as those in Oman and improved resolution by including longer sequences from east India in our analyses (Chapter 3). Results suggest that the majority of entangled olive ridleys originate from east Indian (73%) and Sri Lankan (23%) genetic stocks when no population estimates were included in model design. This meant we could estimate the impact of ghost nets on these populations. Recorded ghost net entanglements may impact yearly recruitment of east Indian populations by 0.48% however a staggering 41% of the Sri Lankan population are thought to be negatively affected by the drifting nets. I then attempted to age ghost gear found drifting in the Maldives, and provided additional evidence to locate possible sites of origin. Percentage cover of biofouling communities and capitulum length of the goose barnacle (Lepas anatifera) provided the most robust metrics to estimate minimum drift times (Chapter 4). Lagrangian simulations (forced by Ocean Surface Current Analyses Realtime OSCAR) could then be utilised to backtrack drifting ghost gear to a putative origin. This analysis highlighted that the origin of these nets overlapped with purse seine (predominantly from Spain and France) and gill net fisheries operating in the area. Moreover, the models show that some of the nets originate close to the Indian and Sri Lanka shorelines, suggesting that small scale artisanal fisheries may provide additional high risk, contributing to ghost nets drifting into the Maldives and entangling turtles. In summary it is hoped that this thesis advances our knowledge on ghost gear significantly. Moreover, this thesis provides the information and tools necessary for the Olive Ridley Project (a British registered charity, tackling this issue face on), along with other stakeholders (government and non-government) in order to better manage resources and combat the ghost gear issue within the Indian Ocean.