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From megafauna to microplastics: Understanding habit use reveals potential threats to Indonesia's manta rays

Germanov, Elitza (2020) From megafauna to microplastics: Understanding habit use reveals potential threats to Indonesia's manta rays. PhD thesis, Murdoch University.

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Manta rays Mobula alfredi are large, charismatic, filter-feeding elasmobranchs that are threatened with extinction. Limited information on populations within Indonesia exists, despite the growing popularity of manta ray tourism. As a wide ranging, coastal dwelling species, the main threats to manta rays come from fisheries. Within Indonesia, directed fisheries are now prohibited, but incidental capture still occurs and indirect threats from entanglement with fishing gear persist. While the impacts are more cryptic, if unregulated, tourism can also pose threats to manta rays through boat strikes and excessive SCUBA diving/snorkeling disrupting natural behaviors and impacting on important habitats. As filter-feeders, manta rays are susceptible to plastic ingestion in areas heavily contaminated by plastic pollution, like Indonesia. The potential threats posed by microplastics to filter-feeding megafauna are reviewed in Chapter 2.

In this thesis, I have investigated the habitat use, demographics and movement patterns of the manta ray inhabiting the Nusa Penida Marine Protected Area (MPA) (Chapter 3) and the Komodo National Park (NP) (Chapter 4), Indonesia. Maximum likelihood techniques and a Markov movement analysis were used to analyze data logged online ( by researchers and citizen scientists. A larger number of manta rays were recorded within Komodo NP than in the Nusa Penida MPA (~1,100 vs. ~ 600). Within the Nusa Penida MPA, manta rays displayed diverse use of habitats with sites being identified as social areas, foraging grounds and potential nurseries. Sex-based differences in site affinity were also identified with females and immature males having higher site affinity than mature males. Within the Komodo NP there was less differentiation in habitat use between the sexes and between juvenile and older individuals, apart from one site, which had high re-sightings of immature individuals. Tourism, measured by the number of boats present at dive sites, increased by 60% in the Nusa Penida MPA (from 2012 to 2017) and by 34% in the Komodo NP (from 2014 to 2017). Further, persistent threats from artisanal fisheries remain, with 14% of manta rays in the Nusa Penida MPA and 5% of those in the Komodo NP observed with line entanglements or permanent injuries from fishing gear.

To assess the abundance of plastic marine debris, including small sized plastics – microplastics, I completed a series of quantitative visual and trawl surveys (n ≥ 6 per season) of feeding grounds for manta rays (Nusa Penida MPA and Komodo NP) and for whale sharks Rhincodon typus (Pantai Bentar, East Java) (Chapter 5). Manta ray feeding grounds were assessed during the two prevailing seasons, the north-west (wet) and south-east (dry) monsoons from 2016 – 2018. Manta ray egested material was collected opportunistically within the Nusa Penida MPA. Floating plastic marine debris was counted visually in transects, while a plankton net trawled for near surface plastics. Plastics were identified, measured and categorized visually from trawl and egested material samples. Linear models found significant seasonal and location differences in plastic abundance with the highest estimates being for the Nusa Penida MPA and coinciding with the wet season. I provided the first theoretical plastic ingestion estimates for both manta rays (up to ~ 63 pieces h-1) and whale sharks (~ 137 pieces h-1) within the Coral Triangle region and confirmed that manta rays both ingest and egest plastic.

The results of the population studies (Chapters 3 and 4) were considered when making management recommendations for manta ray conservation in the region, including mandatory codes of conduct for dive operators, carrying capacity restrictions and management zoning. Knowledge gaps were identified and the value and shortcomings of citizen science data collection were outlined. Further, the plastic marine debris studies (Chapter 5) highlighted the potential threat to manta ray and whale shark populations and identified key times for prioritizing plastic marine debris prevention actions. I underline the need for further investigation into the health impacts of plastic ingestion and exposure to plastic associated pollutants, and provide future research directions for establishing the population level effects to these threatened species (Chapters 2, 5, 6 and Appendix I). Focusing research on these umbrella and flagship species could serve to increase public engagement, awareness and foster greater stewardship for overarching marine conservation issues.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Environmental and Conservation Sciences
United Nations SDGs: Goal 14: Life Below Water
Supervisor(s): Loneragan, Neil, Bejder, Lars, van Keulen, Mike and Marshall, A.
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