Is the species' geographic distribution severely fragmented, or known to exist at a limited number of locations?
Is the area, extent and/or quality of the species' habitat in continuing decline (observed / inferred / projected)?
Is the number of locations or subpopulations in continuing decline (observed / inferred / projected)?
Are there extreme fluctuations in the number of locations or subpopulations of this species?
a. It has been identified that regional populations of manta rays are likely to be isolated from each other due to bathymetric features and oceanic circulation patterns. It is probable that the manta population from east Australia and west Australia are distinct from each other with little connectivity. While oceanic species such as M. alfredi are not subject to the same degree of fragmentation as terrestrial species, the species has site preferences where they can aggregate in numbers (Couturier et al, 2011).
b. Manta ray habitats are in a continuing state of degradation. Coral reefs are well documented as being impacted by loss of climatic habitat caused by anthropogenic emissions of greenhouse gases, bleaching, crown of thorn outbreaks, fresh water run off carrying pollutants, anchor damage from boating and ocean acidification (Hoegh-Guldberg, 1999; Hoegh-Guldberg et al., 2007; Hughes et al., 2003; Selkoe et al., 2009). These issues effect not only corals, but also impact most trophic webs, such as the planktonic food source of M. alfredi (Hays et al., 2005; Richardson & Schoeman, 2004). Unmanaged tourism is also identified as a factor for manta ray habitats’ decline (e.g. Anderson et al 2010). For example in the Maldives, a large number of tourists and boats are impacting critical habitat of manta rays and can be the cause of high stress to the local population. As the diving industry is flourishing, with more and more divers wishing to encounter this species (see Australia Government 2008), aggregation sites and critical habitats of manta ray could quickly be impacted by the influx of boats and divers to these locations if access to these sites is not regulated for recreational activities.
c. The rate of population reduction appears to be high in several regions, up to as much as 80% over the last three generations (approximately 75 years), and globally a decline of >30% is strongly suspected (Marshall et al a). No information is currently available on the population trend of manta rays in Australia.
The M. alfredi population in Australia is probably one of the healthiest worldwide as there are no direct fishing pressures in this area (XXXX XXXX, pers. comm.). However, it is possible that part of the Australian populations migrates to targeted fishing areas and thus be impacted by these activities (XXXX XXXX, unpub. data).
d. Unless a directed-fishery for M. alfredi within Australian waters is created, the species is not likely to undergo extreme population changes (Couturier et al., 2012).
PROTECTED AREAS
Is the species protected within the reserve system (e.g. national parks, Indigenous Protected Areas, or other conservation estates, private land covenants, etc.)? If so, which populations? Which reserves are actively managed for this species? Give details.
Western Australia: Only M. birostris is explicitly protected from any fishing and disturbance or harassment and then, only within marine parks (The Government of Western Australia, 1994). As the species classifications are out of date, the Fish Resources Management Act 1994 does not recognise the two distinct manta ray species, as such M. alfredi is not protected (The Government of Western Australia, 1995).Marine parks in WA cover only a small percentage (<10%) of total known manta ray habitats.
Queensland: M. alfredi occurs with the Great Barrier Reef Marine Park and thus parts of its habitat are covered under the GBR Marine Park Act and the EPBC section on World Heritage areas, although there is no active management for the species.
Threats
KNOWN THREATS
Identify any KNOWN threats to the species, and state clearly whether these are past, current or future threats.
If climate change is an important threat to the nominated species it is important that you provide referenced information on exactly how climate change might significantly increase the nominated species’ vulnerability to extinction. For guidance refer to the Guidelines for assessing climate change as a threat to native species (Attachment B; Part B2).
Directed fishing
M. alfredi is targeted by fisheries around the world. The increase in demand for manta ray product by the Asian Market (mostly for gill raker) has led to the creation of new and highly specialised fisheries. Manta gill rakers are particularly sought for and valued; the trade for this manta product has become more lucrative than the shark-fin trade (Heinrichs et al., 2011; Marshall et al. 2011; Couturier et al. 2012). Individuals are captured and killed by various fishing methods, such as harpooning, netting and trawling (Couturier et al., 2012). Some fisheries target manta rays at aggregation sites using gillnets and can therefore harvest a large number of individuals with relatively small effort (Anon 1997). In artisanal fisheries the species is captured using traditional techniques such as harpoons, hand spears and hooks and lines (Alava et al., 2002).
Direct fisheries have significantly reduced the population abundance of several regions. In Indonesia, over 1500 manta rays are captured each year (Dewar et al. 2002, White et al., 2006). In several fished regions, manta ray populations have fully collapsed, demonstrated by dramatic reductions in catch (Alava et al. 2002; Barnes 2005). In eastern Indonesia, the number of animals caught by local indigenous villagers decreased significantly, dropping from up to 360 per annum, down to zero (Barnes, 2005). The villagers reported that the dramatic decrease in manta catch was due to the appearance of commercial fishing boats in the area (Barnes 2005). The fishing effort for mobulid rays has increased internationally, but the annual landing in many areas is declining (Dewar, 2002; Nair, 2003; White et al., 2006; Couturier et al., 2012). Major fisheries impacting the species were also identified in Indian waters where over 70000 t of elasmobranchs are caught each year (Banerjee et al., 2008). Mobulid rays can represent over 11% of the daily catch in some regions (Zacharia & Kandan 2010). It is important to note that most manta ray fisheries around the world remain unreported and illegal capture of these rays occurs even in protected areas.
It is highly probable that local and regional extinctions will occur in heavily fished areas.
Although no data is currently available, the manta population in WA is likely to swim across international boundaries into the highly pressured Indonesian region and be exposed to local directed fisheries. Figure 6 shows how known aggregation areas in WA and Indonesia are ~500Kms apart. Given the ability of the species to migrate at least 500Kms it is likely that the Australian populations migrate into Indonesian waters. The M. alfredi population in Australia is probably one of the healthiest populations worldwide as there are no direct fishing pressures in this area. However, it is possible that part of the Australian populations migrates to targeted fishing areas and thus is impacted by these activities, as proposed by Couturier et al (2011).
Incidental capture as by-catch
Manta rays and other mobulidsare regularly caught as by-catch in purse seine, trawl and net fisheries throughout their distribution (Couturier et al., 2012). Tuna purse seine fisheries are a major contributor to by-catch, with mobulid species caught in relatively large numbers in most oceans (Romanov, 2002; Couturier et al., 2012). Long line fisheries in the Atlantic Ocean are also regularly land mobulid species (Beerkircher et al., 2002; Beerkircher et al., 2008; Rey & Muñoz-Chápuli, 1992).
M. alfredi individuals are regularly caught in shark control nets off Australian and South African coasts (Sumpton et al., 2011; Young, 2001). In Queensland, 93 mobulid rays were caught in shark control nets between 1992 and 2008 with a mortality rate of 41% for manta rays (Sumpton et al., 2011). Other Threats
Other threats such as entanglement in marine debris, boat strikes, water pollution, habitat degradation, and irresponsible tourism practises impact this species ( Marshall et al. 2011a; Couturier et al., 2012 ).
Manta rays become entangled in marine debris such as mooring lines and lost fishing lines (Deakos et al., 2011; Marshall & Bennett, 2010), including in Australian waters (XXXX XXXX unpub. data). In Maui, Hawaii 10% of the M. alfredi population have amputated or non-functioning cephalic fins likely caused by monofilament fishing line entanglement (Deakos et al. 2011). These injuries are likely to impend on the overall fitness and survival.
This issue directly relates to the Key Threatening Process within the Environmental Protection and Biodiversity Conservation (EPBC) Act: “Injury and fatality to vertebrate marine life caused by ingestion of, or entanglement in, harmful marine debris” (Commonwealth of Australia, 1999).
High concentrations of heavy metals such as platinum, mercury and arsenic are present in manta ray tissues (Essumang, 2009; Essumang, 2010). The effects of the heavy metals on the health of the species remain unknown (Couturier et al., 2012).
While tourism can be beneficial for sustainable use of M. alfredi, rapid and unmanaged growth can be detrimental to the health and behaviour of individuals (Anderson et al., 2011; Deakos et al., 2011; Marshall et al.2011). The presence of a large number of divers in the water can have a negative effect on individual’s behaviour (Anderson et al., 2011), and fitness (i.e. collision with divers, touching by divers, disruption of normal feeding and cleaning behaviours). In addition, the potential for boat strikes is more frequent when numerous boats, needed to carry divers and snorkelers, are occurring in the same area. Manta rays in the Maldives have been observed carrying injuries resulting from boat interaction, although the number of fatalities remains unknown (Anderson et al., 2011; XXXX XXXX Pers. comm).
POTENTIAL THREATS
Identify any POTENTIAL threats to the species.
The ingestion of plastic debris by marine species has been well documented as causing fatalities, ulcerations, intestinal blockages, malnutrition and internal perforation (Boerger et al., 2010; Mascarenhas et al., 2004). Micro-plastic debris is of particular concern as it is of a similar size to the zooplankton, and can weigh up to six times more (Moore et al., 2001). Manta rays are filter-feeders and are most likely unable to discriminate between marine debris and zooplankton before ingestion. The effect of ingested micro-plastic on manta rays remains unknown. This threat is related to the listed key threatening process within the Environmental Protection and Biodiversity Conservation (EPBC) Act: “Injury and fatality to vertebrate marine life caused by ingestion of, or entanglement in, harmful marine debris” (Commonwealth of Australia, 1999).
Climate Change:
M. alfredi are likely to be significantly impacted by loss of climatic habitat caused by anthropogenic emissions of greenhouse gases, due to the predicted impact this phenomenon will have on the manta ray food source, the zooplankton (Hays et al., 2005). As oceanic temperatures are expected to warm by 2–3 °C by 2070 (IPCC, 2007; Poloczanska et al., 2007), the zooplankton community is likely to respond both globally and locally in terms of changes in abundance, timing and productivity. The migration paths and timing of the species is likely to change as the seasonal hotspots of zooplankton are altered (Couturier et al., 2012). Searching for new feeding ground is likely to impact on the fitness of manta rays as individuals may have to swim larger distances, or in random movement to find new productive areas.
