Journal of Threatened Taxa | www.threatenedtaxa.org | 26
November 2019 | 11(14): 14908–14911
New
recruitment of staghorn corals in the Gulf of Mannar
– the emergence of a resilient coral reef
Koushik Sadhukhan
1, Ramesh Chatragadda 2, T. Shanmugaraj 3 &
M.V. Ramana Murthy 4
1,2,3 National
Centre for Coastal Research (NCCR), NCCR Field Office, Mandapam Camp, Ramanathapuram, Tamil Nadu 623519, India.
4 National
Centre for Coastal Research (NCCR), Ministry of Earth Sciences (MoES), Pallikaranai, Chennai,
Tamil Nadu 600100, India.
1 sadhukhan.1985@gmail.com
(corresponding author),
2 chrameshpu@gmail.com,
3 raj@nccr.gov.in, 4 mvr@nccr.gov.in
doi: https://doi.org/10.11609/jott.4767.11.14.14908-14911
Editor: M. Nithyanandan, Kuwait Institute for
Scientific Research (KISR), Salmiya, Kuwait. Date of
publication: 26 November 2019 (online & print)
Manuscript
details: #4767 | Received 16
December 2018 | Final received 31 July 2019 | Finally accepted 01 October 2019
Citation: Sadhukhan, K., R. Chatragadda,
T. Shanmugaraj & M.V.R. Murthy (2019).
New recruitment of staghorn
corals in the Gulf of Mannar – the emergence of a
resilient coral reef. Journal of Threatened Taxa 11(14): 14908–14911.
https://doi.org/10.11609/jott.4767.11.14.14908-14911
Copyright: © Sadhukhan et al. 2019. Creative Commons Attribution 4.0 International
License. JoTT
allows unrestricted use, reproduction, and distribution of this article in any
medium by adequate credit to the author(s) and the source of publication.
Funding: Ministry of Earth Sciences (MoES),
Government of India; National Centre for Coastal Research (NCCR).
Competing interests: The authors declare no competing interests.
Acknowledgements: The authors thank the secretary, Ministry of Earth Sciences (MoES), Government of India and Director, NCCR, MoES, Government of India for the financial support and
facilities. We are thankful to the field
assistants for the field support.
Coral reefs in the Gulf of Mannar (GoM) have faced severe disturbances from the ever
increasing human pressure which resulted in the rapid decline of marine
biodiversity and biomass of the coastal ecosystems over the past few
decades. Despite severe climatic and
non-climatic stresses, scleractinian coral Acropora
sp. in GoM exhibits new recruitment of coral colonies
in recent times. The genus Acropora
is commonly called staghorn corals for the antler like colony form (Johnson et
al. 2011). Bleaching succeptibility
among coral taxa depends on the resistance capacity of corals to subsequent
bleaching episodes and broad geographic ranges, and staghorn corals are thought
to be one of the most vulnerable species in scleractinian
family (IUCN 2009). Acropora sp.
and other corals reproduce both sexually and asexually and both are important
to restore a degraded reef area (Zayasu et al. 2018).
Acropora corals can grow fast by asexual reproduction method called
fragmentation, which leads to the swift recovery of the degraded reef
system. They contribute significantly to
the reef growth (5.23cm2/month) and form dense colonies which
supports in island formation, coastal protection and fisheries (Bruckner 2002;
Johnson et al. 2011). GoM Marine National Park has fringing type coral reefs
which are distributed in 21 offshore islands ranging in between Rameswaram and
Tuticorin (ENVIS 2015). During the 1998
coral bleaching event, shallow water coral reefs of GoM
faced a severe mortality of 75% leading to a significant reduction in the live
coral cover especially of Acropora spp. (Venkataraman 2000; Kumaraguru et al. 2003).
Post bleaching assessment stated that recovery was too slow and live
coral coverage increased to 36.98% during 2003–2005 (Edward et al. 2018).
Thereafter, during 2009, live coral coverage was further increased to
42.85%. But coral bleaching in 2010
resulted in mass mortality of corals in GoM that
reduce the coral cover upto 33.20% and in 2011, coral
again started recovering and percentage of live coral cover increased to 38.86%
(Edward et al. 2012). It was indicated
that the recovery potential of a reef primarily depends on the successful
sexual reproduction followed by coral recruitment and survival of the coral
larvae (Vermeij et al. 2009). Therefore, an occurrence of new recruitment
of corals brings back the reef to its previous state and maintains a healthy
reef ecosystem. This study mainly
describes the in situ observation on new recruitment of staghorn corals at
selected sites of the GoM and also indicates possible
resilient factors linked to this new recruitment.
The National Centre for Coastal Research (NCCR) team carried out several
underwater surveys in Hare Island, and Manoli & Manoliputti islands of the Mandapam group, GoM from August 2018 to October 2018. Newly recruited corals distributed in
different sites were marked with GPS location (Hare Island northern side:
09.2060N, 79.0840E; Hare Island southern side: 09.1900N,
79.0750E; Manoli & Manoliputti
northern side: 09.2190N, 79.1340E; Manoli
& Manoliputti southeastern
side: 09.2060N, 79.1400E) and also photographed. Line
intercept transect and quadrat sampling methods (English et al. 1997) were
performed at the new recruitment sites.
For detailed assessment, a 20-meter long flexible underwater tape was
laid on selected reef areas, roughly parallel to shoreline with three
replicates at each site and covering an area 20m × 2m (1m on each side of the
transect line) for each transect. A total
of four sites were selected and 12 transects and 36 quadrats (1m × 1m) were
employed to estimate the live coral cover and recruitment density.
Recruitment of Acropora sp. was encountered at northern and
southern sides of Hare Island, northern and southeastern
sides of Manoli & Manoliputti
Islands. The recruitment of different
growth forms of Acropora such as Acropora branching (ACB), Acropora
tabular (ACT), and Acropora digitate (ACD) were found on the dead Acropora
branches and coral rubbles. The southern
side of Hare Island, has several dead patches of coral comprising Acropora
rubbles, and dead colonies of massive and submassive
corals with algal growth. In Manoli & Manoliputti, many
dead patches of massive corals were encountered and heavy sedimentation was
also observed. Among the staghorn coral
species, Acropora formosa and Acropora hyacynthus represented the maximum recruitment at both
the study sites. The average coral cover
of Hare Island and Manoli & Manoliputti
Island was 58.4% and 51.5% of which Acropora sp. represented live coverage
of 48.1% and 15.6%, respectively (Figure 1).
The size of each individual coral colony ranges from 0.9 to 8.2 cm.
Tropical reefs are increasingly impacted by multiple stressors that
result in landscape loss of coral cover primarily due to adult coral mortality,
aborted reproduction effort as well as unsuccessful recruitment at the
disturbed habitat (Connell 1997; Graham et al. 2015). Reef resilience can protect these
disturbances either by resisting change or by rapidly recovering to their
pre-disturbed state (Holbrook et al. 2018).
Reefs in GoM were challenged by intensive
coral mining during the 1980s before the Indian Ocean tsunami event, which
might have resulted in the destruction of Acropora sp. forming rubbles
in the benthic substrate. After the
Tsunami, the coral mining was completely stopped in GoM
(Edward et al. 2008). We observed many
new Acropora colonies (0.5–2 cm) on Hare Island coming up on largely
undisturbed dead corals and rubbles (Image 1a).
The percentage cover of live coral in Mandapam group of GoM has also significantly increased from the earlier
report of 22.69% in 2016 (Edward et al. 2018) to 58.6% in the present study
(Figure 1). We don’t have a direct
observation on coral spawning and larval settlement, but the emergence of new
coral communities following disturbances indicates a function of multiple
processes including coral recruitment, growth and survival of new
recruits. As per the earlier report, the
coral spawning of Acropora sp. mainly happened here during March every
year (Raj & Edward 2010). Earlier
studies also revealed that recruitment to reef habitats is dependent on the
ability of juveniles to find a suitable substratum to settle and metamorphose
(Graham et al. 2011). The reef structure
of Manoli & Manoliputti
is mainly made of rubbles and dead massive coral with algal growth. But on the southeastern
side of Manoliputti Island we observed a significant
density (12–20 colonies/m2) of new recruits of Acropora formosa (Image 1b).
The northern side of the Hare Island has a major settlement of Acropora
formosa, A.humilis
and A.digitifera whose colonies range
1.3–15.5 cm (Image 1c).
In degraded reefs, many negative driving forces also result in the
mortality of post larval settlement of corals (Harrington et al. 2004). The major threat to successful coral
recruitment is the excessive growth of algae on which coral planula may have
settled, but later suffered severe mortality (Vermeij
et al. 2009). In our observation, we
have noticed that new recruits have high competition against the growth of
native algal species Turbinaria sp. and Caulerpa sp. on the reef substrate (Image 1d). An earlier study also reported that GoM has faced heavy sedimentation ranging from 1.97
mg/cm2/day to 12.31 mg/cm2/day that was found to be the highest during the
month of August and lowest during the month of April (Mathews & Edward
2006; Kumar et al. 2014). Despite high
sediment deposition at the northern side and southeastern
side of Manoli & Manoliputti
Island, we have observed that the new recruitment of corals in this region is
high. Staghorn corals play an important
role in reef formation but are easily susceptible to environmental and
anthropogenic stress (Johnson et al. 2011).
The density of coral recruits is on the rise in Gulf of Mannar. The average
density of coral recruits in GoM has increased from
4.1/m2 in 2003–2005 to 7.7/m2 in 2011 (Edward et al.
2012). In the present study, recruitment
density was found to be the highest on the southeastern
side of Manoli & Manoliputti
Island with 12–20 colonies/m2.
Observation on the coral reproduction in GoM
is rare, but studies using the experimental set up were carried out to assess
the recruitment rate of coral spats in Krusadai Reef
complex (Raj et al. 2014; Marimuthu et al. 2018). Coral recruitment is one of the indicators of
coral reef health in marine protected areas, and also provide a positive sign
of coral reef recovery of a degraded ecosystem (Acosta et al. 2011). The findings reported here suggest that
increased percentage of coral cover brings a new hope for the researcher to
find out the possible driving forces for the successful post larval settlement
and survival of new recruits, which results in better conservation and
management plan for the coral reefs of GoM Marine
National Park. Therefore, NCCR initiated
long term monitoring plan of the selected reef areas to assess the growth and
survivability of newly recruited corals and investigate factors for reef
resilience.
For
image & figure - - click here
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