Crown-of-Thorns Seastar Surveys Soon to be Underway!
CCC will be assisting PENRMO in conducting surveys to identify Crown-of-Thorns Seastar outbreak areas!
Acanthaster spp ., more commonly known as the Crown-of-Thorns Seastar (CoTS), is the second largest Echinoderm with a maximum diameter of 50cm. CoTS are renowned for their ability to devastate coral reef ecosystems. However, despite more than 1200 research articles focusing on the Seastar’s biology and ecology, knowledge gaps exist that not only reduce the ability of resources to understand and manage the possible impact from CoTS outbreaks but also inhibit the understanding and quantifying the term “Outbreak”. As such, in recent years, there has been a resurgence of interest in the ecology and biology of CoTS and how this can be translated into practical solutions for resource management.
Acanthaster spp . hold a vast reproductive potential with large females producing up to 65 million eggs per season, of which is particularly relevant to large population fluctuations and possible ecological outbreaks. This fecundity, however, is found to be disproportionate to body size, with individuals less than 30cm producing 0.5 – 2.5 million eggs per year and those with a diameter of 40cm producing more than 46 million eggs per year. Male and female Acanthaster spp. must be within proximity and spawn simultaneously to increase chances of success, further identifying that a distance of 15m would result in a 30-50% chance of successful fertilisation.Post-spawning and successful fertilisation, the larval stage of Acanthaster spp . is typically 11 days but can decrease or increase from 9 to 42 days depending on environmental conditions, predominantly driven by phytoplankton abundance. CoTS juveniles and adults have strong preferences for consuming corals, albeit different types of corals.
CoTS are a highly successful predator of Scleractinian corals, often resulting in the death of an entire colony which can prove to be devastating to reef systems on both short- and long-term timescales. The Indo-Pacific, including Sogod Bay (Southern Leyte, The Philippines) and the Great Barrier Reef (GBR), Australia, are experiencing unprecedented outbreaks from Acanthaster spp . on a wide spatial scale which remains to be a major contributor to the declines observed in Scleractinian coral cover. There are great inconsistencies throughout the literature in defining an “Outbreak”, mainly due to the varying scales and localities where large increases in CoTS populations are recorded, despite the conspicuous differences in densities between ‘Outbreak populations’ and ‘non-Outbreak populations’. There are also great differences in the attempted criteria of an ‘Outbreak’ when considering CoTS population quantitively.
A rapid assessment was initiated in 2019 by the Southern Leyte State University to assess the potential for a CoTS outbreak in Sogod Bay. The aim was to identify and to quantify the extent of a CoTS outbreak in selected municipalities of Sogod Bay. Coral Cay Conservation (CCC) were invited by the honourable Ma’am Eva of the Provincial Environment and Natural Resources Management Office (PENRMO) to assist with this assessment. The aim of the report is to provide data on the densities of CoTS in the municipalities of Liloan and San Francisco, Sogod Bay. We will be surveying the reefs from our base in Napantao, San Francisco up to the town of Liloan. It is suggested, from previous CCC data and anecdotal observations, that CoTS densities in Southern Leyte are naturally (and sustainably) high.
The densities of CoTS were estimated in the municipalities of Liloan and San Francisco in Sogod Bay, Southern Leyte using data collected in 2019. Through anecdotal observations in 2019, it was observed that CoTS are present in large numbers around Sogod Bay, and a rapid snorkel assessment presented observations of more than 1 individual CoTS per hour. Data collected from 2013 to 2019 have demonstrated annual fluctuations with the highest density being in 2018. When recorded at the highest density in 2018, CoTS still remained under the various “Outbreak” criteria proposed from the literature, however, it must be noted that CoTS removal dives were conducted on an ad-hoc basis and therefore could have heavily influenced such abundances. This is supported through rapid visual assessments that have recorded much higher densities and large aggregations throughout the system.
In order to identify, manage and remediate possible CoTS outbreaks, two dives (assessment dive and removal dive) will be carried out. As CoTS present an ecological value in maintaining a healthy reef system through the predation of fast growing Acropora spp . and Montipora spp. , the sole purpose of the assessment dive is to provide clarity on whether the CoTS populations maintain or exceed a natural, sustainable level. The removal dive will only be conducted when an assessment dive has concluded an outbreak of CoTS is occurring on the assessed reef.
An assessment pattern that sees divers/snorkelers swimming across a reef in a straight line parallel to the shore whilst recording the number of CoTS observed will be used during the assessment dives with the intention of increasing efficiency and area covered. The surveys will be conducted at depths 4-10m and varied in straight-line distance which will depend on a number of factors, e.g. current strength, dive time, etc. An approximate width of 10m will be surveyed along these straight-line transects. For the removal dive, the CoTS removal team search in a free movement pattern that enables an efficient and targeted search. Whenever a CoTS is observed, a team of two snorkellers/divers will work together in collecting the individual; one person will remove the CoTS with a pair of tongs, and the other person will open the rice bag for the CoTS to be placed in. Prior to collecting the CoTS, one person will record the size of the individual, found at which depth and associated with which substrate. Any signs of immediate predation will also be noted.
The assessments will be carried out over the next few weeks and a follow up report with results from the assessments will be out following that so keep an eye out for that!
