Results from a new dredging study in Western Australia’s Dampier Archipelago provide, for the first time, a framework for assessing likely impacts of dredging on coral populations, and for evaluating the timeframes and likelihood of population recovery from impacts.
In April 2014 and March 2015 studies of coral populations were undertaken at Enderby and West Lewis Islands in the Dampier Archipelago, outside the central Port of Dampier and largely unaffected by shipping or other port related activities.
The corals investigated in this study were Acropora millepora, Turbinaria mesenterina and massive Porites spp. (mainly P. lobata and P. lutea) because they were among the most common coral taxa on reefs of the Pilbara, and on many reefs globally.
Sampling locations in the Dampier Archipelago, Western Australia. Replicate sites with a permanent transects were established within each location at Enderby Island and West Lewis Island (Babcock et al 2017)
For each species, populations were carefully measured, mapped and tagged, then re-located and measured again one year later in order to compile measurements of colony growth, mortality and recruitment. In total 737 colonies were tagged and relocated.
|Tagged colonies of Acropora millepora, Turbinaria mesenterina and Porites spp. Each colony in the study was tagged and measured, then re-located and remeasured after 12 months to establish rates of growth and mortality (Babcock et al 2017)
Coral recruit, A. millepora. All recruits (colonies less than 2cm diameter) within census transects were recorded, to establish rates of recruitment. (Babcock et al 2017)
A. millepora is fast growing and one of the more common Acropora species of the region. Turbinaria mesenterina has a vase-like growth form and is characteristic of inshore reefs in the Pilbara region, such as those commonly affected by development. Massive Porites spp. are encrusting to hemispherical, and slow growing to more than several metres in diameter. Because of their large size and life history strategy, they resist environmental disturbances such as severe cyclone impacts and provide important physical structure to reefs.
Three general disturbance scenarios were simulated, using the models:
i. size dependent mortality of the largest colonies (e.g. due to a cyclone);
ii. severe reductions in recruitment (e.g. due to an extended dredging campaign); and
iii. chronic reductions in recruitment (e.g. due to a shift in underlying environmental conditions).
More specific scenarios were also simulated to evaluate what levels of disturbance (general mortality combined with recruitment failure) could be sustained while still allowing recovery of populations to pre-impact levels within five years, consistent with the definition of the Zone of Moderate Impact in the Environmental Protection Authority’s impact zonation scheme (see Technical Guidance: Environmental Impact Assessment of Marine Dredging Proposals EPA ).
Acropora millepora showed the most rapid recovery from mortality that affected adult colonies, while recovery was slower in T. mesenterina and slowest in Porites spp.
A. millepora however, was most affected by failure of recruitment, or sustained depression of recruitment rates. Recruitment rates did affect Turbinaria and Porites populations but the effects on adult numbers took much longer to manifest and their influence on population maintenance were smaller than in A. millepora.
Lead CSIRO researcher Dr Russ Babcock said modelling of coral populations under a range of scenarios representing likely impacts from dredging showed that, under a best case scenario, recovery within five years was only likely when impacts on live cover of A. millepora and T. mesenterina were less than 15 per cent.
“The results from the simulated impact events present a ‘best case’ scenario in that they represent a single impact,” Dr Babcock said. “In reality, there will usually be a range of impacts affecting individual corals, coral cover, and rates of recruitment, such as cyclones and/or coral bleaching that could affect communities within a decade.”
Babcock R, Gilmour J and Thomson D (2017) Measurement and modelling of key demographic processes in corals of the Dampier Archipelago. Report of Theme 4 – Project 4.7, prepared for the Dredging Science Node, Western Australian Marine Science Institution, Perth, Western Australia, 43pp.
The WAMSI Dredging Science Node is made possible through $9.5 million invested by Woodside, Chevron and BHP as environmental offsets. A further $9.5 million has been co-invested by the WAMSI Joint Venture partners, adding significantly more value to this initial industry investment. The node is also supported through critical data provided by Chevron, Woodside and Rio Tinto Iron Ore.