The single Offshore zone includes all offshore waters within the Wet Tropics NRM marine region. It extends east from the boundary of the mid-shelf water to the boundary of the Great Barrier Reef Marine Park, and is bordered north and south by the Wet Tropics NRM marine region.

Water quality in the Offshore zone is less affected by plumes of sediments, nutrients and pesticides from river discharge than the inshore zones due to its distance from the land. The boundary is typically a minimum of 20 km from the mainland. The closest point to the Offshore zone boundary is Cape Kimberley, just north of the Daintree estuary (approx. 14 km from the mainland).

The Offshore zone includes numerous coral reefs, 15 of which are included in the offshore coral surveys for the Report Card. Unlike the inshore zone where coral reefs often occur in the shallow fringing waters of continental islands, the coral reefs in the Offshore zone predominantly grow on limestone platforms that are relics of past phases of reef growth. These coral reefs provide habitats for an immense diversity of organisms including hard and soft coral species, ascidians, bryozoans, molluscs, fish, turtles and marine mammals.

The overall score for the offshore marine zone has declined from 83 in the previous year to 75 in 2017-18 with the grade dropping from ‘very good’ to ‘good’. 

The diagram below shows the detailed results for each indicator by year. Click on the timeslider to see data from previous years.

Water quality

The overall water quality grade in 2017-18 for the offshore zone remained ‘very good’. 

Water quality grades are based on water quality guideline trigger values established for the waters of the Great Barrier Reef Marine Park that, if exceeded, identify the need for management responses. The guidelines are not targets or assessments of pollutant loads entering the marine environment. The results are from remote sensed water quality indicators of offshore waters sourced from the Marine Water Quality dashboard provided through the Bureau of Meteorology. 

The water clarity score of 98.1 (‘very good’) in the Offshore zone is comprised of a total suspended solids (TSS) score. 

The chlorophyll a score in the Offshore zone was 99.9 (‘very good’). 

These grades have remained consistent with those of 2014-15 and very little change in the scores have occurred. 



The overall coral grade in 2017-18 for the Offshore zone declined from ‘good’ to ‘moderate’ with the score decreasing from 67 to 51. 

The scores for the coral cover indicator decreased from 51 (‘moderate’) in the previous year to 28 (‘poor’). The score for the juvenile coral indicator decreased 95 (‘very good’) to 71 (‘good’), and the score for the coral change indicator decreased slightly from 56 to 53 (remaining ‘moderate’), since the previous year.  

Coral cover in the Wet Tropics offshore zone decreased to the lowest level on record in 2012, following the impact of Tropical Cyclone Yasi in 2011 (Sweatman 2018). Coral on these reefs recovered rapidly up until 2016 (Sweatman 2018). Coral cover is now declining due to the cumulative impacts of multiple, severe disturbances over the recent years, including coral bleaching and crown-of-thorns starfish outbreaks (Sweatman 2018). 

The following key messaging was sourced from Sweatman (2018) and AIMS 2019 ( ) 

  • Intense and extensive mass coral bleaching events occurred in early 2016 and again in early 2017, as a result of extended periods of unusually high sea temperatures, and affected reefs in the Wet Tropics offshore marine zone.  
  • The recent impacts on coral reefs including the 2016 and 2017 bleaching events were not fully represented in the offshore coral indicators for the 2015-16 and 2016-17 reporting periods due, in part, to the survey design for offshore coral reefs which alternates survey sites between years and aggregates results from the most recent survey years (Table 63 and Table 64).  
  • For 2017-18 recent impacts from coral bleaching events are more apparent with the coral condition score dropping substantially from 67 to 51 and the grade declining from ‘good’ to ‘moderate’ (Table 62).  
  • The indicator driving the declining condition score is the substantial decrease of coral cover score from 0.51 in the previous year to 0.28 for 2017-18. Loss of coral which was primarily caused by coral bleaching with additional impacts from the current wave of crown-of-thorns starfish (COTS) outbreaks.  



The fish index is currently under development and will be available in future report cards.



Sweatman, H. 2018. Longterm Reef Monitoring Program Annual summary report on coral reef condition 2017/18. Australian Institute of Marine Science. Townsville. 


See the detailed reports for further information on the methods used to produce the scores and grades and the detailed results