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 slightly in 2019-20 from 74 to 73 but the grade remains 'good'.
The diagram below shows the detailed results for each indicator by year. Click on the timeslider to see data from previous years.
The overall water quality grade in 2019-20 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 97 (‘very good’) in the Offshore zone is comprised of a total suspended solids (TSS) score.
The chlorophyll a score in the Offshore zone was 100 (‘very good’).
These grades have remained consistent with those of 2014-15 onwards with very little change in the scores occurring.
The overall coral condition grade in 2019-20 for the Offshore zone remained ‘moderate’ with the score decreasing from 48 to 42.
The coral condition score has been declining each year since 2015-16.
Coral cover was the lowest scoring indicator with an overall grade of ‘poor’, and of the 15 reefs surveyed four were graded ‘very poor’, seven were graded ‘poor’ and four were graded ‘moderate’. Compared to 2018-19 the coral cover score increased slightly (from 26 in the previous year to 29) with Agincourt Reefs No. 1 and Arlington Reef improving from a ‘poor’ to ‘moderate’ grade.
The coral change score declined from 51 in 2018-19 to 37. The drop in grade of Feather Reef from ‘good’ to ‘very poor’ was a notable contributor to this decline, reflecting the cumulative effects of recent crown-of-thorns starfish outbreaks and the 2016/2017 mass coral bleaching disturbances events.
The juvenile density score decreased since 2018-19 (68 to 62) with several reefs showing notable changes, both positive and negative, although the most substantial change was the decrease at Hastings Reef which declined from ‘very good’ to ‘moderate’.
Crown-of-thorns starfish individuals in the sub-adult and juvenile size classes were detected at Farquharson Reef and Peart Reef, whilst three reefs were classified as ‘recovering’ from the active outbreaks reported in 2018.
During the reporting year the Wet Tropics offshore zone was under the bleaching warning, issued by NOAA Coral Reef Watch on the 13th February 2020. This event coincided with the 2020 coral surveys which recorded moderate levels of bleaching across most reefs with notable bleaching at Thetford Reef.
The fish index is currently under development and will be available in future report cards.
Sweatman, H. 2018. Long‐term Reef Monitoring Program Annual summary report on coral reef condition 2017/18. Australian Institute of Marine Science. Townsville.