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 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 2016-17 for the offshore zone is ‘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 99.3 (‘very good’) in the Offshore zone is comprised of a total suspended solids (TSS) score.
The chlorophyll a score in the Offshore zone is 99.6 (‘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 2016-17 for the Offshore zone is 64 (‘good’).
Intense and extensive mass coral bleaching events occurred in early 2016 and again in early 2017, as a result of unusually high sea temperatures. This affected reefs in the Wet Tropics offshore marine zone.
However these impacts on coral reefs events are not fully represented in the offshore coral indicators for this reporting period due in part to the survey design for offshore coral reefs. Surveys were also conducted before the extent of mortality from bleaching was evident. Therefore, the coral index score for 2016-17 does not fully reflect the impact of the 2017 bleaching event.
Coral cover on reefs in the Wet Tropics offshore zone decreased to the lowest level on record in 2012 following the impact of Tropical Cyclone Yasi in 2011, but was recovering well until 2016 (Schaffelke 2017).
Surveys of offshore reefs in early 2017 found that coral cover had declined due to thermal bleaching in early 2016 and to continuing Crown‐of‐Thorns starfish (COTS) activity in the region (Schaffelke 2017).
The coral loss from bleaching was greatest at survey sites in the northern part of the offshore zone (north of Cairns) whilst reefs to the south experienced less thermal stress. COTS were most numerous on the southern reefs.
The scores for the coral cover indicator decreased from 0.60 in 2015-16 to 0.51. The score for the juvenile coral indicator decreased slightly from 0.96 to 0.95, and the score for the coral change indicator increased slightly from 0.54 to 0.56, since 2015-16.
To reiterate, these changes in the scores under-represent the impact of the recent coral bleaching and COTS activity.
The fish index is currently under development and will be available in future report cards.
Further information on the methods used to produce the scores and grades and detailed results can be found here.