Offshore Wet Tropics: Waterway Health Flowing to the Great Barrier Reef

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.

For 2020-21 an overall score and grade for the offshore marine zone was not available due to insufficient data. To produce an overall grade and score at least two of the three indices are required.

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

WATER QUALITY

For 2021-22 there was no water quality monitoring program in place to allow for reporting on offshore water quality. For previous years offshore water quality results were obtained from the BoM Marine Water Quality (MWQ) dashboard which was decommissioned in 2021. Alternative data sources are to be identified for reporting offshore water quality as from the 2022-23 reporting year.

CORAL

The overall coral condition grade in 2021-22 for the Offshore zone improved from ‘moderate’ in the previous year to ‘good’, with the score increasing from 50 to 62.

The 2021-22 condition assessment is based on surveys of 9 offshore coral reefs. Offshore coral condition is assessed from the juvenile density, coral cover, and coral change indicators.

Hard coral cover increased to its highest level since 2016-17 for the offshore zone across the nine surveyed reefs. During the summer high sea surface temperatures occurred in the offshore zone. The next round of surveys will further assess the extent of heat stress and coral bleaching on coral condition.

The 2021-22 reef surveys recorded no potential, incipient or active crown of thorns starfish outbreaks in the offshore zone. All reefs have shown a general improvement in coral cover following impacts from heat stress and crown-of-thorns starfish between 2016 and 2018.

The sampling design for the monitoring program covering offshore coral was updated from 2021-22 onwards. For the Wet Tropics region, the monitoring program previously included 15 reefs with a subset monitored in alternating years. The updated sampling design has reduced the number of surveyed reefs to nine and conducts surveys at all reefs every year. Details of the changes to the reefs that are surveyed are presented in the methods technical report.

Whilst this change reduces the number of reefs monitored it has the distinct advantage of increasing the frequency of sampling from a two-year to one-year cycle. The previous design involved rolling scores forward for reefs not sampled in a given year, and meant that there was a lag in the condition assessment for reefs not surveyed for the reporting year.

The offshore coral results up until 2020-21 present the results and grades from the previous sampling design, whilst the results from 2021-22 present the results and grades from the updated survey design.