ENVIRONMENTAL
RESULTS
ABOUT THE ZONE
The Mid Harbour zone is the second largest of the harbour zones, and is bounded by Facing, Curtis and Boyne Islands. Most shipping enters the harbour along the Gatcombe channels in the southern end of this zone. The northern boundary of this zone also marks the boundary of the Great Barrier Reef Marine Park.​
WATER & SEDIMENT
The Mid Harbour received an overall water quality score of 0.80 (B).
Sediment quality of the Mid Harbour was very good with an overall score of 0.96 (A).
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The following graphs compare the Water and Sediment scores reported for 2024 (top line) to those from 2023.
WATER
SEDIMENT
Very good (0.85-1.00)
Poor (0.25-0.49)
Good (0.65-0.84)
Satisfactory (0.5-0.64)
Very Poor (0.00-0.24)
No data available
Overall, Water quality in Mid Harbour scored 0.80 and received a good grade (B), similar to the 2023 report card. This score was calculated by aggregating the three sub-indicator and associated measure scores (Physicochemical – pH and turbidity, Nutrients – total nitrogen, total phosphorus and dissolved metals, and dissolved metals – aluminium, copper, lead, manganese, nickel and zinc).
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Within the Physicochemical sub-indicator, Mid Harbour received a score of 1.00 and a very good grade (A) for pH, indicating that the average pH was within the guideline value range. This zone received a score of 0.60 and a satisfactory grade (C) for turbidity, indicating that average turbidity was within the guideline value range.
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Within the Nutrient sub-indicator group, total nitrogen (0.41) received a poor grade (D), indicating that concentrations for this measure were above guideline values. In contrast, total phosphorus (0.88) improved a grade to very good (A), and chlorophyll-a (0.51) declined a grade to satisfactory (C). Both measures were below the guideline values.
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All six Dissolved metals received high scores (1.00) and very good grades (A) indicating that metals were well below the water quality guideline values.
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For Sediment quality, Mid Harbour received an overall score of 0.96 and a very good grade (A), a similar result to the previous year. This score was calculated by aggregating measure scores (Metal and metalloids – arsenic, cadmium, copper, lead, mercury, nickel and zinc).
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In 2024, six of the seven Metal and metalloid sub-indicator measures received very high scores (1.00) and very good grades (A), while one measure – arsenic – received a score of 0.69 and a good grade (B). These results reflect that all concentrations were below the guideline values.
HABITATS
SEAGRASS RESULTS
Meadow 43
Meadow 48
Mid Harbour has two monitored meadows adjacent to the south-east corner of Curtis Island. Meadow 43, known locally as Pelican Banks, is the largest and most productive seagrass meadow assessed for the report card. It is also the only meadow where all three indicators are classed as stable or highly stable. Pelican Banks (Meadow 43) is an intertidal meadow, while Meadow 48 is a subtidal meadow neighbouring the eastern side of Quoin Island.
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The overall condition of the Mid Harbour Seagrass was graded poor (D) with a score of 0.49. This is a decrease from the previous report card (2023: 0.57; satisfactory grade, C) and reflects a declining trend since 2022 (0.67; good grade, B). In Meadow 43, changes in species composition from the foundation species Zostera muelleri ssp. capricorni (the species which were abundant during the 2022 report card reporting period), to smaller, less persistent Halodule uninervis and Halophila ovalis during this period contributed to continuing low meadow biomass score. Additionally, Meadow 43 is subject to high levels of herbivory from dugongs and turtles that may be altering the species composition and restricting recovery. The overall grade for Meadow 48 was satisfactory (C), a decline in grade from the previous year.
CORALS - ZONE SCORE
CORALS - HARBOUR SCORE
The 2024 Gladstone Harbour Report Card scores for the Mid Harbour zone are based on data collected from four reefs: Facing Island, Farmers Reef, Manning Reef and Rat Island. The overall score for Coral in the Mid Harbour zone was 0.17 and graded very poor (E), indicating a very poor condition.
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Compared to the 2023 results, coral cover scores remained similar at Manning Reef, Rat Island, Facing Island and Farmers Reef. Minor score variations have been observed over the past six years; however, these scores are still substantially lower than the 40% threshold required to receive a satisfactory grade (C) with both harbour zones graded very poor (E) for coral cover.
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Scuba surveys indicated that the bio-eroding sponge Cliona orientalis continues to impact the coral community across the whole harbour and in particular, colonies Porites and Cyphastrea on reefs in the Mid Harbour zone. Sponge prevalence is likely contributing to a lack of coral cover recovery across the harbour. In 2024, its presence was also recorded for the first time at Manning Reef.
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Farmers Reef was the only reef to receive a very good grade (A) for macroalgal cover due to an improved score from the previous years (2022: 0.28, 2023: 0.56, 2024: 0.92), indicating lower macroalgal cover. All other reefs scored a very poor grade (E) for the ninth consecutive year. Macroalgal communities are more variable at reefs in the Mid Harbour zone, where cover and composition vary for both from year-to-year. In 2024, communities at the four Mid Harbour reefs were dominated by the red macroalgae Asparagopsis or the brown macroalgae Lobophora.
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The harbour-wide density of juvenile corals has continued to decline, receiving a very poor grade (E) for the fifth consecutive year. All reefs at Mid Harbour either decreased scores or remained similar, retaining very poor grades (E). High coral cover in 2009 was linked to abundant representation of Acropora juveniles – a fast-growing, branching coral; however, in 2024, this juvenile genus was notably lacking across the harbour. Recovery of corals may be limited until this family reappears and survives.
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Overall, in Gladstone Harbour changes in hard coral cover has been graded poor (D) for the past seven years. In 2024, the change in hard coral cover score for Mid Harbour (0.23) remained very poor (E) for the second year in a row. In general, the ongoing low scores and very poor grades demonstrate demonstrate negligible recovery of coral communities in Gladstone Harbour to date.
MANGROVE RESULTS
Mid Harbour has two Mangrove monitoring zones – one at Curtis Island, and another at Facing Island. In 2024, the overall grade for Mangroves at Mid Harbour was poor (D), a decline in grade from a satisfactory (C) in 2019. In 2024, mangrove extent declined from a poor (D) grade in 2019 to a very poor (E) grade at Mid Harbour, indicating a net loss of mangrove area. The grade for canopy condition at Mid Harbour remained a satisfactory (C) in 2024. Shoreline condition of mangroves at Mid Harbour improved in grade from a satisfactory (C) in 2019 to a very good (A) in 2024.
FISH & CRABS
FISH RECRUITMENT RESULTS
Fish recruitment was assessed for two species: Yellowfin bream Acanthopagrus australis and Pikey bream Acanthopagrus pacificus. At Mid Harbour, Fish recruitment improved from poor (D) in 2023 to satisfactory (C) in 2024.