ENVIRONMENTAL
RESULTS
ABOUT THE ZONE
Colosseum Inlet is an estuarine zone that is sheltered by Hummock Hill Island. Colosseum Inlet connects to both the Outer Harbour and Rodds Bay zones. It is outside the Gladstone Port Limits and inside the Great Barrier Reef World Heritage Area. The inlet has several large tributaries branching off the main creek and all are lined with mangroves and salt pan areas.
WATER & SEDIMENT
Colosseum Inlet received an overall water quality score of 0.79 (B).
Sediment quality of Colosseum Inlet was very good with an overall score of 1.00 (A).
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The following graphs compare the Water and Sediment scores reported for 2025 (top line) to those from 2024.
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 Colosseum Inlet scored 0.79 and received a Good (B) grade, similar to the 2024 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 chlorophyll-a, and dissolved metals – aluminium, copper, lead, manganese, nickel and zinc).
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Colosseum Inlet received a Very Good (A) grade for pH (1.00) a Good (B) grade for turbidity (0.71), and a Satisfactory (C) for total phosphorus (0.64). All of the six of the dissolved metals received a Very Good (A) grade, indicating that average concentrations for these measures were within the guideline values for this zone. Total nitrogen (0.48) and chlorophyll-a both received a Poor (D) grade.
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For Sediment quality, Colosseum Inlet received an overall score of 1.00 and a Very Good grade (A), the same result as the previous year. This score was calculated by aggregating one sub-indicator and associated measure scores (Metal and metalloids – arsenic, cadmium, copper, lead, mercury, nickel and zinc). The results reflect that all concentrations were below the guideline values for this zone.
HABITATS
MANGROVE RESULTS
The 2025 Gladstone Harbour Report Card incorporates the 2024 Mangroves results. Mangrove condition has historically remained stable due to strong buffering capacity, and since 2019, monitoring has been undertaken every five years. In 2024, the overall grade for Mangroves at Colosseum Inlet improved from a Satisfactory (C) in 2019 to a Good (B).
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The mangrove extent sub-indicator, which measures changes in canopy cover relative to saltmarsh and saltpan areas, retained a Very Good (A) grade. The canopy condition indicator, based on remote sensing of canopy density and health, declined from a Good (B) to Satisfactory (C). Shoreline condition, assessed through aerial surveys of mangroves bordering Gladstone Harbour’s water quality zones, declined from Satisfactory (C) to Poor (D) in 2024.
FISH & CRABS
FISH RECRUITMENT RESULTS
Fish recruitment was assessed for two species: Yellowfin bream Acanthopagrus australis and Pikey bream A. pacificus. Fish recruitment remained at a Good (B) grade in 2025, with a similar score received across years (2024: 0.68, 2025: 0.67). The catch number of Yellowfin bream decreased (2024: 41, 2025: 38), although Pikey bream (2024: 12, 2025: 45) improved.