ENVIRONMENTAL
RESULTS
AREA: 129.25 km2
ABOUT THE ZONE
The Narrows is the northern outlet of the harbour. It connects the harbour to Keppel Bay near the mouth of the Fitzroy River and separates Curtis Island from the mainland. Curtis Island has a number of conservation zones including national parks, regional parks and state forests and is considered to have significant environmental and cultural value. The Narrows is lined by mangroves and saltmarsh, it provides sheltered water and is an important area for recreational and commercial fisheries. This zone has one monitored seagrass meadow—an intertidal meadow comprising aggregated patches of seagrass near Black Swan Island.
This zone has six water and sediment monitoring sites, one seagrass monitoring meadow, four fish monitoring sites, one mud crab monitoring site and two mangrove monitoring sub-zones.
WATER & SEDIMENT
WATER
SEDIMENT
The Narrows received an overall water quality score of 0.84 (B). This score was based on the scores for physicochemical (0.85), nutrients (0.66) and dissolved metals (1.00).
Sediment quality of The Narrows was very good with an overall score of 0.92 (A).
MEASURES RESULTS
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
pH = acidity TN = Total nitrogen TP = Total phosphorus Chl-a = Chlorophyll-a Al = Aluminum Cu = Copper Pb = Lead Mn = Magnesium Ni = Nickel Zn = Zinc
As = Arsenic Cd = Cadmium Cu = Copper Hg = Mercury Pb = Lead Ni = Nickel Zn = Zinc
The Narrows received a very good score of 1.00 for pH indicating that the average pH was well within the guideline value range. This zone received a good score for turbidity (0.69) indicating that average turbidity was below the guideline value.
All six dissolved metals received very good scores (1.00) indicating that average concentrations were well below the water quality guideline values.
The Narrows received a good overall score for nutrients (0.66). Both total phosphorus (0.83) and total nitrogen (0.68) had good scores, indicating that average concentrations for these measures were below the guideline values for this zone. In contrast, chlorophyll-a had a poor score (0.48) as the average concentration was above the guideline value.
HABITATS
SEAGRASS (MEADOW 21)
The Narrows has one monitored meadow at Black Swan Island. It is an intertidal meadow with variable biomass.
The overall condition of the seagrass meadow in this zone was very good (0.90) for the first time since 2009—a continued improvement from the poor condition in 2018. Improvements in overall meadow condition from the previous year were largely driven by increases in seagrass biomass—which more than doubled—and presence of the historically dominant species (Z. muelleri) which accounted for 96% of the mean biomass. The overall score was the result of very good conditions for biomass (0.90), area (0.97) and species composition (0.97).
MANGROVE RESULTS
Very good (0.85-1.00)
Good (0.65-0.84)
Satisfactory (0.5-0.64)
Poor (0.25-0.49)
Very Poor (0.00-0.24)
No data available
The overall mangrove condition score was 0.65 (B) indicating a good condition for mangroves. Mangrove extent (0.79) was in a good condition and shoreline condition (0.61) and canopy condition (0.55) were satisfactory.
The overall score in The Narrows improved from 0.56 in 2018 to 0.65 in 2019. However, as with the previous year, evidence of shoreline erosion and indications of scouring and erosion of upland marginal vegetation were observed.
FISH & CRABS
FISH HEALTH RESULTS
HARBOUR SCORE
VISUAL FISH CONDITION: HARBOUR SCORE
FISH HEALTH ASSESSMENT INDEX: HARBOUR SCORE
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
Fish Health (Overall)
The overall score calculated for fish health in 2021 was 0.82 (B), which is an average of the scores for Visual Fish Condition 0.74 (B) and the Health Assessment Index 0.90 (A).
Visual Fish Condition:
The overall score for the visual fish condition is an average of 0.97 (A) for Fish Visual Assessment (FVA) and 0.50 (C) for Fish Body Condition (FBC).
The high scores for FVA are a result of a low incidence of poor visual health. All fish species assessed for this metric received very good scores ranging from 0.94 (A) to 0.98 (A). This result was similar to the HAI scores for external measures where a very low number of external health issues were recorded.
The satisfactory score for FBC in 2021 was a result of poor scores for yellow-finned bream (0.47) and pikey bream (0.48). Both species assessed had a mean body condition below the long-term average (2003 – 2020). However, the remaining three species assessed all had satisfactory scores ranging from 0.54 to 0.55.
Fish Health Assessment Index (HAI):
The overall HAI score for Gladstone Harbour was 0.90 (A).
This was comprised of scores from five fish species, barramundi 0.98 (A), barred javelin 0.90 (A) and blue catfish 0.81 (B), bream 0.98 (A) and mullet 0.81 (B).
In general, the surveyed fish species showed very few signs of external health issues, a similar result to the Visual Fish Condition sub-indicator. Scores for internal organs were also low indicating good to very good fish health.
FISH RECRUITMENT RESULTS
Fish recruitment was assessed for two species: yellow-finned bream Acanthopagrus australis and pikey bream Acanthopagrus pacificus. The overall score for 2021 was 0.62 (C) similar to the 2020 score of 0.64. The final scores were measured against a 2012 to 2020 baseline. The 2021 score for fish recruitment indicates a season with higher recruitment rate (increased catch rate) relative to the mean reference level determined over the baseline period. The total number of bream caught in the 2021 reporting year was 626, 329 yellow-finned bream and 297 pikey bream. Pikey bream tended to dominate in the northern sites while yellow-finned bream tended to dominate in the southern sites.
MUD CRAB RESULTS
The Narrows is lined by mangroves and saltmarsh, provides sheltered water for mud crabs and is an important area for recreational and commercial mud crab fishers.
The overall zone score for The Narrows was 0.64 indicating a satisfactory condition for mud crabs. This score was relatively comparable to the score from the previous four years (0.60–0.66). Sub-indicator patterns were mostly stable as abundance received a very good score (1.00) and sex ratio received a very poor score (0.00). The score for prevalence of rust lesions (0.92) was very good. While higher than in 2020, the prevalence of rust lesions score was comparable to those received from 2017 (0.90–1.00).