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
The Narrows received an overall water quality score of 0.79 (B).
Sediment quality of The Narrows was very good with an overall score of 0.95 (A).
The following graphs compare the Water and Sediment scores reported for 2023 (top line) to those from the previous year.
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 The Narrows scored 0.79 and received a good grade (B), similar to the 2022 report card. This score was calculated by aggregating the three sub-indicator and associated measure scores (Physiochemical – pH and turbidity, Nutrients – total nitrogen, total phosphorus and dissolved metals, and dissolved metals – aluminum, copper, lead, manganese, nickel and zinc).
Within the Physiochemical sub-indicator, The Narrows received a score of 1.00 and a very good grade (A) for pH indicating that the average pH was well below the guideline value. This zone received a score of 0.61 and a satisfactory grade (C) for turbidity indicating that average turbidity was within the guideline value range.
Within the Nutrient sub-indicator both total nitrogen (0.61) and total phosphorus (0.58) had satisfactory grades (C), indicating that average concentrations for these measures were within the guideline values for this zone. In contrast, chlorophyll-a had a lower score (0.47) and a poor grade (D) as the average concentration was above the guideline value.
All six Dissolved metals received high scores (1.00) and very good grades (A) indicating that average concentrations were well below the water quality guideline values.
For Sediment quality, The Narrows received an overall score of 0.95 and a very good grade (A), also a similar result to 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).
In 2023, six of the seven Metal and metalloid sub-indicator measures received very high scores (0.89–1.00) and very good grades (A), while one measure – nickel – received a score of 0.75 and a good grade (B). These results reflect that all concentrations were below the guideline values for this zone.
HABITATS
SEAGRASS (MEADOW 21)
The Narrows has one monitored Seagrass meadow at Black Swan Island. It is an intertidal meadow with variable biomass.
The overall grade for this meadow was very good (A) and received a score very similar to the previous year. All three sub-indicators received very high scores – demonstrating the high meadow biomass, a very good meadow area and a high presence of the historically dominant species.
MANGROVE RESULTS
No mangrove assessment was conducted in 2023, so the above results are from monitoring conducted in 2019. 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 RESULTS
The Fish indicator is comprised of two sub-indicators, Fish Health and Fish Recruitment.
Fish recruitment was assessed for two species: Yellowfin bream Acanthopagrus australis and Pikey bream Acanthopagrus pacificus. The overall score for 2023 was lower than the previous year (2022: 0.64, 2023: 0.37) and declined one grade to
poor (D). This decline in score is a result of a decrease in number of bream caught during the 2022–23 reporting period
(2022 report card: 139 fish, 2023 report card: 75 fish).
Fish health is assessed though two sub-indicators, Fish condition and Fish Health Assessment Index (HAI).
The overall score for Fish health in 2023 was 0.81 and received a good grade (B). The 2023 results were calculated by the average of the Fish HAI score (0.90; results carried over from 2021) and new assessment of Fish condition which scored similar to the previous year (2022: 0.74, 2023: 0.73).
MUD CRAB RESULTS
The overall zone score for The Narrows was 0.50, indicating a satisfactory grade (C). The abundance sub-indicator decreased slightly from the previous year and declined to a satisfactory grade (C), a reflection of a lower catch rate. The sex ratio remained unbalanced (low score), while the prevalence of rust lesion remained low (high score).