Astles K, West G, and Creese RG (2010) Estuarine habitat mapping and geomorphic characterisation of the Lower Hawkesbury river and Pittwater estuaries. Industry & Investment NSW – Fisheries Final Report Series No. 117. Cronulla, NSW, Australia. 229pp.

Summary

The Hawkesbury Nepean Catchment Management Authority and the Hornsby Shire Council have recently finalised management plans for the lower Hawkesbury estuary (LHE). However, a better understanding of the distribution of estuarine habitats and the potential threats to those habitats from human activity within the LHE was sought to assist with the implementation of the plans. This project developed and trialled a methodology for doing this.

Eight different estuarine habitats were mapped during the project: seagrass, mangroves, saltmarsh, mudflats, sandflats, rocky reef, foreshore habitat and water column. Pittwater had the largest area of seagrass beds including extensive beds of the vulnerable Posidonia australis. Mangrove Creek had the largest area of mangrove forest and saltmarsh community. The fluvial delta and riverine channel also had large areas of mangroves. Mooney Mooney Creek had the most extensive areas of mudflats, but sandflats were most extensive in Pittwater and the marine reach. Rocky reef that was mapped had the largest extent in the fluvial delta, but it may have been under-estimated in other areas. The dominant foreshore habitat was naturally occurring soft sediment and was mostly found in the upper reaches of the estuary including Mangrove Creek, riverine channel, Berowra Creek, fluvial delta and Mooney Mooney Creek. Natural hard foreshore habitat was predominantly found in the lower portion of the LHE including Cowan Creek and Berowra Creek.

A qualitative ecological risk assessment method was used to evaluate the risks from human activities on the estuarine habitats in the LHE. The purpose of the risk assessment was to determine which habitats were at what levels of risk from which human activities and then to identify the issues that need to be addressed if these risks were to be reduced. The LHE was divided into sub-catchments and reaches and the risk assessment was done on each of these separately, enabling specific issues to be identified for each area. Seven human activities were assessed – recreational fishing, aquatic recreation, foreshore development, stormwater/catchment run-off, sewage, dredging and sedimentation and commercial vessel movements. Risk assessments for commercial fishing in the LHE had been done previously under the environmental impact assessment process for commercial fisheries in NSW and those results were also incorporated into this project.

The risk assessment identified substantial knowledge gaps with regard to the magnitude, frequency and duration of various components of many of the human activities. These knowledge gaps themselves contribute to the risk to habitats because the level of stress from these human activities on habitats could be large but there is insufficient information to appropriately manage them. In particular, knowledge gaps were found in the amount of recreational boating (non-fishing) throughout the LHE, nutrient loads at the sub-catchment and reach scales from stormwater, upper catchment run-off and non-point source pollutants, the proportion of contaminated sediments and subtidal erosion and accretion of sediments around habitat edges.

Foreshore development and some elements of recreational fishing generally posed the greatest risks throughout the different sub-catchments and reaches. The most important issue arising from these human activities overall is the need to examine the extent of the interactions (intensity and location) between these human activities and the habitats. Where the interactions are most intense, investigation of the condition of those habitats is needed to determine the most effective and efficient management responses.

The information collected during this project can now be used to generate spatial layers in a Geographical Information System (GIS) at the sub-catchment scale. In turn, this will highlight areas where risks are high and where further investigation, and possibly management intervention, is warranted. These GIS tools will greatly assist in better informed and more targeted management of the estuarine resources of NSW.