Objectives

• To develop guidelines for floodgate specifications and management.
• To assess the behaviour of estuarine fish in relation to the tidal flows through floodgates and the behavioural responses of recruiting juveniles to low concentrations of acid sulfate soil drainage water.
• To develop and implement an extension program to communicate the guidelines to agricultural groups, local government and other agencies that manage land and water resources in coastal floodplains.

Summary

Coastal floodplains and wetlands provide essential nursery habitats for many species of fish and prawns, several of which are important to commercial and recreational fishers. In eastern Australia, farming practices associated with grazing and intensive cropping can have detrimental effects on coastal floodplains, two of the most important for fisheries being the drainage of acid sulfate soils and the alienation of significant habitat areas for fish.

Artificial draining of floodplains is often achieved through the construction of floodgates and their associated drains. These structures act as barriers that restrict fish distribution and block migration by juvenile fish, particularly for migratory species such as Australian bass, sea mullet and school prawns. Artificial draining of floodplains with acid sulfate soils results in saturated soils becoming exposed to the atmosphere. This can cause a build-up of harmful chemicals such as sulfuric acid, iron and aluminum, which are washed into waterways during heavy rain, causing increased 'acidity' in the water which may then cause fish diseases or significant fish kills.

This study examined the effectiveness of two options for reducing the impacts of floodgates on fish in the Clarence River floodplain: different opening regimes of floodgates and different gate structures. Assemblages of estuarine species were consistently very different between systems with and without gates, with gated systems often being dominated by the exotic fish, Gambusia. Variables of concern in gated systems were the elevated concentrations of nutrients, the abundance of grasses and rushes, and the absence of mangroves. Different types of gate (e.g. mini-sluice gates or vertical lift-gates) did not improve things. An increase in the opening frequency of the gates, however, resulted in a significant increase in species richness. Juveniles moved into drainage systems with opened floodgates, regardless of whether the system was a modified natural creek or a man-made drain. Frequent and regular opening of floodgates should be implemented where possible, because any improvements to fish passage quickly disappeared when floodgates were not opened for prolonged periods of time. Opening of floodgates resulted in significant improvements in water quality, and also caused the disappearance of waterlillies, grasses and rushes due to tidal influx. Further improvements can be expected from tide-activated, automatic floodgates, but only a preliminary assessment of these was possible during this study.

Laboratory experiments examined the possible existence of acid avoidance behaviour in juvenile Australian bass, snapper, yellowfin bream and adult school-prawns. The results showed that all species avoided acidified water, with snapper showing the strongest responses and school-prawns the weakest. The acidic component of acid sulfate run-off alone has the potential to affect migration of these species in the field.

The project contributed to the ecologically sustainable management of coastal floodplains in eastern Australia by providing information on the relationships between floodgate management, species assemblages, water quality and habitat condition. This research helped in the development of new guidelines entitled 'Guidelines for managing floodgate and drainage systems on coastal floodplains' produced in collaboration with NSW Agriculture. As these guidelines are implemented, fishers, farmers and landholders should benefit from improved tidal flushing and better water quality in coastal drainage systems.