Habitat modification in salt marsh: long term impacts and selected complementary research

Abstract

The salt marsh mosquito, Aedes vigilax (Skuse), is a major vector of polyarthritic diseases such as Ross River and Barmah Forest virus. These are not fatal but are debilitating (MacKenzie et al. 1998, Russell 2002)). Ross River virus (RRV) has been the major arbovirus in Australia over recent decades with up to 8000 cases each year. Between 1993 and 2007 there were 62815 notifications of RRV in Australia and 52% (32789) of these occurred in Queensland (data from the Commonwealth Department of Health and Ageing). The disease exacts a social and economic cost to society that has been estimated to be $AUD1,000 - 2,500 per person (Boughton 1994, Harley et al. 2001, Ratnayake 2006). Runnelling is a minor form of Open Marsh Water Management that can be used to manage larval populations. It was implemented at Coomera Island in Queensland in 1985 and has been described in (Hulsman et al. 1989). Runnels are shallow (less than 0.30 m deep and 0.90 m wide (three times as wide as deep) spoon-shaped channels that connect isolated salt marsh pools to the tidal source. This increases connectivity and tidal flushing increasing predator access and flushing larvae from the marsh. In the broad area of salt marsh management for mosquito control (Dale and Hulsman 1990) reviewed a suite of habitat modification methods, concluding that runnelling had the least impact, a view supported by later research comparing its impacts with those of other water management methods (Dale and Knight 2006). As with all forms of mosquito control it is necessary to assess impacts on non-target aspects of the environment. (Latchford 1997) assessed short term impacts in Western Australia, and found minor effects. There have been several assessments of the impact of runnelling at the reference site at Coomera Island in south east Queensland. These have covered relatively short term impacts (Breitfuss 2003, Dale and Dale. 2002, Dale et al. 1993, Dale 1988) and have used novel approaches such as detecting change using remote sensing (Dale et al. 1996), assessing impacts on process rather than the state of variables (Dale and Dale 2002) and have considered detailed impacts such as on oviposition (Dale et al. 2002), crabs (Breitfuss et al. 2004, Chapman et al.2004). The most recent research focussed on field variables monitored at the Coomera site (Dale 2008). The general conclusion has been that there were no significant effects of runnelling on marsh function in modified marshes. The aim here is to outline the changes over time that occurred since runnelling was implemented in 1985 and until the end of 2005. It will also illustrate the results of some related research demonstrating an interdisciplinary approach to maximise the overall scope of the monitoring.