About fish kills
Opening Intermittently Closed and Open Lakes and Lagoons (ICOLL), like Belongil Creek or Tallow Creek, to the ocean is dependant on the:
- water level
- weather
- tidal conditions
- state of the beach berm and other factors.
When an ICOLL is opened, there may be a fast release of water from the creek to the ocean. This is called the drawdown. In most instances Council operations attempt to minimise drawdown by opening the creek mouth as the tide is on its way in.
However, each ICOLL opening is different. The drawdown, or rush of water out of the creek generally causes a rapid decrease in oxygen in the water, by drawing the stagnant water out of the catchment and into the main channel of the creek.
Three known reasons for fish kills:
- Drawdown depletes the oxygen from the water and without it fish and marine life cannot survive.
- The creek has been open to the ocean and the quality of the water is poor, resulting in a fish kill.
- Unexplained fish kills have been caused by pollution or storm-water run off.
Fish kills are distressing and the top priority for any artificial opening is to try to minimise the risk of this happening by having a flexible and adaptable ICOLL opening strategy - one that considers as many factors as possible prior to opening.
Minimising Fish Kills
A fish kill is distressing and minimising a fish kill is the priority when Council decides (in concurrence with authorities) to intervene or open the Belongil Creek or Tallow Creek mouth. Efforts to minimise fish kills during an opening include:
- Promotion of natural openings over mechanical openings by scraping the beach berm prior to heavy rainfall
- Ensuring mechanical openings coincide with an incoming tide
- Ensuring mechanical openings coincide with rainfall (if possible)
- Winter (cooler conditions) are better than opening during Summer
- Windy days are better to open rather than hot, still days.
Tallow Creek Fish Kill 2019
In June 2019, Council decided to mechanically open the mouth of Tallow Creek due to high water levels which had reached the trigger point of 2.2 metres AHD (above sea level) for many weeks. This decision was made after the Bureau of Meteorology predicted rain.
The opening of Tallow Creek occurred on an incoming tide but unfortunately minimal rain eventuated. The result was a large scale fish kill which was a very distressing event. While scientists are still working out exactly what causes fish kills, rainfall appear to be one of the main contributing factors.
There has since been internal and external investigations into why this fish kill occurred and Council has taken into the account the many outcomes and recommendations of these investigations. Council staff continue to work closely with NPWS, Marine Park Authority, NSW DPI Fisheries, Universities and Arakwal to improve the management of these important systems into the future.
The management of ICOLLs is a complex and difficult task. No two ICOLLs are the same and agencies, Councils and stakeholders involved in ICOLL research need to be flexible and adaptable when it comes to their management.
Council is committed to improving management and is:
- Applying lessons learnt
- Consulting with agencies and key stakeholders
- Partnering with universities and the Department of Planning, Industry and Environment on research projects
- Embarking on the state process to develop a Coastal Management Program for these systems.