Townsville SOE home page

  Unfavourable conditions in Tropical waterway systems
can lead to large and small fish kills

Fish Kills in Tropical Waterways

  Fish of tropical waterways in Northern Australia are usually extremely tolerant of low oxygen conditions in comparison to temperate fish. They are generally untroubled by oxygen levels around 1mg/L and many only show a behavioural reaction when there is almost no oxygen left. Behavioural responses include: stop feeding; breathing just under the surface or by gulping the surface film; jumping out of the water; disorientated swimming; lying on the bottom and gasping; and finally, death. Generally, when tropical fish (especially highly tolerant fish such as barramundi) start behaving in this manner this situation has gone past where actions can prevent a fish kill.


There are several documented causes of fish kills in tropical systems:

Council Responses & Actions You Can Take

1. Lack of oxygen
What is Dissolved Oxygen? (895kb .pdf)

a. Storm wash of organic matter in the water: early wet season storms sometimes wash large amounts of organic matter and other soil materials into water bodies, which can raise the Biochemical Oxygen Demand (BOD) of the water causing anoxic (extremely low oxygen) conditions.

b. Storm wash of oxygen depleted water: Storms can wash anoxic water from the organic rich flood-plain into water-bodies.

c. Storm induced mixing of the water: Late in the dry season, the upper layer of water heats up and the deeper water can become depleted of oxygen. Wind and rain can cool the surface layer and mix the water, reducing the oxygen levels. The mixing also stirs up organic matter from the bottom, further reducing oxygen to critical levels.

d. Oxygen consumption by plants: During the day, aquatic plants produce O2 as a by-product of photosynthesis, raising the level of O2 in the water. However, at night, the O2 is used up by the respiration of these same plants reaching a minimum at dawn. Plant decay during the wet season will aggravate this effect. High temperatures and declining water levels at the end of the dry season may result in there being little or no oxygen during the pre-dawn period.

2. Naturally toxic water

a. Acid sulphate soils and aluminium toxicity

b. Toxic plants

3. Disease

Fish suffering from disease are commonly seen in isolated water holes at the end of the dry season. It is assumed this is partly a result of increased stress from higher water temperatures (and hence lower oxygen levels) and crowding from declining water levels. External lesions, often diagnosed as red-spot disease are common on barramundi and catfish. This often causes disorientated swimming, blindness and death.

In the situation of Townsville waterways, large fish kills are likely to be a combination of low oxygen conditions (1) coupled with a relatively high biomass of fish exacerbating the problem.

Actions You Can Take:

Council Responses (TCC Urban Stormwater Quality Management Planning - USQMP, & Community Waterway Educator)

Associated Links

Fishweb (State Government - EPA & DPI Fisheries)

EdFish logo

Townsville Fishwatch