Think you're sick of the rain? Think of the fish | Oceanwatch Australia

Think you’re sick of the rain? Think of the fish

Is the current climate of constant rain in NSW getting to you? Is that side ways rain getting under your umbrella? Well, spare a thought for the fish that perish because of large downpours.

This morning oyster farmers on the Macleay River and community members at Hastings Point have been reporting fish kills occuring.
So what are fish kills and who or what is killing them?

Fish kills are the significant and sudden death of aquatic animals. Many fish species cannot tolerate sudden changes in water conditions and their death is often an indication of problems in the environment.

Humans have modified catchments (particularly floodplains), often promoting the formation of acid sulfate soils and blackwater.

Acid sulfate soils are naturally occurring sediments that are deposited under estuarine conditions. They contain iron sulfides, most commonly as pyrite, and/or the products of iron sulfide oxidation. When exposed to oxygen through drainage or disturbance, these materials produce sulfuric acid, often releasing toxic quantities of iron, aluminium and other metals.

Blackwater events Occur as water inundates the flood plain, it picks up carbon in the form of plant matter (leaves, grass and bark), animal faeces and other debris. Floodwater picks up these carbon sources when it floods any dry floodplain – including forests and cleared floodplain like farmland.

As these massive inputs of carbon decompose and leach into the water turning it black in colour – thus the name. Decomposition of this material by microbes also, sucks the dissolved oxygen from the water making it hypoxic (low in dissolved oxygen), and potentially deadly to anything that needs to breathe under the water.

Although acid sulfate soil and blackwater can form under natural conditions, through the construction of floodgates, weirs, dams, channels and irrigation, we have changed the flow and buffering effect of many of our natural systems.


As NSW and many other locations in Australia are currently experiencing heavy downpours keep an eye out for fish kills and report any concerns to the Fishers Watch 24 hour hotline 1800 043 536.

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