COMMENT: Fish kill at Clarrie Hall Dam

ON July 5, reports came in of hundreds of dead bass floating belly up at Clarrie Hall Dam. The next day Tweed Shire Council workers confirmed that about 500 Australian bass were found dead. Many keen bass anglers are asking how this ‘fish kill’ happened? And how will it affect the fishing in the dam?

How did it happen?

‘Fish kills’ as the name suggests is just a general term used for the events where a large number of fish have simply died. Many things can cause fish kills such as pollution and temperature spikes, but the most common cause is de-oxygenation of the water.

Dissolved oxygen is naturally stripped from the water during the process of decomposition, which is carried out by millions of microorganisms. These little guys have what’s known as a high oxygen demand, which they need in order to break down organic matter such as dead aquatic vegetation and leaves.

This decomposed matter can accumulate on the lake bed and can become very acidic and the surrounding water can become anoxic (depleted of dissolved oxygen). Many lakes including Clarrie Hall experience a seasonal ‘turn over’ of the different layers of temperature within the water column. During winter, as the water at the surface of a lake cools, it sinks to the bottom of the water column, displacing and overturning the warmer layers below. It is likely this anoxic water has been disturbed and caused the fish kill at Clarrie Hall Dam.

How will it affect the fishing?

These turn over events typically happen in deeper water. So, fortunately, it appears this fish kill has been isolated to a small area in the deepest part of the dam, near the dam wall. The area where the dead fish were found makes up a very small percentage of the dam. But the death of 500 bass is certainly concerning. So, the question is; what percentage of the population has just died?

Clarrie has had some impressive stocking figures over the years. For example, between 2001 and 2009 more than 180,000 bass have been stocked. Even when taking into consideration mortality rates from predators such as birds, other fish and even fisherman, there should be a considerable population of bass left in the dam. So aside from predators, what other factors could affect the survival and therefore the abundance of the stocked bass?

It could be said that the population of any organism is limited by the ‘carrying capacity’ of the environment. In other words, the amount of resources available (food and habitat) that an organism needs to survive, is a limiting factor of the abundance of a species.

At Clarrie Hall Dam I believe the main food source for the bass are firetail gudgeon. These little baitfish are found in nearly every square meter under the lily pads that line the margins of the lake. The lily pads provide the bass with an excess of shelter and endless opportunities to ambush their prey. With no shortage of food and shelter, the bass in Clarrie Hall are fat, fit and healthy.

In these ideal conditions, the likelihood of stocked bass growing out to maturity is better than average. With its plentiful resources, Clarrie Hall has and will continue to support a high stocking density of bass. As such, the recent fish kill is most likely to be a small dent in the overall population of bass at Clarrie Hall Dam.

Simon Fitzpatrick runs a bass fishing charter at Clarrie Hall Dam called Northern Rivers Sportfishing.

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