Flathead facts: let the big fish go!

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Patrick Linehan releasing the metre-plus monster croc.

FOLLOWING the catch and release of a 105cm dusky flathead last week and the social media comment and opinion frenzy that ensued, Fisho sheds some light on the spawning capacity of monster-size dusky flathead.

According to NSW DPI Fisheries research, dusky flathead grow relatively quickly. In fact, dusky flathead can reach 40cm total length after just three years. Female flatties mature at around 55cm total length while males mature much earlier at about 22cm total length.

To date, the oldest (and largest) dusky studied by NSW DPI Fisheries researchers is a 7.5kg, 98.5 cm fish estimated at 16 years of age.

As with almost all fish species, age varies enormously with size. At 105cm, last week's monster flathead could have been 15-plus years old.

Flathead age varies enormously with size. Graph: NSW DPI Fisheries

Research conducted by NSW Fisheries suggests female dusky flathead produce about 700 eggs per gram of body weight. Interestingly, Fisheries research says this ratio doesn't appear to change with body size indicating even very large female duskies remain viable breeders.

In NSW, dusky spawning season extends from November to March annually, possibly around the new or full moon periods. Dusky flathead are known as "multiple batch spawners”, meaning they have the capacity to spawn up to once per month if environmental conditions are favourable.

When it comes to Patrick Linehan's 105cm fish, let's conservatively estimate it as weighing 9kg (an estimate backed by Patrick and his mate who netted the fish). Using the eggs per gram ratio, this equates to the production of 6.3 million eggs per spawning event.

Under favourable conditions, this fish could have up to five spawning events a season, which could potentially result in the production of about 31.5 million eggs annually!

If 0.1% of these survive year on year, this single flathead has the capacity to potentially produce 30,000-plus flatties each year.

It is clear to see, the opinion that monster-size flathead could be "too old to breed" is far from fact.

Big flatties are far too valuable to kill. Let the big fish go!

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