A 1.7m bull shark has been spotted in the Maroochy River swimming close to a spot becoming increasingly popular with holiday-makers, ski and SUP paddlers.

The sighting on Tuesday morning, which has been reported on the Queensland Shark Report website, Dorsal, is a reminder to swimmers and surfers to be wary of entering the water at dawn and dusk and when visibility is low and conditions overcast.

The shark was spotted swimming close to the end of the sand spit that extends from the Cotton Tree Caravan Park out into the mouth of the Maroochy River.

Earlier this month marine research scientist Dr Jodie Haig, of Ocean and Coast Research, who has helped tag and track 80 bull sharks to three metres in length since 2006, said as the top predator they played an important role keeping ecosystems healthy, picking off weak crustaceans and fish and in the process weeding out the bottom of the gene pool.

"They pick off weak animals, ensuring only strong genes are passed on,” Dr Haig said.

"That's how natural selection works.

"As they move to sea they have the same effect.

"They form an important part of the system and keep dolphin, turtle and fish populations healthy.”

Shark diver Tony Isaacson has warned that bull sharks have a predisposition to bump, feeling out prey, in murky conditions and researchers have found river systems to be an essential element of their life cycle.

Bull sharks born in Sunshine Coast river systems have been tracked as they mature travelling south and connecting with other animals off the Gold Coast and inside its waterways.

They follow distinct chemical cues from each river system and return to where they were born to give birth to their own pups.

Dr Jonathan Werry, of Ocean and Coast Research, told the Sunshine Coast Daily in 2015 that animals from 1.5m in length regularly move between the Sunshine Coast and the Gold Coast.

Individuals have been tracked as far north as to Cairns and south to Bondi, in Sydney.

Research has discovered bull sharks stay in the upper reaches of river systems from birth where salinity is low, moving down the systems as they mature before leaving in search of mates offshore as they reach six to seven years of age.

A female, measuring 2.7m in length, has been tracked entering the Noosa River during pupping seasons, staying two days before heading back to sea, presumably after giving birth.

Dr Werry said the animals appeared to be similar to marine turtles in that they returned to where they were born to give birth.

The sharks have been tracked using internal acoustic tags.

They are present in every south-east Queensland river system.

Despite their wide distribution and apparent prevalence, bull shark numbers are in decline globally.