99 Crew dive club members hit out at the rubbish in and around Taranaki Wharf.

Clever detective work by school children and technical assistance from adults solved a mystery plaguing the beaches of Taranaki.

The students of Oakura School, southwest of New Plymouth, and Highlands Intermediate, in New Plymouth itself, were undertaking projects to understand and protect important species on their coast. While on beach visits, they repeatedly came across plastic shotgun wads.

These wads separate the gunpowder from the metal pellets (the shot) inside a shotgun cartridge. They have other purposes but when the gun is fired, they are pushed out the end of the gun and fly through the air for some distance before gravity pulls them back to earth.

The cartridges themselves – also plastic – typically fall to the feet of the shooter and are easily gathered.

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So what were the wads doing on Taranaki's beaches?

Oakura and Highlands schools were enrolled in an initiative called Project Hotspot, which uses citizen science to "better protect coastal threatened species". It's funded by Curious Minds, a central government initiative to encourage engagement with science.

PROJECT HOTSPOT Students found these plastic shotgun wads on Taranaki beaches. How did they get there?

Project Hotspot was able to call upon outside assistance, says Highlands science teacher Pat Swanson.

Taranaki Regional Council was already involved and turned to Allen Stancliff​ of the Taranaki Fish & Game Council. He suspected the wads were being carried down the Manganui River, which flows into the Waitara River, and then out to sea.

There's a gun club on the Manganui and some of its "traps" – or shooting ranges – are on the riverbank and wads could easily have been shot out over the river and fallen into the water and been carried down to the coast.

To test this idea, a privately owned company called MetOcean Solutions was called upon for it expertise in modelling and oceanography.

If the wads were entering the sea at the Waitara River mouth, then well-observed currents would deposit them on beaches in predictable places.

"There's a number of reasons why stuff washes ashore in certain places," said Mariana Horigome, the oceanographer who worked the project. "The wind and currents are the drivers, but the coastal aspect and shoreline profile also has to be right for objects such as the plastic wads to beach and not get refloated on the next tide."

She ran the model – computer software – and sure enough it showed the wads "spread widely to locations both north and south of New Plymouth, nicely replicating where the wads had been found" by the students.

"It was great to get some help determining where the plastic came from," said Dr Emily Roberts of Taranaki Regional Council. "The modelling confirmed our suspicions."

Most of this happened in 2016 but was barely reported. New generations of students are now carrying on with similar work, and 27 Highlands students, plus teacher Pat Swanson won an award from TRC, for studying more plastic litter on regional beaches.

PROJECT HOTSPOT The wads separate the gunpowder from the metal pellets inside a shotgun cartridge. When the gun is fired, they fly through the air and fall back to earth.

Among other plastics, they found small numbers of glow sticks used by the fishing industry. The disposable plastic lights are attached to long-line fishing gear and attract fish to bait.

They can fall off or get dumped and some wash up in Taranaki. Students were tracing the origins of the glow sticks, which might have been transported thousands of kilometres.

The class had also engaged with the NZ fishing industry.

"I've discovered that kids are very effective advocates," Swanson said. "I'm proud of them."

The gun club was switching to biodegradable wads.