"I just loved some of the aquatics, some of the tropical fish. Back in those days, there weren't a lot of people doing it so I had tropical marine fish at home.
"You couldn't buy them locally here so, myself and a friend, we used to buy them and have them flown in."
After leaving school at 15 he started working for the Napier City Council parks and reserves department in 1968, moving to the Aquarium in 1974.
In those days, the facility was known as the Hawke's Bay aquarium, and featured "little picture box windows with small tanks and fish", and even a crocodile, under what is now the Napier Conference Centre.
In the decades since he has shaped what is now the National Aquarium of New Zealand, having a hand in many of its milestones.
When the facility moved to its current site on Marine Parade, the building that opened in 1976 had design input from Mr Yarrall. He was involved in making the aquarium the first in New Zealand to house piranha, and the first to spawn groper and snapper in captivity.
It was a career filled with highs - from breeding tuatara, delivering education to "thousands and thousands of schoolchildren", to being part of the team that went out on the James Cook research ship.
"We brought the first live orange roughy up from 1400ft, and kept them alive. We were up there for days getting them up so they didn't get the bends, before we could take them back [to the aquarium]".
The Napier resident will be a familiar face to the 150,000 people who now come through the facility each year - not to mention the many families who have visited it in the past three decades.
He has also had the pleasure of giving tours to various prime ministers over the years, and even Prince Edward, whose favourite attraction was the ocean tank.
While some might think spending nearly half a century is a long time to work in one place, there was never a dull day, he said, with a mix of jobs to keep things interesting.
As well as being a manager, and exhibitions curator, Mr Yarrall has also held the role of the first fulltime diver, diving on a daily basis throughout the 1970s and 1980s.
In more recent years, he oversaw the relocation of the penguins from Marineland, set up the aquarium's popular living coral display, and been part of the kiwi recovery, and kiwi breeding programmes.
"Also we've rebuilt the whole aquarium twice and now we're in the process of planning to rebuild again."
The council has proposed a $45million expansion for the aquarium, with high-profile partners including Weta Workshop coming on board.
"It'll be a fantastic complex if this all goes ahead. It will be great for Napier but incredible for the region as well."
Although he will be resigning from fulltime work in December, Mr Yarrall will still be around the facility. "I love this place, but I've been here a long time."
After 50 years of a "seven-day-a-week job", he was looking forward to spending more time with his family, who he said had supported him throughout his career.
"This will be the first Christmas where I won't have to worry about what's happening ... well I probably still will but I'm not supposed to be.
"I guess I'm just lucky to have been able to work most of my life doing something that I have a passion for."