It was a blow that the couple said they couldn't have come back from without the support of the wider Horowhenua community.
Owlcatraz put Shannon on the world map when a picture of its 2.8-tonne cattle beast named Big Red went global in 1997, appearing in newspapers across Europe and Asia.
The giant, reputedly the world's biggest cattle beast, drew international tourists to Owlcatraz to see him up until his death in 2008.
Mrs Campbell said even today visitors to the park ask to see Big Red, his taxidermied head now proudly on display in the park's reception.
Owlcatraz is home to six moreporks (ruru) and is New Zealand's only morepork breeding programme, also caring for and rehabilitating moreporks with secondary poisoning from 1080.
The ruru is a Maori taonga (treasure), believed to have originated from the underworld and strongly associated with the spirit world.
"I love the owl. It's an extraordinary bird with more intelligence than most," Mr Campbell said.
He said the ruru fly through the canopy of the bush. The darker it is the better it hunts.
"It uses infrared to find its prey.
"We feed them 30,000 mice per year that we buy frozen then heat so that the ruru can find them."
From his own observations, Mr Campbell said due to 1080 secondary poisoning ruru populations in the Tararuas have dropped from 109 pairs to 16 pairs.
"Owls can only have two eggs at once and one clutch per year. The population can't bounce back from the poisoning."
Mr Campbell said the North Island weka population is also on a steep decline and in trouble due to 1080 drops.
He believed their breeding programme at Owlcatraz was the only one of its kind.
The couple said they credit the success of their business to the personality they inject, saying a successful business is 50 per cent product and 50 per cent personality.
To celebrate their 20th birthday they have printed commemorative t-shirts with the slogan '1997-2017 20th Anniversary, Owl be Back', which can be bought at the wildlife park, and a 'two children for the price of one' offer will run throughout June.