An 8kg hapuku dosed with chlorine, formalin and peroxide is missing from the National Institute of Water and Atmospheric Research (NIWA) aquaculture centre's quarantine and disease investigation unit at Ruakaka in New Zealand.
And scientists fear kingfish involved in a sex reversal study were taken.
NIWA regional manager Michael Stobart told the Norther Advocate that scientists studying sex reversal had put hormones including oestrogen and other veterinary medicines into the kingfish.
"It's impossible to specify what would happen, but eating too much kingfish treated for sex reversal is the most scary aspect of it," he told the paper.
"We don't give the fish large doses, but the medicines we give them render them unfit for human consumption."
The Norther Advocate said that the burglar came over a 2m barbed-wire topped fence facing Ruakaka Beach on the former Marsden A power station site on Friday. Using nets on the site, the hapuku was taken from a tank where it was being held alone for breeding purposes.
There was evidence the intruder had been into other tanks, but NIWA staff will not know whether anything more was taken until the fish are counted during regular weight assessments.
Warning over Stolen Fish
NEW ZEALAND - Scientists from a New Zealand research institute where fish were stolen have issued a warning that they should not be eaten.