The New Zealand Sea Lion Trust (NZSLT) is hoping a $12,000 reward will lead to someone providing information, which may result in a conviction.
"Someone must know who is responsible," NZSLT co-chairman Shaun McConkey said.
The trust initially offered a $5000 reward after a sea lion pup was found dead from shotgun wounds in the Catlins area, on September 22 last year.
Two weeks later, Jade, the pup’s mother, was also found shot dead.
Since then, support from the Otago Peninsula Eco Restoration Alliance (Opera) and donations from community members has allowed NZSLT to increase the reward to $12,000.
A necropsy also found a third sea lion may have been stabbed at Waipati Beach in November and left to die.
The necropsy, conducted at Massey University, found the sea lion had fractures to her left humerus and ninth thoracic vertebra, and the cause of the injuries was "most consistent with a human-induced stabbing".
Department of Conservation rangers were notified about the wounded sea lion on November 11, but her injuries were so severe that she had to be euthanised.
Mr McConkey said the $12,000 reward now also applied to the death of the third, unnamed female sea lion.
He said the trust was "deeply distressed" by the third human-induced death of a female sea lion in the Catlins in just a few months.
"The death of any female sea lion is devastating for the recovery of this nationally endangered species, which only started breeding again on mainland New Zealand 30 years ago.
"These three deaths have set the Catlins population back many years," Mr McConkey said.
An Opera spokesperson said the "senseless", "tragic" and "abhorrent" killings of Jade and her pup were unacceptable and heartbreaking reminders that humans often posed the most dangerous threat to New Zealand’s vulnerable and endangered wildlife.
"This crime must not go without consequences."
Mr McConkey said the statute of limitations on any prosecutions under the Wildlife Act was one year.
"We’re getting close to nine months now, so there’s a potential that we might run out of time."
Given the incidents all happened in the same area, he believed they might be connected.
"Clearly there’s the same intent there.
"At the trust, we believe they’ve all been deliberate killings of sea lions and so I would expect the same sort of attitude and potentially the same person.
"We believe that someone will know something.
"Obviously the perpetrator knows. We’re hoping that someone else might know as well and that’s the intention of the reward — to try and encourage people to do the right thing and to come forward."
Mr McConkey urged anyone with information to make contact.
"Any information will be dealt with in the strictest confidence."
Since the reward had been offered, there had been no more incidents of harm to sea lions in the area, he said.
"If nothing else, we hope that the high levels of disgust shown by people in responses to our posts, and the fact that at least $2000 of that reward money has come from the public, has discouraged this behaviour.
"We hope that this level of support for sea lions pushes the message home that this sort of behaviour isn’t OK and that most people are not going to accept it.
"Hopefully, those responsible are much more aware that someone in the community is watching now."
