Mr Nathan said there were four large boats targeting sharks in the harbour this morning.
With his powerful telescope, Mr Nathan said he could see the men using two-foot long barracudas as bait - much too large to be targeting anything but sharks.
"They're not fishing for kingfish or snapper. They're definitely after sharks."
Mr Nathan said a big game fishing boat had also joined in, "looking for a bit of sport".
While bronze whalers were "fair game", Mr Nathan said he'd never seen anyone fish for them before.
"It's funny that this has happened and all of a sudden there are guys out here fishing for sharks.
"That's the nature of the beast. It's mob mentality."
However, the president of a Whangarei fishing club says it is far-fetched to suggest people were maliciously hunting sharks in the harbour today following the fatal shark attack at Muriwai Beach.
Whangarei Deep Sea Anglers president John Robbins said there was no evidence to suggest anyone was maliciously targeting sharks.
"No fisherman that I'm aware of in their right minds would go and hunt sharks as a revenge attack," he said.
Mr Robbins said some sports fishermen caught, tagged and released sharks, and at least one research organisation had been tagging hammerheads off Whangarei recently.
"There would be people fishing possibly for sharks in Whangarei Harbour every day of the week, but they're not fishing them necessarily to kill them," Mr Robbins said.
"I honestly don't [think there would be people hunting sharks after the attack at Muriwai. Those particular sharks would be nowhere near this area."
Forty-six-year-old father Adam Strange was killed at Muriwai Beach, west of Auckland, yesterday afternoon while swimming.
Police shot the shark, believed to be a great white, but it is not yet known whether it was killed.
- nzherald.co.nz and APNZ
