Pine passed them in his truck as they stopped for lunch.
Half an hour later they found a set net with no-one looking after it, in contravention of the Whitebait Fishing Regulations.
Under that legislation it was illegal not to remain within 10m of a net once set.
Pine arrived at the spot five minutes after the rangers and admitted he had left his post for 40 minutes, claiming to have used the toilet and had a cup of tea.
Judge Michael Crosbie fined him $200 but deferred payment until more than a month after the defendant was due to be released from prison.
Robert Hugh Faddes (64) also landed in the dock this week on a similar breach.
On September 12, Doc rangers found him on the Matau branch of the Clutha where he had set up a screen from the riverbank to the far edge of his stand.
"The screen created a barrier to whitebait from the riverbank, forcing fish to swim towards the net," prosecutor Pene Williams said.
At 8.3m, the screen significantly exceeded the maximum length of such apparatus of 6m.
While it theoretically gave Faddes an unfair advantage, the court heard it did not play out that way.
"He never caught any fish that day," counsel Steve Turner said.
"This is a case of an amateur getting involved in a heavily regulated activity."
Judge Crosbie fined Faddes $400 and ordered confiscation of the net seized that day.
"This type of offence is quite an important one in our country because the resource is a scarce one," he said.
