MPI obtained their cellphone numbers, call data and text messages which indicated the brothers were taking large quantities of kina from Northland, processing and placing them into baskets for sale on the black market in Auckland.
Both made two trips to the City of Sails between February 1 and March 31, 2015 and sold the kina to Davis.
Between February 1 and 7, they went diving and harvested an excess of 14 sacks of kina.
Davis onsold the kina at $12 per basket and earned an estimated $636.
MPI also discovered Davis deposited $648 into Reo Uerata's bank account on February 26, 2015.
Reo Uerata said the offending had been playing heavily on his conscience and that he wanted to make some money as he was going through hard times.
The other two declined to comment.
Judge McDonald said since the introduction of the quota management system that limited the quantity of fish to be taken, the country has had a serious problem with unlawful fishing to service a large black market trade, particularly in Auckland.
He said although kina were in a healthy state, any illegal commercial harvesting had a direct impact on MPI's ability to accurately assess stock levels.
He fined the brothers $4000 each.