The Heiltsuk Nation began blockading a Department of Fisheries and Oceans field office on Denny Island, near Bellow Bella, on Sunday night while others camped outside the building.
By Monday afternoon, about 70 people were protesting, said Chief Coun. Marilyn Slett.
The band said there's not enough herring for a commercial fishery, but the federal government maintains the stock can support a modest harvest.
The Heiltsuk said commercial boats took about 625 metric tonnes during a two-day seine fishery last week.
Members are concerned that another gillnet fishery could open soon, leaving them without enough herring for sustenance, Slett said.
"We were pushed into this position. Our people are so afraid that the herring is being fished to levels that will devastate the stocks."
Fisheries Department spokesman David Walters said in an email that scientific forecasts have shown that the herring stocks can support "modest" harvesting.
In some areas, those forecasts support a harvest rate that's twice as high as the current level, he said.
"DFO is engaged in a continued dialogue with the Heiltsuk First Nation," Walters said.
"The department was open to compromise on setting aside or keeping closed key areas of the Heiltsuk First Nation for food, social and ceremonial and commercial Spawn on Kelp fisheries. This offer was turned down by attending Heiltsuk representatives."
Staff continue working with the Heiltsuk to monitor stocks, and RCMP and Fisheries officers remain on scene, Walters said.
Slett said she and the band's resource management director slept on the field office floor on Sunday night and kept warm with their coats and jackets.
"Comfort wasn't part of the plan," she said, adding the occupation happened quickly and followed failed talks on Sunday with the DFO's regional director general, Susan Farlinger.
Band members and Denny Island residents who are camping outside, and in some cases drumming and singing, have provided her and the management director with hot meals and sandwiches, Slett said.
The band has been communicating with Farlinger, who may be travelling to the office for a meeting, Slett said.
The Fisheries Department declined to confirm her travel plans.