The Parties to the Nauru Agreement or PNA says it will object to any new membership to the Western Pacific Central Fisheries Commission.
The Tuna commission, which is the governing body for the world's largest tuna fishery, is meeting this week in Nadi in Fiji.
Several South American countries have recently expressed interest in becoming members of the Commission, one of which is Ecuador.
The chief executive of the PNA, Ludwig Kumoru said one of the issues that PNA is strongly opposing to is having new members, who have minimal interest in the fishery, admitted to the commission.
He said having countries without direct fishing interests admitted to the commission would weaken island countries influence on the management of the fishery.
"For PNA we always look at, look most of these waters in the Commission area belongs to the coastal states [they are] not the high seas," Mr Kumoro said.
"So when you bring in more and more [members] they are actually coming here [to the WCPFC] to make a decision on Fish that is in our waters.
"So you are inviting other people, most of them have no interest [In actually fishing in the Pacific] to come and make decision for us how we should run our business in these waters. That is not right."