Fiji's Fisheries Ministry says it has seized 1.2 tonnes of fish of which 600 kilograms are of the banned species donu (grouper) and kawakawa (mackerel tuna).
The temporary fishing bans came into effect on 1 June and ends on 30 September this year.
In June last year, a study showed the kawakawa and donu populations had declined by 70 percent over the last 30 years.
The ministry said in a statement three penalty notices had been issued.
It said the fish seized by the ministry's enforcement personnel had been preserved for evidence in legal proceedings.
The ministry warned that any person or business found violating the four-month ban period could receive fines of up to $US20,000 for individuals and $US40,000 for corporations.