Sixteen of 19 crew members ate fish on board the vessel, but only 14 have developed food poisoning. The fish was obtained in international waters.
Both the ship's captain and cook are suffering from food poisoning.
Saint John Regional Hospital implemented its code orange mass casualty plan on Saturday after 13 people with food poisoning arrived at the emergency room.
As of Saturday evening, three crew members were in the intensive care unit.
The crew told New Brunswick health officials they ate the fish around noon on Saturday, and went to the emergency room three hours later.
Because there are only five healthy crew members remaining, they have to stay on board the vessel for safety reasons.
According to the Saint John Port's website, the Balsa 85 is carrying potash for foreign ports.
A U.S. Coast Guard document lists the vessel's owner as Hiong Guan Navegacin Co. Ltd., a Japanese company. The company will have to fly in replacements for the ill crew because they will be kept in observation for several days.
The Balsa 85 was loaded and ready to leave port when the men fell ill on Saturday.
The sick crew do not live in Saint John and there is no danger to the city's residents, New Brunswick's medical officer of health, Cristin Muecke, told CBC News.