TOKYO — The Fisheries Agency has decided to dramatically cut the nation’s catch of immature Pacific bluefin tuna, in a bid to replenish the rapidly falling population of the prized fish.

Beginning in 2015, the agency will reduce the allowed catch of bluefin tuna that are 3 years old or younger — too young to spawn — by 50 percent from the average in 2002-04.

Immature Pacific bluefin tuna are referred to by names including “mejimaguro” and “yokowa,” and are popular among mass marketers as a comparatively inexpensive tuna. Prices may rise if catches are sharply reduced.

The number of adult bluefin tuna capable of spawning has been falling year by year due to overfishing. Experts have said they may drop below the record low recorded in the 1980s of about 18,000 tons.

Last December, Japan, the United States and other members of the Western and Central Pacific Fisheries Commission agreed to cut their catches of immature Pacific bluefin tuna in 2014 by 15 percent or more from the 2002-04 average.

However, the Fisheries Agency decided it would be difficult to restore the fish’s numbers unless larger cuts were made.

The agency’s decision to strengthen restrictions on Japan’s bluefin tuna fishing is an independent move by Japan. By taking the lead as the world’s largest consumer of tuna and demonstrating its intention to reduce the amount of fish caught, Japan hopes to encourage other nations to strengthen their restrictions as well.