One in three Japan Coast Guard vessels is being used beyond its operational time limit, constraining the service’s capacity at a time when its mission is growing in importance amid Chinese muscle-flexing around disputed islands in the East China Sea, officials said Saturday.

The Japan Coast Guard officials attributed the aging of the fleet to a slow replacement process following the last mass fleet expansion decades ago after Japan extended its territorial waters and fishing zone in 1977.

The coast guard has set a 25-year use period for relatively large patrol ships in its fleet, and a 20-year use limit for its smaller patrol boats. Of the total 366 patrol vessels 129, or 35 percent, had exceeded their operational time limits by the end of March.

Six more vessels will have exceeded their time limits by the end of next March. Over a 10-year period after that, 98 more vessels are expected to reach their use period limits.

In 1977 Japan extended its territorial waters from 3 nautical miles (about 5.5 kilometers) from the shore to 12 nautical miles (22 km), while establishing a 200-mile (322 km) fishing zone.

The changes vastly expanded the area of coast guard activities, prompting the maritime law-enforcement authorities to build 107 vessels between 1977 and 1980.

The fleet began entering a replacement period around 2000, but because of budgetary constraints the coast guard has found it difficult to replace retiring ships with a matching number of new vessels, the officials said.

With Chinese ships frequently spotted around the Japanese-administered, Chinese-claimed Senkaku Islands in the East China Sea, called Diaoyu in China, the coast guard is also trying to procure new patrol vessels.

However, it costs over ¥10 billion to build the larger ships at a time when the service’s initial budget for new patrol vessels and aircraft is around ¥30 billion a year.

The coast guard is thus struggling to address the question of how to ensure it has enough vessels to carry out its mission amid budgetary constraints and how it can operate its fleet in an efficient way.

Older ships show their age in the most glaring of ways such as in engine performance, coast guard officers say.

Such ships are often reassigned to roles requiring lower speeds. They also have a tendency to break down, rendering it difficult to carry out assigned work, a coast guard officer said.