BUCHAREST, Feb 19 (Reuters) - Ukrainian grain shipments through Romania's Black Sea port of Constanta fell 38% year-on-year in January, port data showed, raising concern among operators that export routes created since Russia's invasion will languish as Kyiv uses its own ports more.

Ukraine is one of the world's biggest grain exporters, and Constanta has become Kyiv's largest alternative export route since Russia's full-scale invasion in February 2022, with grains arriving by road, rail and barge across the Danube.

Its exports through Constanta, non-existent before the war, stood at 8.6 million metric tons in 2022 and surged to 14 million tons last year, aided both by European Union-funded investment in the port and by operators adding equipment.

But its transit volumes fell to 436,000 tons in January from 700,000 tons in January 2023, the port authority told Reuters.

"The perception among operators is that volumes are dropping, Ukraine's corridor through its own ports is working," Viorel Panait, the manager of Constanta port operator Comvex , told Reuters.

Ukraine created a shipping corridor from its own ports in August, which hugs the western Black Sea coast near Romania and Bulgaria, shortly after Russia withdrew from a U.N.-brokered Black Sea grain export deal.

Panait, who is also president of the Constanta Port Business Association, said new grain export control mechanisms introduced by Kyiv last year to prevent tax avoidance were contributing to the slowdown.

He added Romanian port operators, logistics and railroad companies had all invested to increase capacity to handle Ukrainian grain, while EU-funded support schemes were also underway.

"There is a concern that these export routes, established in the last two years, will be lost. Given the equivalent shipping costs through Ukrainian ports and Constanta, a rational way to preserve the new flows ... would be prudent," he added.

However, some market analysts said they expected volumes to kick back up in the spring.

"We are seeing a market lull, Ukrainian farmers are hesitating to sell at such low prices, but exports will resume in March-April," Cezar Gheorghe of Romanian grain market consultancy AGRIColumn told Reuters.

He estimated Romanian farmers would have an exportable surplus of 20-22 million tons of grains in the 2024/25 season.

In January, the U.S. State Department's Assistant Secretary for European and Eurasian Affairs Jim O'Brien said he expected Romania would remain Ukraine's largest alternative export route for grains and other goods, behind its own ports. (Reporting by Luiza Ilie Editing by Mark Potter)