ROME May 11 (Reuters) - Italian police arrested seven people on Wednesday for running a people-smuggling ring in which Somali boat migrants who reached Italy by boat were held prisoner until their families paid their passage further north, a statement said.

A court in Catania in eastern Sicily ordered that 13 people be detained for running the smuggling operation, but only seven were picked up. The others were thought to be living abroad.

The investigation, dubbed "Somalia Express", raided nine apartments in and around Catania used by the group to hold migrants in lieu of payment. Thirty-seven Somalis were freed from the apartments when the arrests were made, including three minors, the police statement said.

Families used pre-paid credit cards, or hawala, an informal payment system based on personal relationships, to pay off the smugglers, who would pick them up from migrant shelters in Sicily and neighbouring Calabria, at the southern end of the Italian peninsula.

The money was then used to buy bus or train tickets for the migrants to send them to their final destination further north in Europe or within Italy, and for fake documents that allowed them to move freely, police said.

"These organised networks could continue to grow. We're not optimistic," Catania police chief Marcello Cardona told reporters after the arrests. Catania investigators broke up a similar group focused on Eritrean migrants in 2014.

As of May 10, 31,250 migrants had reached Italy by boat this year, a 14 percent decline from the same period last year, according to the Interior Ministry. This year, about 2,500 Somalis have reached Italy, compared with 12,176 last year.

Italy is one of the front-line countries in Europe's worst migration crisis since World War Two. More than 320,000 came to Italy by boat in 2014-15, fleeing poverty and persecution at home and seeking a better life in Europe. (Reporting by Steve Scherer; Editing by Mark Heinrich)