RECENT weeks have seen an abrupt shift in the pattern of irregular migration in the Mediterranean. A deal last year between the European Union and Turkey all but sealed the eastern sea route to Greece. Italy then received the lion’s share of arrivals, with more than 100,000 people entering the EU in 2016 from the south, most having set off from Libya. Now that route appears to have been disrupted too. Arrivals in July were down more than 50% on last year. The fall this month looks as if it will be even steeper: by August 22nd, 2,745 migrants had landed without authorisation in Italy, compared with 21,294 in the whole of August last year. The number of arrivals in Spain, however, is rising: on August 16th, more than 600 migrants were rescued—the highest one-day figure since 2014. What is going on?

Various explanations have been offered. The EU’s border control agency, Frontex, has pointed to changeable weather, increased activity by the Libyan coastguard and violence in Libyan city of Sabratha, lately the main centre for migrant smuggling. Others have cited Italy’s tougher controls. The government has provided equipment for and trained the Libyan coastguard, as well as deploying its own naval vessels in support. It has offered funding to tribes in southern Libya to block the flow of economic migrants and political refugees. And it has imposed restrictions on NGOs carrying out search-and-rescue operations off the Libyan coast.

A report by Reuters last week offered a different, though not incompatible, explanation: that an armed group in Sabratha—possibly one calling itself Brigade 48—was stopping migrants from leaving and, in some cases, locking them up. If that is the key cause of the drop in numbers, the lull may be temporary. Since the fall of its late dictator, Muammar Qaddafi, Libya has become a patchwork of territories controlled by murderously competitive armed bands. The ascendancy of the particular group said to be clamping down on the traffickers may not last.

Leaving the responsibility of policing the Mediterranean to Libya has won enthusiastic backing in Italy and the rest of the EU. But it is at least as morally murky as last year’s deal with the increasingly authoritarian government of Turkey. The most decent argument in its favour is that it stops people risking their lives on what the International Organisation for Migration calls “the deadliest migration route in the world”, where almost 15,000 people have perished since 2014. There are two main counter-arguments. The first is that although the majority of those attempting the crossing are economic migrants, a substantial minority are fleeing persecution (often in Libya itself). Penning them up in Libya denies them their right to seek asylum. The second objection centres on the appalling conditions in the Libyan camps to which migrants are confined. German journalists who visited a women’s camp in June said a detainee claimed she had been raped repeatedly and “showed us her blood-stained clothing”. A UN report last year described conditions as “generally inhuman”. It described a “widespread pattern of guards beating, humiliating and extorting migrants”. Small wonder migrants are ready to risk their lives at sea.