Around 119,000 people arrived by sea in 2017 compared to 181,126 a year earlier, taking the number back to pre-2014 levels, the agency said.
The downward trend began in July following a 20 percent spike in arrivals between January and June. Some 10,400 landed at Italy's ports during the last three days of June alone, prompting the country to introduce controversial measures after EU neighbours refused to share the burden.
As a result of the measures, which included clamping down on NGO rescue ships and boosting the Libyan coastguard's ability to intercept boats, arrivals dropped by 70 percent in the second half of the year.
There have also been moves to tighten Libya's southern borders, accelerate repatriations directly from Libya and measures to stem the flow of migrants from sub-Saharan Africa through transit states such as Niger and Sudan.
However, harrowing accounts emerged of desperate migrants throwing themselves overboard in order to avoid being sent back to the chaos in Libya.
Migrants intercepted or rescued by the Libyans are usually held in detention centres to await repatriation, but waiting times are often long and conditions deplorable.
International outrage over the situation was stoked in November by a CNN television report on migrant Africans being sold as slaves in Libya.
It got to the point that the EU's decision to help Libya intercept migrants trying to cross the Mediterranean and return them to detention centres was condemned as "inhuman" by the United Nations human rights chief, Zeid Ra'ad Al Hussein.
Meanwhile, tens of thousands of asylum seekers languish in large shelters in Italy, feeding into the mutual distrust of surrounding neighbourhoods.