The proportion of refugees and migrants arriving in Europe by hidden routes is on the rise, a new study suggests.
Researchers estimate that 330,000 people will reach the continent this year by sea through "overt" channels, normally across the Mediterranean.
This is a sharp fall compared to last year, but a report from think tank, the Overseas Development Institute, said these arrivals are only part of the picture.
The paper suggests that while fewer people will arrive this year on well-known routes, in many cases refugees and migrants may be taking alternative "covert" routes.
It said the projected number of new asylum applications is still very high, at 890,000 by the end of 2016, adding: "This large discrepancy between new arrivals and new asylum applications suggests that there are many people whose journeys to Europe we know little about."
These refugees and migrants travel to Europe through a variety of "covert" channels and means which can include travelling by plane using false documents, concealed in vehicles or by over-staying visas, according to the study.
It calculated that at least 1.7 billion euro (£1.4 billion) has been spent on internal deterrent measures including border controls since 2014, while governments have also committed 15.3 billion euro (£13 billion) outside Europe on bilateral agreements and trust funds to increase economic opportunities at home or in neighbouring countries in an attempt to deter refugees and migrants from setting off on their journeys.
While effective individual national border controls have reduced the number of new, overt arrivals, they have not stopped the large movement of people to Europe, the report said.
Author Marta Foresti said: "While on the surface, the number of people arriving in Europe has fallen, the rate of those taking hidden routes to Europe has not been affected and is likely to increase.
"These covert routes can be more dangerous and make it harder for governments to monitor migration and design effective responses."
She added that increasing legal routes is "the only way to be able to predict flows" and make "pragmatic decisions" about how to better manage migration.