The prefecture of the French port town said 7,414 people had been arrested between January and June this year against 3,129 a year earlier.
"We have noted that there are considerably more illegal migrants, and during our checks we find more," a source at Calais port told AFP.
In the first two weeks of July alone, some 1,200 people were arrested but most of them were released because the detention centres are already overcrowded.
French police have been trying for years to dislodge migrant camps in and around Calais.
The Local has reported how French riot police have pulled down numerous migrant camps in recent weeks, much to the anger of humanitarian aid groups, who are demanding France come up with a new policy for its treatment of migrants.
“We have to take into account the reality of the situation and this zone is a part of a circuit for people seeking asylum in Britain. It’s not going to stop tomorrow,” Jean-Claude Mas, secretary general of immigration support group La Cimade, told The Local recently.
“Instead of discouraging people who have nothing to lose, and who are willing to do anything to make the crossing at Calais, why not get organized?”
Last week the mayor of Calais passed a decree that banned the setting up of camps in certain areas, which he said was causing a problem for public order and sanitation.
The flow of migrants has remained persistent, with many migrants hoping to hide in trucks or other vehicles crossing to Britain, where they believe conditions are better for would-be refugees than in France.
Most of the immigrants come from the Horn of Africa, especially from Sudan, Somalia and Eritrea, a source close to the case said.
"They arrive in Lampedusa (Italy's southernmost tip) and come to France by train or bus and get to Calais," the source said.
At the three points where there are Channel crossings – Calais, Dunkirk and the Channel Tunnel – 10,500 migrants were arrested in the first half of 2014, against 5,133 a year earlier.