(CNN) -- U.S. authorities are replacing a border fence that stretches into the Pacific Ocean after having zeroed in on weaknesses of the barrier.
Construction on the new 1,300-foot fence began in October, and will replace a fence built in 1993 that time and the elements have shifted, San Diego sector Border Patrol spokesman Michael Jimenez told CNN.
In recent years, the need for a replacement fence arose as migrants attempted to walk around it during low tide or avoided it by boat.
Currently, there is landing-mat fencing along the land border up to the beach, where steel posts filled with concrete stretch out into the ocean to demarcate the boundary.
But the posts have shifted over time, creating spaces where people can walk through, Jimenez said. In other cases, storms blew part of the fencing away, he said.
As a result, "during low tide, people can attempt to walk around the fence," he said.
The new, $4.3 million fence will stretch 300 feet into the ocean and will also consist of steel posts with concrete, but these will be coated with a material resistant to the effects of salt water. The posts in the water will be dug some 15 feet into the ground, but about 18 feet will still be above ground, Jimenez said.
As border enforcement has intensified throughout the majority of the boundary, smuggling corridors for people and drugs have moved to the edges of the border.
"We've seen an increase in this type of activity," Jimenez said.
In 2008, 230 people were apprehended in the Pacific trying to circumvent the fence. It nearly doubled to 400 apprehensions in 2009, and doubled again to 867 apprehensions in 2010.
The new fence is expected to be completed by March, and has an estimated lifespan of 30 years, Jimenez said.