FORT CRONKHITE, Calif. - A recent surge in weakened and malnourished sea lions found along the Northern California coast is mystifying scientists and keeping workers hopping at the newly expanded Marine Mammal Center here.

"We're way ahead in the numbers this year. We have twice as many animals as we should," marine veterinarian Bill Van Bonn said after examining Charcoal, a sick harbor seal.

Experts at the nonprofit center, situated on wind-swept Marin headlands just north of the Golden Gate Bridge, said they believed the perplexing spike in malnourished sea lions along several hundred miles of coast could be due to a decline in populations of smaller fish that young seals and sea lions eat while developing.

"It's likely a problem with the food web, something lower in the food chain that is affected, but we are not sure what it is yet," Van Bonn said.

For 35 years, the seaside hospital has treated and studied ailing elephant seals and other coastal mammals in bathtubs and makeshift facilities. Today, as more and more sick animals are needing attention, a new $32 million building will be unveiled that expands the center's capacity and technical ability at a crucial time.

On a recent afternoon, center staff wearing rubber boots and yellow slickers busily tended to about 130 critters lolling in the center's new pens, which are shaded by solar panels and include pools with freshly filtered water.

The center treats an average of 600 marine mammals a year, but last year more than 800 were rescued. In a recent week, staff rescued 10 more sea lions a day than usual.

"It's concerning," said Jeff Boehm, the center's executive director.

The only bright side, Boehm said, is that the center is now better equipped to help solve the riddle.

It has "a state-of-the-art lab, a state-of-the-art suite for performing science and doing that pathological work, which helps us understand diseases," Boehm said. The center also has surgeons to repair broken flippers or remove cataracts.

Elephant seals, harbor seals, and California sea lions make up the bulk of the patients, but the center also is called upon to help untangle whales caught in fishing nets, or to perform necropsies on dead animals that wash ashore.

Only about 50 percent of the animals rescued make it out alive, but all of them help in the center's scientific mission: More than 14,000 genetic and tissue samples have been stored.

The new building allows for more public access, which is free. Visitors can view the animals in their pens, watch a necropsy, or attend classes.

The Marine Mammal Center has also sought to leave a lighter environmental footprint in its new home: Ceiling tiles are made of seaweed, and structural beams are composed of partly recycled materials. The solar panels used to shade the pens also provides about 10 percent of the center's electricity.

These days, as the ocean's acidity rises because of climate change, much of the center's work will focus on how this changing sea chemistry affects the mammals that live within its 600-mile reach. Only about 8 percent of the center's patients are injured by hazards such as fishing nets, tackle, or boats.

On a recent sunny afternoon, a group of the malnourished California sea lions barked as two volunteers held one of them down to insert a feeding tube.

The staff force-fed the sea lion, known as Robin, with a yellowish mash of herring, water, and salmon oil, hoping to bolster Robin's strength so she can eventually return to the sea.