The sea lions -- a juvenile weighing about 200 pounds, and an older animal weighing roughly 400 pounds -- were spotted on San Francisco's Pier 39 with strands of wire or plastic wrapped around their necks, Marine Mammal Center spokesman Jim Oswald said.
The animals were last seen on Tuesday, and while they were in good condition at the time, would-be rescuers fear that being entangled in trash will prevent the animals from foraging, leaving them vulnerable to disease and predators.
The rescuers are also concerned that the strands will cut into their skin as they grow, he said.
Oswald said they are hoping to find the sea lions on a beach, which would make it easier to capture them.
Rescuers tried to reach the animals four times while they rested on the pier, but their attempts were unsuccessful.
"Once the animals are in the water, they're in their own element," he said.
Oswald said rescuers are ready to try again -- hopefully on solid ground this time -- if the sea lions reappear.
"It's a wait-and-see game at this point," he said.
The public is asked to report any injured sea lion sightings to the Marine Mammal Center's 24-hour tip line at (415) 289-7325.