Only parts of the exhibit have mesh, while at the Point Defiance Zoo in Tacoma the entire walrus enclosure is covered in netting. Seagulls and other birds will swoop down and steal fish if they have access to an uncovered pool.
In Portland the mesh is what keeps the sea lions in. Supposedly, Steller Sea Lions are capable of amazing leaps out of the water (much more so than California sea lions), and the front of the pool is only inches lower than the public area at this vantage point.