The fish hatchery has possibly been filled in, and I'm assuming that the steps were at one time a "ladder" for the fish to pass across. There is a very similar cement development that was constructed at Seattle Aquarium and is actually still there:
Farthest from the truth. The fish hatchery was a WPA project and built on the city park land to stock nearby lakes. This continued until the zoo took over the park land and the hatchery ceased operations. Much of the hatchery still remains including the pictured stairs and four man-made ponds in the valley of the zoo. The pictured stairs were used by people to move in and out of the valley to maintain the ponds located in the valley below. One of the ponds shown in the far background of the picture has a large, overgrown with vegetation, square island in the center (now it looks like a moated exhibit).
@snowleopard: Thanks for your response. Your answer seemed credible and I appreciate the effort, and the picture of the pelican exhibit.
@gerenuk: Thanks much for the historical information. I looked at this picture originally as a good target for absurdist comedy (or at least an attempt) and ended up learning some cool history.
