A true test of this hypothesis must make observations as the tourist population varies. The San Francisco tourist season is year-round, although it does have peaks and valleys. However, the effects of tourists and weather would be difficult to disentangle. At the same time, however, sea gulls prefer to be close to the San Francisco shore line, while sea lions are unaffected by the distance from the shore. Perhaps the difference in sea gull and sea lion preference is a result of their differing main modes of transportation.
Sea lions travel by swimming, in general, while sea gulls travel mainly through the air.
Whether or not swimming mammals would prefer to be far from the pier, unaffected by the shoreline, while flying birds would prefer to be close to the shoreline, unaffected by the pier, is unknown. Spatial preferences alone could produce the observed effect, in which sea lions and sea gulls are rarely observed together on the same platforms. A true test of whether or not sea lions and sea gulls dislike occupying the same platforms would monitor the behavior of specific individuals. One could count how often sea gulls exit platforms containing no sea lions and land on platforms containing sea lions, to see if they have a preference for lion-less platforms. One could perform similar counts with sea lions as well.