Patterns of habitat use by harbor porpoise (Phocoena phocoena) in Central San Francisco Bay
This research takes an innovative approach to modeling distribution of a marine predator explicitly in the temporal domain. Harbor porpoise (Phocoena phocoena) are a small cetacean seen in San Francisco Bay year round. Porpoise presence at the entrance of the Bay varies from zero sightings to over 100 in an hour. The solitary social and foraging behavior of this non-migratory species makes it an especially useful indicator of habitat patchiness along the west coast and of tide-dependent ecological processes in central Bay. Bathymetry data of the Golden Gate channel show steep shelf breaks and complex outcroppings that effect water flow and create spatially stable areas where tidal fronts occur. Oceanographic features associated with tidal fronts are recognized by marine predators as areas in which prey biomass accumulates. Sighting frequencies were hypothesized to vary according to changes in the same spatially consistent, but temporally variable, tidal factors that correlate to lower trophic level congregating mechanisms. Circular statistics were used to describe sightings data over a 24 hour tidal period. Sightings were fit to circular models based on tidal segments that correlated to tidal state: ebb or flood, and changes in current velocity. One year of data show a greater number of porpoises are present during a flood tide, but there are more sightings during an ebb tide. Porpoise sighting frequency showed multimodal distribution and best fit a model with a specific mean direction at the 95% confidence interval with [F=0.34, p = 0.001]. Most sightings occurred within three hours after maximum inflow current velocity on the north side of the channel. The time at which the most sightings occurred over a 24 hour tidal period correlated to the time at which the most defined shear zones occur in central Bay with a circular correlation coefficient of -0.15 (p=0.0006). A three tiered, nested ANOVA found significant variation in porpoise foraging behavior correlated to tidal phase or tidal front presence. Foraging behavior varied significantly according to tidal state with [F=9.96, p = 9.38 x 1 O'08]. The data show that it is the patch in tidal progression, rather than geographic space, which is significant to variations in porpoise sightings and foraging behavior in the Golden Gate. The results produce a temporal habitat model for a federally protected, upper trophic level predator in the Bay. Models like this are an efficient way to inform management in a highly anthropogenic influenced area.