Modeling the effects of El Nino, density-dependence, and disturbance on harbor seal (Phoca vitulina) counts in Drakes Estero, California: 1997-2007

Harbor seal (Phoca vitulina) haul-out site use may be affected by natural or anthropogenic factors. Here, we use an 11-yr (1997-2007) study of a seal colony located near a mariculture operation in Drakes Estero, California, to test for natural (El Nino-Southern Oscillation (ENSO), density-dependence, long-term trends) and anthropogenic (disturbance or displacement related to oyster production activities) factors that may influence the use of haul-out subsites. Annual mariculture related seal disturbance rates increased significantly with increases in oyster harvest (r(s) = 0.55). Using generalized linear models (GLMs) ranked by best fit and Akaike's Information Criteria, ENSO and oyster production (as a proxy for disturbance/displacement) best explained the patterns of seal use at all three subsites near the mariculture operations, with effects being stronger at the two subsites closest to operations. Conversely, density-dependence and linear trend effects poorly explained the counts at these subsites. We conclude that a combination of ENSO and mariculture activities best explain the patterns of seal haul-out use during the breeding/pupping season at the seal haul-out sites closest to oyster activities.