Hotspot Analysis of Humpback Whales along Cordell Bank and Gulf of the Farallones, Central California

Abstract

Understanding habitat preferences for threatened and endangered species is a high priority for spatial management strategies to ensure minimum conflict between human uses of the ocean and wildlife conservation. The purpose of this study was to identify environmental variables that predict occurrences of humpback whales (Megaptera novaeangliae) within Cordell Bank and Gulf of the Farallones National Marine Sanctuaries, California, to assess potential conflict with vessel traffic. I used data collected by the Applied California Current Ecosystem Studies (ACCESS) conducted from 2004 to 2011. Using zero inflated negative binomial regression, I developed predictive models and identified locations repeatedly used by whales to characterize humpback whale habitat within the Sanctuaries. I designated whale encounter rates at 3-km bin intervals as the dependent variable, and bathymetric, surface and mid-water oceanographic data as the independent variables. The resulting models: 1) a reduced model using only surface hydrographic variables and 2) a full model using both surface and mid-water variables; were compared and contrasted. The full model performed significantly better than the reduced model, which underestimated the amount of whale habitat in the northern half of our study area. We compared resulting habitat areas to current and proposed San Francisco Bay Area shipping lane layouts to explore whether proposed changes in routes excluded humpback whale habitat. Our results showed that proposed shipping lane layouts reduced vessel traffic within Sanctuaries and high-use habitat by about 70%, reducing threats to this endangered species.