Relating the Distribution of Humpback Whales to Environmental Variables and Risk Exposure

Abstract

Humpback whales (Megaptera novaeangliae) have been increasing in abundance globally since they faced near-extinction in the 1900s. As the species recovers, it is returning to areas it historically occupied. The Salish Sea in Washington state and British Columbia is an area where humpback whale sightings are increasing and is an important summer feeding ground. Here, I examined the dramatic increase in sightings in the Salish Sea since 1989. To better understand where the whales are going and why, I related whale sightings to chlorophyll-a (a proxy for primary productivity) and bathymetry. To understand the potential interactions with vessels, I analyzed the overlap of sightings with ship traffic and ferry routes in the region. Whale sightings were associated with increased chlorophyll-a in coastal waters and in Puget Sound during portions of the year and with depths generally shallower than the bathymetric averages. Humpback whales appeared to be using habitats with substantial traffic in commercial vessels and car ferries, suggesting an increasing risk of ship strikes as humpback numbers rebound. Sightings data indicate that the humpback whale critical habitat proposed by the National Marine Fisheries Service (NMFS) in 2019 appears to cover the area of highest use by the whales. While this study indicates associations, there may be a sighting bias caused by the opportunistic public sightings in areas closer to developed population hubs.