Research Document 2017/007

Assessing the risk of lethal ship strikes to humpback (Megaptera novaeangliae) and fin (Balaenoptera physalus) whales off the west coast of Vancouver Island, Canada

By Nichol, L.M., Wright, B.M., O’Hara, P., and Ford, J.K.B.

Abstract

Vessel strikes are a source of mortality and injury for baleen whales that can have population-level impacts. Spatial analysis of whale distributions and marine traffic provide a valuable approach for identifying zones of high relative collision risk. We conducted 34 systematic aerial surveys (2012-2015) in fall and winter to estimate humpback and fin whale densities off the west coast of Vancouver Island, Canada. This region includes approaches to major shipping lanes in Juan de Fuca Strait, a gateway to the ports of southern British Columbia and Puget Sound, Washington. To predict whale densities (per km²) in the study area, we fit negative binomial Generalized Additive Models (GAMs) to whale sightings data, incorporating survey effort as an offset and depth (m), slope (deg) and latitude (northing, UTM) as environmental covariates. Humpbacks were primarily observed on the continental shelf, with highest predicted densities along the beginning of the shelf edge (~200 m), whereas fin whales were largely predicted to occur west of the shelf in deeper water (>450 m).

We then mapped AIS-reported ship traffic densities (2013) over the same study area and combined these data with the GAM-predicted whale densities to estimate the relative risk of whale-ship collisions. Since vessel speed is an important determinant of collision lethality, we also calculated the relative risk of lethal injuries, given the probability that a strike occurs. Humpbacks were most likely to be struck along the shelf edge, the inshore approaches to Juan de Fuca Strait, and within the western portion of strait itself. Fin whales were most likely to be struck in the offshore approaches to Juan de Fuca Strait and inside the western portion of strait. Ship traffic is predicted to increase as a result of port expansions and developments in both BC and Washington State. We therefore tested future shipping projections from two sources and incorporated these predicted increases in ship traffic into our models to estimate the change in relative risk of ship strike and lethal ship strike by 2030. Our study is the first to assess ship strike risk to large whales off the west coast of Vancouver Island. The estimates of strike probability and risk of lethal strike we present here are minimum estimates, as our models did not account for species specific vulnerabilities and future projections did not account for whale population growth or anticipated increases in ship size.

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