Differential Vulnerability to Ship Strikes Between Day and Night for Blue, Fin, and Humpback Whales Based on Dive and Movement Data From Medium Duration Archival Tags

John Calambokidis,James A Fahlbusch,Brandon L Southall,Dave E Cade,Angela R Szesciorka,Jeremy A Goldbogen,Ari S Friedlaender

doi:10.3389/fmars.2019.00543

Abstract

We examine the dive and movement behavior of blue, fin, and humpback whales along the US West Coast in regions with high ship traffic where ship strikes have been identified as a major concern. All three species are known to feed in coastal waters near areas of high ship traffic. We analyzed data from 33 archival tag deployments representing over 3,000 hours of data that were attached with suction-cups or short darts for periods >24 hours and recorded depth (≥1 Hz), fast-lock GPS positions and other sensors. There were clear differences among the three species but all showed a distinct diurnal difference in diving behavior. While dive depth varied among animals based on where prey was located, whales spent a high proportion of their time closer to the surface where they would be more vulnerable to ship strikes at night than in the day. This was most pronounced for blue whales where vulnerability was twice as high at night compared to the day. We also found differences in movement patterns of whales between day and night. Movements were more localized to specific areas in the day near prey resources while at night these movements often involved directional movements (though sometimes returning to the same area). We show how in several specific areas like the Santa Barbara Channel, these differences in movements and locations translate to a very different overlap with shipping lanes at night compared to the daytime locations, which is the basis for most sighting data.

Highlights

Ship strikes of larges whales have become a growing concern in many areas around the world (Panigada et al, 2006; Williams and O’Hara, 2010; Silber et al, 2012b)
This included at least four fin whale ship strikes documented in the Pacific Northwest in 2002 (Douglas et al, 2008), and at least four blue whales documented struck by ships off southern California in Fall 2007 (BermanKowalewski et al, 2009)
We have been conducting tag deployments in the eastern North Pacific along the US West Coast on blue, fin, and humpback whales using a variety of archival tags since the 1990s

Summary

Introduction

Ship strikes of larges whales have become a growing concern in many areas around the world (Panigada et al, 2006; Williams and O’Hara, 2010; Silber et al, 2012b). While reported numbers of ship strikes have been of concern, these likely dramatically underrepresent the true number of ship strikes occurring due to the low proportion of strikes documented or carcasses recovered (Williams et al, 2011; Rockwood et al, 2017). This problem has been known for many years, solutions have proved challenging though some options have been put into place. Other strategies including voluntary ship slowdowns (McKenna et al, 2012) or use of acoustic alarms (Nowacek et al, 2004) have been shown to be ineffective or of limited use

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Results

Conclusion