Abstract
Mortality from collisions with vessels is one of the main human causes of death for large whales. Ship strikes are rarely witnessed and the distribution of strike risk and estimates of mortality remain uncertain at best. We estimated ship strike mortality for blue humpback and fin whales in U.S. West Coast waters using a novel application of a naval encounter model. Mortality estimates from the model were far higher than current minimum estimates derived from stranding records and are closer to extrapolations adjusted for detection probabilities of dead whales. Our most conservative model estimated mortality to be 7.8x, 2.0x and 2.7x the U.S. recommended limit for blue, humpback and fin whales, respectively, suggesting that death from vessel collisions may be a significant impediment to population growth and recovery. Comparing across the study area, the majority of strike mortality occurs in waters off California, from Bodega Bay south and tends to be concentrated in a band approximately 24 Nm (44.5 km) offshore and in designated shipping lanes leading to and from major ports. While some mortality risk exists across nearly all West Coast waters, 74%, 82% and 65% of blue, humpback and fin whale mortality, respectively, occurs in just 10% of the study area, suggesting conservation efforts can be very effective if focused in these waters. Risk is highest in the shipping lanes off San Francisco and Long Beach, but only a fraction of total estimated mortality occurs in these proportionally small areas, making any conservation efforts exclusively within these areas insufficient to address overall strike mortality. We recommend combining shipping lane modifications and re-locations, ship speed reductions and creation of ‘Areas to be Avoided’ by vessels in ecologically important locations to address this significant source of whale mortality.
Highlights
Widespread hunting of whales during the nineteen and twentieth centuries has left many whale populations severely depleted [1]
The current state of knowledge leaves several important gaps: 1) we still do not know with any precision how many whales are killed or where those deaths occur, 2) we lack a synoptic understanding of the problem and cannot prioritize locations and approaches nor manage strike risk well on the scales where regulations are made and enforced, and 3) there is no way to set mortality limits for smaller jurisdictions such as National Marine Sanctuaries
We report summary values and spatial patterns of our model results here and compare them to documented stranding records
Summary
Widespread hunting of whales during the nineteen and twentieth centuries has left many whale populations severely depleted [1]. The current state of knowledge leaves several important gaps: 1) we still do not know with any precision how many whales are killed or where those deaths occur, 2) we lack a synoptic understanding of the problem and cannot prioritize locations and approaches nor manage strike risk well on the scales where regulations are made and enforced, and 3) there is no way to set mortality limits for smaller jurisdictions such as National Marine Sanctuaries. To fill these important needs, we calculate strike mortality using a quantitative framework based on encounter theory [37,38] and allocate the results to local and regional jurisdictions. Our results enable the design of a suite of marine spatial plans and regulations that have the potential to mitigate mortality from vessel collisions on the west coast
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