Abstract
Recent estimates of blue (Balaenoptera musculus) and humpback (Megaptera novaeangliae) whale ship-strike deaths on the US west coast are above the Potential Biological Removal limit determined by the National Marine Fisheries Service. Beginning in 2015, the National Oceanographic and Atmospheric Administration requested voluntary Vessel Speed Reductions (VSR) in the designated shipping routes off San Francisco, California, USA, in order to decrease whale mortality from ship strikes. We applied a ship strike model based on whale density and Automatic Identification System (AIS) vessel data. We bootstrapped speeds from vessels that transited when no VSR was in place to assess the effect of the VSR on strike mortality rates. Finally, we calculated the expected mortality for hypothetical compliance scenarios by programmatically imposing speed caps. Average predicted mortality for the region was 2.7 blue whales and 7.0 humpback whales in a 4 month period. Compared to years prior to the VSR (2012-2014), vessel speeds during the VSR were slower. This lowered blue whale deaths within the shipping lanes by 11-13% and humpback whale deaths by 9-10% in 2016-2017. If 95% of mariners adhered to recommended 10 knot (kn) limits in the shipping lanes alone, we predicted twice as many blue whale and 3 times as many humpback whale deaths would be avoided relative to current adherence. Adding a 10 kn speed limit (with 95% cooperation) at the ends of each of the lanes would result in about 5- and 4-fold reductions in blue whale and humpback whale mortality, respectively, relative to current practices. Our approach can evaluate ship strikes and mitigation measures for whale populations around the globe.
Highlights
Deaths from collisions with ships have been identified as one of the top 2 human threats to whale populations around the world (Clapham et al 1999)
Our approach provides broadly applicable tools that can help reduce the occurrence of strikes and estimate strike mortality to help decision makers choose what actions, if any, are needed to ensure healthy whale populations
Our research identifies regions beyond the terminus of the Traffic Separation Schemes (TSSs) that could significantly decrease strike mortality if Seasonal Management Areas (SMAs) with established speed limits were introduced
Summary
Deaths from collisions with ships (ship strikes) have been identified as one of the top 2 human threats to whale populations around the world (Clapham et al 1999). Our approach, based on an encounter model, can gauge speed limit program success, establish expectations for temporal variability in effectiveness and evaluate how increasing cooperation or expanding speed limit areas will change mortality from ship strikes. We applied these approaches to a case study of the shipping approaches to the San Francisco Bay, California, USA. This biologically important whale area, known as the Gulf of the Farallones, has seen numerous documented instances of ship strikes and there were 24 reported whale mortalities from 2008 to 2018
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