Abstract
Collisions with ships (ship strikes) are a pressing conservation concern for fin whales (Balaenoptera physalus) along western North America. Fin whales exhibit strong diel patterns in dive behavior, remaining near the surface for most of the night, but how this behavior affects ship-strike risk is unknown. We combined diel patterns of surface use, habitat suitability predictions, and ship traffic data to evaluate spatial and temporal trends in ship-strike risk to fin whales of the California Current System (CCS). We tested a range of surface-use scenarios and found that both increased use of the upper water column and increased ship traffic contribute to elevated ship-strike risk at night. Lengthening nights elevate risk during winter throughout the CCS, though the Southern California Bight experienced consistently high risk both day and night year-round. Within designated shipping lanes, total annual nighttime strike risk was twice daytime risk. Avoidance probability models based on ship speed were used to compare the potential efficacy of speed restrictions at various scales. Speed reductions within lanes may be an efficient remediation, but they would address only a small fraction (13%) of overall ship-strike risk. Additional speed restrictions in the approaches to lanes would more effectively reduce overall risk.
Highlights
Of the great whales, fin whales (Balaenoptera physalus) were hunted in the highest numbers (Aguilar, 2009) and today they are among the most often struck by ships (Laist et al, 2001)
From 2009 to 2015, there were 10 documented fin whale mortalities attributed to ship strike along the coast of California, eight of which occurred in the Southern California Bight (NOAA, unpublished data)
Ship-whale encounter models for the U.S west coast Exclusive Economic Zone indicated that the ship-strike mortality rate for fin whales is twice that of blue whales (Balaenoptera musculus) and 2.4× that of humpback whales (Megaptera novaeangliae), and is estimated to be 2.7× above the Potential Biological Removal limit for non-natural mortality currently set by the National Marine Fisheries Service (Rockwood et al, 2017)
Summary
Fin whales (Balaenoptera physalus) were hunted in the highest numbers (Aguilar, 2009) and today they are among the most often struck by ships (Laist et al, 2001). Collision with ships (ship strike) is currently considered the most pressing conservation issue for fin whales in the eastern North Pacific (National Marine Fisheries Service [NMFS], 2010; Carretta et al, 2018), where productive coastal ecosystems overlap with busy shipping areas. Ship-whale encounter models for the U.S west coast Exclusive Economic Zone indicated that the ship-strike mortality rate for fin whales is twice that of blue whales (Balaenoptera musculus) and 2.4× that of humpback whales (Megaptera novaeangliae), and is estimated to be 2.7× above the Potential Biological Removal limit for non-natural mortality currently set by the National Marine Fisheries Service (Rockwood et al, 2017)
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