Modeling Whale Deaths From Vessel Strikes to Reduce the Risk of Fatality to Endangered Whales

Abstract

Vessel strikes have been documented around the world and frequently figure as a top human cause of large whale mortality. The shipping lanes in the Santa Barbara Channel, California and nearby waters have some of the highest predicted whale mortality from vessel strikes in the United States waters of the eastern Pacific. Beginning in 2007, National Oceanographic and Atmospheric Administration requested voluntary vessel speed reductions (VSRs) for vessels greater than 300 GT traveling in the Santa Barbara Channel shipping routes to decrease whale mortality from ship strikes. We employed a ship strike model using whale density data and automatic identification system (AIS) vessel data to estimate mortality under several management scenarios. To assess the effect of the VSR on strike mortality, we bootstrapped speeds from vessels greater than 19 m long that transited when no VSR was in place. Finally, we calculated the predicted mortality for hypothetical cooperation scenarios by artificially adding speed caps post-hoc to real vessel transits. For 2012–2018, we estimated that in our study area on average during summer/fall (June–November) 8.9 blue, 4.6 humpback, and 9.7 fin whales were killed from ship strikes each year (13–26% greater than previously estimated). We evaluated winter/spring (January–April) humpback mortality for the first time, resulting in an estimate of 5.7 deaths on average per year. Poor cooperation with the VSR led to low (5% maximum) to no reductions in the estimated number of strike mortalities. Evaluating potential scenarios showed that if 95% cooperation occurred in the lanes, whale deaths there would decrease by 22–26%. Adding VSRs with similar cooperation levels at the northern end of the Santa Barbara Channel and south of Channel Islands National Marine Sanctuary could decrease estimated strike mortalities in those areas by 30%. If VSRs were added and cooperation reached 95% there and in the lanes, we estimate a 21–29% decrease in vessel strike mortalities could be attained relative to estimated mortality in the entire study area. To decrease the vessel strike related whale mortalities in this region, we recommend expanding the VSR areas and increasing cooperation voluntarily, or considering mandatory speed limits if desired cooperation levels cannot otherwise be met.

Evaluating Adherence With Voluntary Slow Speed Initiatives to Protect Endangered Whales

“…Our analysis of vessel traffic across 2010-2019 shows that cooperation levels with NOAA voluntary VSR requests were under 50%. Though a 20% increase in cooperation levels with the NOAA voluntary VSR was recorded in 2019 compared to 2018 levels-likely influenced by the increase in cooperation recorded by enrolled vessels in the Incentive Program in those yearsvoluntary adherence to the NOAA voluntary VSR is not reaching 80-95% cooperation levels that are estimated to be needed to reduce vessel strike mortality to levels that do not inhibit reaching and maintaining optimal sustainable whale populations (Rockwood et al, 2021).…”

“…California Bight may maximize benefits to whales (Redfern et al, 2019), and that costs to industry accrued by slowing down vessels in these areas are minimal (Gonyo et al, 2019). Additional work to continue to separate vessels and whales and streamline vessel traffic patterns, such as ATBA and adjustments to Traffic Separation Schemes, are being considered and would be beneficial to supplement VSR initiatives (Redfern et al, 2019;Rockwood et al, 2021). Future measures for the region include a proposal currently in review by the IMO to expand the ATBA and extend the TSS that overlap with important blue and humpback whale feeding areas.…”

“…Models estimate that a total of 83 (18 blue, 22 fin, and 43 humpback) whales are killed by vessel strikes along the U.S. west coast between May and September each year (Rockwood et al, 2017), suggesting that total annual vessel strike mortality often exceeds blue and humpback Potential Biological Removal (PBR) management thresholds, which are defined under the MMPA as the maximum number of animals, not including natural mortalities, that may be removed from a marine mammal stock while allowing that stock to reach or maintain its optimum sustainable population (Rockwood et al, 2017;Carretta et al, 2020). In the Santa Barbara Channel and the surrounding waters of the Southern California Bight alone, an estimated average of 8.9 blue, 4.6 humpback, and 9.7 fin whales are killed by vessel strikes between June and November each year (Rockwood et al, 2017;Rockwood et al, 2021).…”

“…Recent modeling work suggests that a cooperation level of 80% or higher with the southern California VSR request could reduce the estimated vessel strike mortality by 20-30% to blue, fin, and humpback whales (Rockwood et al, 2021). However, past studies on the voluntary VSR requests in southern California have shown adherence to be limited (0-13%) across large vessel traffic in the years examined (Mckenna et al, 2012;Freedman et al, 2017).…”

Western Gull Foraging Behavior as an Ecosystem State Indicator in Coastal California

“…A particularly concerning issue in the California Current system is severe whale injury or mortality as a result of entanglements in fishing gear and ship strikes (Williams and O'Hara, 2010;Redfern et al, 2013;Rockwood et al, 2021). With the urgent need to utilize ecosystem data to advise marine resource management, working groups were established to develop mitigation strategies and understand drivers of increased whale entanglement (Santora et al, 2020;Feist et al, 2021;Ingman et al, 2021) and ship strikes (Hazen et al, 2017;Abrahms et al, 2019;Blondin et al, 2020;Rockwood et al, 2020).…”