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Mitigating Vessel Strikes

Over the past several months large whale strandings along the northeast US have been widely publicized in the media and have generated substantial interest from the public. During the three-month period between December 1, 2022, and March 1, 2023, at least 16 humpback whales washed ashore between New York and Virginia. During this same time period, two North Atlantic right whales—one in Virginia and one in North Carolina—were found deceased along the coastal beaches. Two additional whales—one minke whale and one sperm whale—were also found deceased along New York beaches. While alarming, this spike in strandings is part of two larger Unusual Mortality Events (UMEs) for the US Atlantic—one for humpback whales and one for North Atlantic right whales—that have been ongoing since 2017.

The National Marine Fisheries Service (NMFS) declares a UME when illness, strandings, or mortalities in a marine mammal population meet one or more of seven criteria that together help determine a UME. Through their investigations of these stranding events, NMFS concluded that many of the recent large whale mortalities were caused by collisions with vessels, also known as vessel strikes. Dozens of peer-reviewed research papers have described the issue of ship strikes and the threat posed to marine mammals and sea turtles. The threat is particularly acute for species with small population sizes like the North Atlantic right whale (NAWR)—where fewer than 350 individuals remain—which is a decline in the overall abundance of nearly 30% since 2011. Vessel strikes have been identified as one of the leading causes of right whale mortality.

Vessel Strike Prevention

To understand and ultimately reduce vessel strike risk, the physical characteristics of the vessel—size, draft, speed—as well as the animal’s occurrence and behavior—seasonal distributions, dive patterns, geographic-specific activities, and presence of cow-calf pairs—must all be considered. However, putting that knowledge into practice to inform actual mitigation actions is a greater challenge.

Vessel speed regulations have long been recognized as a logical method for mitigating strike risk. Slower vessel speeds can reduce the severity of injuries if a collision occurs, but more importantly, slowing down the vessel provides an increased opportunity for both the vessel and the animal to detect one another and take evasive action. However, there is not a good understanding to date regarding how and when animals detect vessels as a threat and what behaviors are initiated when an animal tries to avoid a collision—change direction, dive, increase swim speed. All these factors are likely species- and context-specific. NMFS mandates vessel speed restrictions for some vessels in particular geographic locations during certain times of the year when and where right whales are more likely to be present. A proposed rule was announced by NMFS in 2022 to expand the scope of the speed rule in an effort to further reduce NARW mortalities; the Proposed Rule remains pending at the time of this publication.

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The risk assessment model user interface with the GIS program, ArcMap. Green areas indicate BOEM's Wind Energy Areas on the Atlantic outer continental shelf. (Image credit: CSA)

Another avoidance measure that can be implemented is stationing a dedicated and trained lookout on vessels whose sole responsibility is looking ahead on the track of the vessel for marine species. If spotted, the lookout informs the bridge of animal presence, which would then trigger one or more mitigative actions like reducing speed or changing course. Reduced vessel speed improves the effectiveness of trained lookouts significantly by increasing the distance of detection and allowing aversion actions to be taken by both the vessel and the animal. However, trained, dedicated lookouts are only posted on a relatively small number of vessels working under permitted activities in US waters, such as on all offshore wind farm vessels and military vessels. Observers are not required and are generally not used on commercial shipping vessels, the latter of which represents most of the vessel traffic along the US eastern seaboard.

Alternative technologies that can assist with marine mammal detection from a moving vessel, such as infrared cameras with auto-detect software algorithms, are quickly advancing with the expectation that continuous, effective monitoring will decrease potential ship strike risk. While these technologies are currently emerging, a fundamental, analytical assessment of ship strike risk relative to planned vessel transit routes is yet another tool that can and should be employed.

Implementing a Solution

Identifying areas of high overlap, where vessels are likely to be occupying the same ocean space as marine animals, is important for assessing risk in planning vessel routes or activities and in evaluating mitigation measures such as reduction in vessel speed with the risk of vessel strikes. CSA was awarded a contract, in 2020, to collaborate in the creation of a GIS-based ship strike risk model for marine mammals and sea turtles by the Bureau of Ocean Energy Management (BOEM). CSA’s model was formulated to address vessel transit scenarios arising from the development of offshore wind energy but can be readily adapted to any other vessel operation areas where the data exist.

The model is currently available for use by wind developers upon request from BOEM. The version of the model delivered in 2021 covered wind development areas along the Atlantic coast from Massachusetts to Florida. The risk assessment tool and underlying model determine the predicted number of animals—by species—at risk for vessel encounters allowing transit analysis and comparative testing in an easy-to-use, spatially registered framework. Given a set of vessel scenarios that can be applied over a large geographic area, the tool allows a user to apply speed and routing variables to better understand when and where high strike risk may occur and where mitigation measures should be focused.


This graphic shows an example of the number of expected animal encounters as a heat map along a hypothetical vessel route. (Image credit: CSA)

Following the initial publication of the risk assessment tool and accompanying report, CSA conducted additional sensitivity testing and incorporated updates based on the best available scientific data. An updated tool was released in 2022.

Since the tools update in 2022, CSA has given several presentations to inform industry leaders and stakeholder groups not on the existence of this model but on the application and value of assessing vessel strike risk prior to project implementation. The model provides quantifiable predictions of ship strike risk that are useful for impact assessments and mitigation development. Further development and application of these types of geospatially articulated tools—combined with detection technology and mitigation—provide an emerging opportunity to ask questions, predict consequences, and ultimately reduce the impacts of vessel activities coinciding with vulnerable marine species.

To learn more about CSA’s Offshore Renewables department and their services, visit:

This feature appeared in Environment, Coastal, & Offshore (ECO) Magazine’s 2023 Deep Dive 1 edition, to read more about Ocean Observation access the magazine here.

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Corporate Headquarters

8502 SW Kansas Ave.
Stuart, FL 34997