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New Directions in Marine Mammal Behavioral Noise Exposure Criteria3Marine Research of the National Academy of Marine ResearchDOI: 10.29677/MR.202406_4(1).0001 Subsequent and broad-scale public awareness of noise impacts on marine mammals was later amplified by a series of mortal mass stranding events of cetaceans (predominately odontocete cetaceans, especially beaked whales), associated with tactical mid-frequency active sonars (MFAS) (Filadelfo et al., 2009; Simmonds et al., 2014; Anciento et al., 2021). These events drove the issue onto the front page of newspapers around the world as well as into court rooms with litigation over the absence or substance of regulation and mitigationrequirements for these powerful sound sources. Given the near-total absence of data on the behavior and behavioral response of marine mammals to these sound sources, these events drove major research efforts to obtain quantitative data on exposure and response to MFAS, which focused initially on beaked whales but subsequently extended to other species. The substantial body of science that resulted has provided some key insights and direct applications in regulatory assessments in moving beyond initial ideas about discrete RL thresholds. The development and implementation of high-resolution archival tags with movement and acoustic sensors was a key enabler for experimental field studies to generate detailed measurements of individual behavioral responses of marine mammals to various sources, notably military sonar signals (Southall et al., 2016). For instance, Tyack et al. (2011) quantified behavioral responses of a beaked whale (Blainville's beaked whale, Mesoplodon densirostris) at relatively low received levels to the kinds of tactical mid-frequency (3.5 kHz) sonar signals involved in earlier stranding events. DeRuiter et al. (2013) applied similar experimental methods and showed strong behavioral responses (including cessation of foraging and energetically intense avoidance responses) at even lower received levels for simulated sonar signals for Cuvier's beaked whales (Ziphiuscavirostris). Goldbogen et al. (2013) and Southall et al. (2019b, 2023a) used similar approaches to document similar responses in some contexts in blue (Balaenoptera musculus) and fin (B. physalus) whales. Miller et al. (2015) similarly found that a northern bottlenose whale (Hyperodon ampullatus) responded clearly to anoperational military sonar exposure at relatively higher frequency (6-7 kHz) by altering foraging, acoustic, and diving behavior following exposure to relatively low received level signals. From these and other studies, Experimental studies have demonstrated that within contexts, RL is an important driver of the relative probability of response. For instance, Miller et al. (2014) demonstrated the killer whales (Orcinus orca) exposedto an approaching sonar stimulus responded differently than approaches with no sonar, and that response probability increased following a specific relationship as a function of noise exposure level. But recent findings have demonstrated the extent to which contextual factors of noise exposure can also be critically important, including behavioral state, spatial proximity (e.g., range, relative motion), exposure repetition, and the relative familiarity of individuals with noise sources (Ellison et al., 2012). For instance, for blue whales (Balaenoptera musculus) exposed to simulated military sonars in controlled exposure experiments have shown that deepfeeding whales are much more likely to respond to noise at comparable levels than shallow-feeding or traveling whales (Goldbogen et al., 2013). Kastelein et al. (2011) demonstrated an increasing probability of response for captive harbor porpoises (Phocoena phocoena) at similar received sound levels with repeated stimulus presentation (i.e., behavioral sensitization). Various experimental and observational methods have been applied in evaluating these relationships, many of which have relevant applications to understanding, managing, and hopefully reducing the potential negative effects of human noise on marine mammals. probabilistic nature of response as well as contextual, species, and source type differences. Southall et al. (2007) found that while there was not a uniform and ubiquitous relationship of received level and response probability, within certain species groups and especially behavioral states (e.g., feeding versus traveling baleen whales) there was a generally increasing probability of response with increasing noise exposure received levels. These early efforts at establishing systematic criteria clearly called into question the scientific basis for simple all-ornothing threshold approaches. They also called for more systematic observational and controlled research to further understand response probabilities and contextual dependencies and further refine criteria assessments.