Discrete‐space continuous‐time models of marine mammal exposure to Navy sonar

Abstract

Assessing the patterns of wildlife attendance to specific areas is relevant across many fundamental and applied ecological studies, particularly when animals are at risk of being exposed to stressors within or outside the boundaries of those areas. Marine mammals are increasingly being exposed to human activities that may cause behavioral and physiological changes, including military exercises using active sonars. Assessment of the population-level consequences of anthropogenic disturbance requires robust and efficient tools to quantify the levels of aggregate exposure for individuals in a population over biologically relevant time frames. We propose a discrete-space, continuous-time approach to estimate individual transition rates across the boundaries of an area of interest, informed by telemetry data collected with uncertainty. The approach allows inferring the effect of stressors on transition rates, the progressive return to baseline movement patterns, and any difference among individuals. We apply the modeling framework to telemetry data from Blainville's beaked whale (Mesoplodon densirostris) tagged in the Bahamas at the Atlantic Undersea Test and Evaluation Center (AUTEC), an area used by the U.S. Navy for fleet readiness training. We show that transition rates changed as a result of exposure to sonar exercises in the area, reflecting an avoidance response. Our approach supports the assessment of the aggregate exposure of individuals to sonar and the resulting population-level consequences. The approach has potential applications across many applied and fundamental problems where telemetry data are used to characterize animal occurrence within specific areas.

Quantifying the response of Blainville's beaked whales to U.S. naval sonar exercises in Hawaii

“…Relatedly, we excluded data collected between training activity within an SCC (13% of the available data) as we did not consider it to be true baseline data since naval activity and/or MFAS had recently (within hours or days) been present. It would be interesting to explore the complete data set, including these interim periods, to investigate the timescales on which beaked whales respond to naval activity (e.g., Jones‐Todd et al, 2021; Joyce et al, 2019). We might expect that time since training activity or MFAS could lead to recovery of p (GVP) towards baseline levels, perhaps modulated by the cumulative exposure to training and MFAS.…”

Using individual-based bioenergetic models to predict the aggregate effects of disturbance on populations: A case study with beaked whales and Navy sonar

“…Simulated whales were assumed to inhabit and move between discrete geographical areas (spatial units) that differed in habitat quality and MFAS use. Movement was modelled as a continuous-time Markovian process with transition rates derived from satellite telemetry data of tagged Zc and Md at SOAR and AUTEC, respectively [ 31 , 42 , 49 ] (Fig S2 in S1 File ). To model the effect of behavior on energetics and life history, a general energy-budget model [ 50 ] for mammalian species previously applied to the long-finned pilot whale ( Globicephala melas ) [ 51 , 52 ] was reparametrized for Zc and Md .…”

“…A similar situation occurs at the Atlantic Undersea Test and Evaluation Center (AUTEC), which is a U.S. Navy training range in the Bahamas and used by a resident population of Blainville’s beaked whales ( Mesoplodon densirostris , hereafter Md ) [ 41 ]. Joyce et al [ 31 ] studied the movement and dive behaviors of Md at AUTEC during repeated, frequent and intense MFAS use as part of naval exercises and found clear and sustained displacement away from the core MFAS area in the majority of tracked whales [see also 42 ]. Dive behaviors during exposure were comparable to behaviors post- and pre-exposure, although the proportion of time spent at depths consistent with foraging decreased during initial exposure to MFAS [ 31 ].…”

“…Dive behaviors during exposure were comparable to behaviors post- and pre-exposure, although the proportion of time spent at depths consistent with foraging decreased during initial exposure to MFAS [ 31 ]. Passive acoustic monitoring using bottom-mounted range hydrophones at AUTEC and at the Pacific Missile Range Facility (PMRF) in Hawaii also indicated that Md either leave the range, or reduce foraging vocalization upon exposure to sonar [ 33 , 43 – 45 ], and that it may take 1–3 d post exposure before call rate returns to pre-exposure level [ 42 , 46 ]. Taken together, these are strong indications that MFAS use disrupts the behavior of beaked whales, leading to ‘lost foraging dives,’ either through cessation of foraging or displacement away from preferred foraging areas.…”

Combining bioenergetics and movement models to improve understanding of the population consequences of disturbance

“…A species' movement ecology will affect the aggregate exposure of individuals (i.e. the total duration and intensity of exposure to a given stressor, sensu Jones-Todd et al 2022) to disturbance from a particular activity. Keen et al (2021) described three broad categories of movement in marine mammals (resident, nomadic and migratory) that result in different probabilities of exposure to a localised source of disturbance (Fig.…”