Context-dependent variability in the predicted daily energetic costs of disturbance for blue whales

Abstract

Assessing the long-term consequences of sub-lethal anthropogenic disturbance on wildlife populations requires integrating data on fine-scale individual behavior and physiology into spatially and temporally broader, population-level inference. A typical behavioral response to disturbance is the cessation of foraging, which can be translated into a common metric of energetic cost. However, this necessitates detailed empirical information on baseline movements, activity budgets, feeding rates and energy intake, as well as the probability of an individual responding to the disturbance-inducing stressor within different exposure contexts. Here, we integrated data from blue whales (Balaenoptera musculus) experimentally exposed to military active sonar signals with fine-scale measurements of baseline behavior over multiple days or weeks obtained from accelerometry loggers, telemetry tracking and prey sampling. Specifically, we developed daily simulations of movement, feeding behavior and exposure to localized sonar events of increasing duration and intensity and predicted the effects of this disturbance source on the daily energy intake of an individual. Activity budgets and movements were highly variable in space and time and among individuals, resulting in large variability in predicted energetic intake and costs. In half of our simulations, an individual’s energy intake was unaffected by the simulated source. However, some individuals lost their entire daily energy intake under brief or weak exposure scenarios. Given this large variation, population-level models will have to assess the consequences of the entire distribution of energetic costs, rather than only consider single summary statistics. The shape of the exposure-response functions also strongly influenced predictions, reinforcing the need for contextually explicit experiments and improved mechanistic understanding of the processes driving behavioral and physiological responses to disturbance. This study presents a robust approach for integrating different types of empirical information to assess the effects of disturbance at spatio-temporal and ecological scales that are relevant to management and conservation.

Emerging themes in Population Consequences of Disturbance models

“…A population's sensitivity to disturbance will be strongly influenced by the importance of the disturbed area for foraging, reproduction and migration, as shown in several PCoD models [ 13 , 49 , 57 , 61 , 95 , 96 ]. For example, simulations carried out by Pirotta et al [ 61 ] found that disturbance within important foraging areas had a more dramatic effect on adult female northern elephant seal energy budgets than a similar disturbance located in less important habitat within the population's range.…”

“…moving or stationary), the distance between the source and receiver, and the nature of the sound itself (e.g. source frequency and intensity) [ 94 , 104 ], which all affect the consequences of disturbance in PCoD models [ 57 , 96 ]. This research has led to the development of analytical tools such as dose–response functions, which provide a framework for relating an individual's probability of responding to some metric of exposure (e.g.…”

“…For example, PCoD models show that species [ 17 ], sex [ 55 ], age class [ 13 ] and body condition [ 61 ], as well as context (e.g. current behavioural state [ 96 ] or energetic state [ 32 ]), can influence the effect of disturbance on individual health and vital rates. An individual's experience can also influence the severity of response, although changes in responsiveness have yet to be incorporated into PCoD models.…”

“…As outputs of behavioural response studies become available, they should be incorporated into PCoD models and risk assessments (e.g. via improved dose–response functions) and used to develop mitigation and monitoring protocols that validate predictions [ 96 , 104 ].…”

Far-Field Effects of Impulsive Noise on Coastal Bottlenose Dolphins

“…Further research is now required to test this hypothesis and describe the extent of any change in more detail, given that moderate modifications of vocal behavior have the potential to affect foraging and individual vital rates (Southall et al, 2007). The challenge in future research will be linking modifications of vocal behavior to changes in energetic costs (Booth, 2020;Pirotta et al, 2021) and, ultimately, population level consequences (Pirotta et al, 2015a;Reed et al, 2020).…”