Modelling the biological significance of behavioural change in coastal bottlenose dolphins in response to disturbance

Abstract

Summary 1.Behavioural change in response to anthropogenic activities is often assumed to indicate a biologically significant effect on a population of concern. Disturbances can affect individual health through lost foraging time or other behaviours, which will impact vital rates and thus the population dynamics. However, individuals may be able to compensate for the observed shifts in behaviour, leaving their health and thus their vital rates and population dynamics, unchanged. 2. We developed a mathematical model simulating the complex social, spatial, behavioural and motivational interactions of coastal bottlenose dolphins (Tursiops truncatus) in the Moray Firth, Scotland, to assess the biological significance of increased rate of behavioural disruptions caused by vessel traffic. 3. We explored a scenario in which vessel traffic increased from 70 to 470 vessels a year in response to the construction of a proposed offshore renewables' facility. Despite the more than sixfold increase in vessel traffic, the dolphins' behavioural time budget, spatial distribution, motivations and social structure remain unchanged. 4. We found that the dolphins are able to compensate for their immediate behavioural response to disturbances by commercial vessels. If the increased commercial vessel traffic is the only escalation in anthropogenic activity, then the dolphins' response to disturbance is not biologically significant, because the dolphins' health is unaffected, leaving the vital rates and population dynamics unchanged. 5. Our results highlight that behavioural change should not automatically be correlated with biological significance when assessing the conservation and management needs of species of interest. This strengthens the argument to use population dynamics targets to manage human activities likely to disturb wildlife.

Variation in harbour porpoise activity in response to seismic survey noise

“…Less overt effects can arise from trade-offs that animals make [3], such as deciding to remain in disturbed areas and tolerating higher exposure levels where prey are abundant [4]. However, these trade-offs may also influence activity budgets, potentially affecting fecundity and survival [1,5] or having biologically significant consequences at a population level [6][7][8][9].…”

Increased proximity of vessels reduces feeding opportunities of blue whales in the St. Lawrence Estuary, Canada

“…Disruption of breathing sequence and thus impairment of oxygen replenishment over short periods of time may possibly be compensated by increased feeding rates at a later time, as proposed for some odontocete species (e.g. New et al 2013). The likelihood of effects on the fitness of individuals depends on their ability to compensate and varies according to specific ecological and social conditions (New et al 2013).…”

Predicting the effects of human developments on individual dolphins to understand potential long-term population consequences

“…We developed an individual-based model that combined the results of previous studies on dolphin ranging patterns in the Moray Firth [28], on spatio-temporal distribution of foraging activity [29], on the distribution of boat traffic [22,28] and on the effect of boat interactions [33] and construction activities [32] on dolphin behaviour (figure 1). The model-simulated dolphin behavioural dynamics under the assumption that each individual had a set of two motivational states (to acquire energy and to spend energy) that resulted from the integration of internal and external stimuli and that regulated its behavioural decisions [19,20].…”

“…In addition, we replace the tuned parameters used in [22] with ecological parameters estimated from empirical data. The model is described using the updated overview, design concepts, details protocol [35,36].…”

“…Previous work has shown that behavioural temporal dynamics can be modelled successfully through the integration of an individual's motivational states, which combine the effects of external stimuli and physiological needs [19 -21]. Because motivations are unobservable, mechanistic models are required, and agent-or individualbased models have been used for this purpose [22,23]. Predicted changes in an animal's internal states can then be linked to its energy balance and condition to understand how its allocation of energy to survival or reproduction will be impacted [8,10].…”