Context-dependent variability in the
predicted daily energetic costs of
disturbance for blue whale – A
Critical Review
“Context-dependent variability in the predicted daily energetic costs of disturbance for blue
whale” Pirotta et al. (2021) is a research article that was published in 2021 in the Journal of
Conservation Physiology. The article is about a study that was conducted in order to
investigate the effect of low level, sub-lethal anthropogenic disturbance on a wild population
of blue whales (Balaenoptera musculus) that was exposed to military active sonar on the
West coast of the United States (Pirotta et al., 2021).
The level of disturbance was measured by the frequency of foraging activity in the form of
the number of lunges per hour and the depth of lunges of 27 blue whales, who were all
attached with tags that measure location, depth and 3D accelerometry data (Pirotta et al.,
2021). All of which was later on integrated into a population model, acting as parameters to
measure their level of foraging in the presence of said disturbance (Pirotta et al., 2021). The
effects of the disturbance on foraging activities were measured using two temporal subsets
which were July and October, and spatially using two subsets of latitudinal ranges which are
33.8°N-34.4°N and 37.6°N-38.4°N that were used to model individual movement in previous
studies (Pirotta et al., 2018b; Pirotta et al., 2019; Pirotta et al., 2021). Markov chain
algorithm was used to estimate transition probabilities between hourly lunging/feeding state
and Minimum convex Polygon were used to estimate the area a whale ranged a day (Pirotta et
al., 2021).
In term of movement under the effects of disturbance, Pirotta et al. (2021) utilised a method
from a study in the past to create Controlled Exposure Experiments (CEEs) (Southall et al.
2012; Southall et al. 2016; Southall et al. 2019a), which recorded the fine scale, 3D
movements of individual whales which were then analysed in order to detect behavioural
changes (Pirotta et al., 2021). Various simulations within a 100km x 100km rectangle that
matches the spatial resolution mentioned above were created, manipulated and monitored in
order to collect foraging behaviour and exposure response (ER) in accordance to the level of
disturbance (Pirotta et al., 2021). There was an attempt to investigate differences in krill
density in shallow and deep patches, however, it was unsuccessful due to sampling
limitations (Pirotta et al., 2021).
The data of individual were collected and statistically analysed with the consideration of
variables such as krill density, time of year, and geographical location. It was concluded that
the daily energetic costs of disturbance on individual blue whales were highly variable and its
distribution has long tails, which indicate possible dramatic consequences across different
scenarios (Pirotta et al., 2021). Despite the meticulous collection and analysis of data, it was
concluded that data for individual is inapplicable to the proposed population model and more
research need to be done in order for it to work (Pirotta et al., 2021).
This particular study is relatable to some of the previous studies in the literature. Not only on
the effects of anthropogenic disturbance on cetacean and marine mammals, but it extends to
other taxa as well. For example, Beale and Monaghan (2004) concluded that human
disturbance resulted in a decline in breeding successes in sea birds. Despite coming from a
different taxon, it is highly applicable to what this current study in which 50% of the
simulations saw net energy intake of individual blue whales decreased, 11% of which was
negative (Pirotta et al., 2021). Despite not having populational implication, it is still
significant for those individuals who saw their net energy intake negatively affected by the
disturbance. This could in turn affects their breeding successes and/or their ability to care for
calves, which coincides with what was concluded by Beale and Monaghan (2004) in their sea
birds study.
Booth (2019) conducted a study which shows the disruptive effects of noise disturbance on
feeding behaviour of harbor porpoise and thus affected their energetic requirement for
growth, homeostasis and reproduction. This study is highly similar and comparable to the
study in focus as it is on another species of cetacean, being affected by the same type of
disturbance which was shipping noise and measuring the same variable that is net energy
intake. McKenna et al. (2013) stated that the volume of shipping sound can reach up to 180
dB, which is not far off from the volumes of CEEs that was used in the focused study (160
dB), which implies that not only controlled, frequent sound can disturb marine mammals, but
the sound of ships moving at a constant rate over the ranges of these animals could have a
detrimental effect on their foraging behaviour and successes.
Furthermore, Goldbogen et al. (2011) stated that the lunge manoeuvre perform by blue
whales is extremely costly in term of energy, hence it is critical that they successfully engulf
a patch of krill that is high in density in order to “justify” the large amount of energy spent
for such manoeuvre. Because as much as 50% of the simulations ran showed a decrease in
net energy intake (Pirotta et al., 2021). This could have serious implication on feeding
behaviour of blue whales, as their lunge is energetically costly (Goldbogen et al., 2011), it is
crucial that the amount of energy intake from a manoeuvre at least equal to the amount of
energy required in order for them to maintain body function.
Lastly, through a series of CEEs which played random sound at random volumes and
frequencies in popular blue whale foraging grounds, Friedlander et al. (2016) concluded that
blue whales responded strongly and clearly to CEEs, in which it decreases the depth of their
lunge for krill. In relation to the study in focus, it expanded from the study of Friedlander et
al. (2016) by being more specific on the type, frequency and intensity of the disturbance,
showcasing the adverse effect on foraging behaviour from a sound of a certain volumes (75160 bB) coming from a common occurrence in the form of military exercises.
The results of this study can provide important clues and information on the conservation of
blue whales and cetacean in general. According to Cooke (2018), blue whale is officially
classed as an endangered species, with their numbers being threaten by climate change and
various anthropogenic activities such as fishing and shipping. Based on what was concluded
and gathered in the focus study, integrated with aforementioned past studies, conservation
plans and strategies could be developed in order to preserve, protect and possibly increase
their population, which at the time of writing is from 5,000 to 15,000 individuals (Cooke
2018). Since it is now known that sounds within the range of 160-180 dB can seriously
disturb foraging behaviour and ultimately their daily net energy intake, which could result in
decrease in reproductive successes and the ability to care for young of blue whales and other
cetacean, which will only further damage their population. Perhaps an alternative shipping
routes, away from popular whales foraging grounds could be used during appropriate time of
year, such as during breeding season which falls under autumn and winter (Mizroch et al.,
1984), ensuring that their foraging activities remain undisturbed during this crucial period.
An alternative solution could be the development of a quieter ship engine, or more
sophisticated sound proof system in order to either reduce the volume of ship engines or
muffle the sound that it makes while travelling through whale feeding areas. This could be
costly and difficult, however, if successful, it could be the long-term solution against noise
disturbance, and not only will it benefit blue whales but other marine animals as well.
The result of the study is significant in term of the effects the disturbance, in form of CEEs,
had on individual blue whales and their foraging activity, however, as the researchers
mentioned, it is inapplicable to a population due their environment being highly variable
(Pirotta et al., 2021). This could be a limitation of the study because the results, despite being
significant on an individual level, were insignificant on a population level, hence it could be
difficult to use it to develop a conservation strategy or as an evident of the adverse effects
disturbance should it need to be presented to the public or related government bodies.
Another and perhaps more significant limitation is that the study failed to consider the effects
that the disturbance has on krill density in those areas which might significantly affect blue
whales foraging behaviour and ultimately the result of the study. As mentioned by Goldbogen
et al. 2011 rorqual whales (which include blue whales) do not usually dive deep while
foraging but choose to engulf massive amount of krill-filled water instead. Hypothetically
speaking, if the disturbance cause krill to occur deeper than their usual depth and that of blue
whales foraging range, it could be the factor that cause the large number of negative net
energy intake that were collected in the study. This could also mean that the noise disturbance
might not have affected the whales as much as the researchers have found.
Despite its limitations, the focused study is still significant, as not only it expanded on
previous work with regard to CEEs as a tool to measure and investigate behavioural
responses in marine mammals, it also provides a snapshot on the effects of frequent loud
noise on foraging activity of individual blue whales, which when further studied and
researched could prove to be useful in future conservation efforts and studies.
When studying wild animals, especially measuring and investigating their behaviours through
control experiments, it is extremely difficult to avoid ethical complications. In the focused
study, 27 individual blue whales were tagged and exposed to loud noises for a prolonged
period of time, twice in a year, while they were foraging for food (Pirotta et al., 2021). 50%
of the simulations resulted in negative net energy intake, with 11% of those being negative
(Pirotta et al., 2021). The obvious short-term effect on the animals is not getting enough food
and energy in order to be healthy and maintain bodily function in that period of time. In the
long term, it could see the whales avoiding that area all together, which might mean that they
have to travel further in order to forage for undisturbed, for the same amount of energy, and it
could possibly further damage their ability to successfully produce. As mentioned above, the
breeding season of blue whale is around autumn to winter (Mizroch et al., 1984) and their
gestation period is around 12 months (Sears & Perrin 2018), these whales could be carrying
their young during the first temporal subset (July) of the experiment or preparing and
stocking up for their trip to mate during the second temporal subset (October) (Pirotta et al.,
2021).
In conclusion, despite its flaws and limitations, “Context-dependent variability in the
predicted daily energetic costs of disturbance for blue whale” by Pirotta et al. (2021)
provided various valuable insight on the effects of constant noise disturbance on individual
blue whales. Not only does it further develop methods and materials used in past studies, it
also solidifies and specifies some of its old findings. Further studies in the field, with more
developed technique and considered variable, using this as a ground work could further prove
its value to the literature and the landscape of marine mammals study as well as future
conservation efforts.
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