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Abstract

The great whales (baleen and sperm whales), through their massive size and wide distribution, influence ecosystem and carbon dynamics. Whales directly store carbon in their biomass and contribute to carbon export through sinking carcasses. Whale excreta may stimulate phytoplankton growth and capture atmospheric CO2; such indirect pathways represent the greatest potential for whale-carbon sequestration but are poorly understood. We quantify the carbon values of whales while recognizing the numerous ecosystem, cultural, and moral motivations to protect them. We also propose a framework to quantify the economic value of whale carbon as populations change over time. Finally, we suggest research to address key unknowns (e.g., bioavailability of whale-derived nutrients to phytoplankton, species- and region-specific variability in whale carbon contributions).
Opinion
Whales in the carbon cycle: can recovery
remove carbon dioxide?
Heidi C. Pearson ,
1,
*
,@
Matthew S. Savoca,
2,@
Daniel P. Costa,
3
Michael W. Lomas,
4
Renato Molina,
5
Andrew J. Pershing,
6,@
Craig R. Smith,
7
Juan Carlos Villaseñor-Derbez,
2,8
Stephen R. Wing,
9
and Joe Roman
10
The great whales (baleen and sperm whales), through their massive size and
wide distribution, inuence ecosystem and carbon dynamics. Whales directly
store carbon in their biomass and contribute to carbon export through sinking
carcasses. Whale excreta may stimulate phytoplankton growth and capture
atmospheric CO
2
; such indirect pathways represent the greatest potential for
whale-carbon sequestration but are poorly understood. We quantify the carbon
values of whales while recognizing the numerous ecosystem, cultural, and
moral motivations to protect them. We also propose a framework to quantify
the economic value of whale carbon as populations change over time. Finally,
we suggest research to address key unknowns (e.g., bioavailability of whale-
derived nutrients to phytoplankton, species- and region-specic variability in
whale carbon contributions).
Whales and the oceanic carbon cycle
The ocean is an important carbon sink (see Glossary), absorbing approximately 22% of anthropo-
genic carbon emissions during 20102019 [1], some of which is exported to the deep sea [2]. A
better understanding of the role of its largest biota, the great whales,inthecarbon cycle is needed
to better inform ocean management and carbon dioxide removal (CDR) strategies. We discuss
the importance of whales to marine ecosystems and their role in the biological carbon pump
(BCP), assess their potential to contribute to carbon storage and carbon sequestration,and
consider how whale carbon services could play a role in climate-change mitigation strategies.
Whales and marine ecosystems
Blue (Balaenoptera musculus)andn(Balaenoptera physalus) whales are the two largest animals
to ever exist on Earth [3,4]. The gigantism of lter-feeding baleen whales results from high energetic
efciency facilitated by feeding on dense, high-energy prey patches with physical adaptations that
allow them to engulf large amounts of prey [5,6]. Their size and longevity allow great whales to exert
strong effects on the carbon cycle by: (i) storing carbon more effectively than small animals [7],
(ii) ingesting extreme quantities of prey [7], and (iii) producing large volumes of waste products
[8]. Considering that baleen whales have some of the longest migrations on the planet [9], they
potentially inuence nutrient dynamics and carbon cycling over ocean-basin scales [1012].
Industrial whaling is estimated to have reduced great whale biomass by 81% (L.B. Christensen,
MSc thesis, The University of British Columbia, 2006), resulting in the removal of an estimated
0.017 Gt carbon stored in baleen whale biomass [7]. In addition, depletion of the great whales
may have altered top-down forcing [1315], which further impacted carbon-cycling dynamics
[8,1618]. Whereas industrial whaling had the largest impact on great whale population
Highlights
As climate change accelerates, there is
increasing interest in the ability of whales
to trap carbon (i.e., whale carbon), yet it
is currently undetermined if and how
whale carbon should be used in
climate-change mitigation strategies.
Restoring whale populations will en-
hance carbon storage in whale biomass
and sequestration in the deep sea via
whale falls, though the global impact will
be relatively small.
Whale-stimulated primary productivity
via nutrient provisioning may sequester
substantially more carbon, though there
is uncertainty regarding the carbon fate
in these food webs.
Recovery of whale populations via re-
duction of anthropogenic impacts can
aid in carbon dioxide removal but its in-
clusion in climate policy needs to be
grounded in the best available science
and considered in tandem with other
strategies known to directly reduce
greenhouse gas emissions.
1
Department of Natural Sciences,
University of Alaska Southeast, Juneau,
AK, USA
2
Hopkins Marine Station, Stanford
University, Pacific Grove, CA, USA
3
Ecology and Evolutionary Biology
Department, University of California
Santa Cruz, Santa Cruz, CA, USA
4
Bigelow Laboratory for Ocean
Sciences, East Boothbay, ME, USA
5
Rosenstiel School of Marine,
Atmospheric, and Earth Science and
Miami Herbert Business School,
University of Miami, Miami, FL, USA
6
Climate Central, Inc., Princeton, NJ, USA
7
Department of Oceanography, University
of HawaiiatMānoa, Honolulu, HI, USA
Trends in Ecology & Evolution, Month 2022, Vol. xx, No. xx https://doi.org/10.1016/j.tree.2022.10.012 1
© 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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TREE 3074 No. of Pages 12
abundance [19] and body size [20], modern populations continue to face threats from sheries
entanglement [21], climate-change-induced shifts in prey distribution and abundance [22],
noise pollution [23], ship strikes [24], marine debris [25], and, in some areas, continued commer-
cial whaling [26].
The BCP
The oceans carbon pump is a complex combination of biological, chemical, and physical
processes that control the transfer of carbon into the ocean interior where it is removed from
exchange with the atmosphere for long time periods [27,28]. The BCP dominates the total
sequestration of carbon in the ocean [29] and it can vary regionally by nearly an order of magnitude.
Carbon export generally decreases from the poles to the equator and nearshore to the ocean gyres.
The BCP can be further separated into the gravitational export pump and particle injection
pumps [27],whichinaggregateareestimatedtotransfer412 Gt C year
1
to the deep sea [2,30,31].
Diel vertical migrations (DVMs) of mesopelagic shes and zooplankton drive the mesopelagic-
migrant pump (one component of the particle injection pump), comprising the largest migration of
animals on Earth in terms of abundance and biomass [32]. Consequently, the DVM of mesopelagic
nekton can actively transport nutrients and organic matter between the epipelagic and deep ocean
[32,33]. DVM transport of carbon and nitrogen from the epipelagic, where many copepods,
euphausiids, and salps feed at night in shallow, phytoplankton-rich waters, to depths where they
excrete waste products can be a large component of carbon and nitrogen export [34,35]. Nonethe-
less, in some circumstances, the uxes of organic matter and nutrients may be upward. For
example, Antarctic krill (Euphausia superba) can forage on the benthos and then migrate into
surface waters, releasing high concentrations of new and remineralized iron into the photic zone
[34,36,37]. Further, marine mammals (e.g., whales), seabirds, and large shes forage extensively
on vertically migrating zooplankton and mesopelagic sh [38,39], releasing nutrient-rich
excreta into surface waters and stimulating primary production [40,41]. These connections comprise
a potentially important feedback for carbon export and storage.
Carbon persistence
The nomenclature of carbon persistence differs by timescale. Carbon storage refers to
short-term carbon retention in organismal tissues [42]. For longer-lived animals such as
whales, carbon storage can be on the order of decades to centuries. For organisms in the
great whale food web (phytoplankton, zooplankton, forage sh, and squid), carbon storage
is days to years.
Carbon sequestration, however, extends beyond the time horizon of carbon storage and it often
follows from more complex dynamics. For example, great whale excreta are highly enriched in
limiting nutrients, including nitrogen, phosphorus, and trace metals (e.g., iron) that baleen
whales recycle within the epipelagic (see Whale pump section; Figure 1A) [11,40,4348]. In the
case of sperm whales (Physeter macrocephalus), the transport of these limiting nutrients from
the deep ocean can enhance phytoplankton production in the photic zone. A portion of this
whale-stimulated primary and secondary productivity is recycled within the upper-ocean detrital
food web. The amount that escapes recycling within the upper ocean can be exported below the
maximum mixing depth and sequestered on timescales of centuries to millennia. As such, deep
carbon export leading to sequestration is an important sink for CO
2
, yet the inuence of whales on
deep carbon export and the BCP is largely unknown. Export efficiency (which can lead to long-
term sequestration) spans three orders of magnitude (range: 0.00010.24 [49,50]) due to
variability in productivity across the global ocean. It is also a process particularly difcult to
inuence and measure.
8
Bren School of Environmental Science
& Management, University of California
Santa Barbara, Santa Barbara, CA, USA
9
Department of Marine Science,
University of Otago, Dunedin,
New Zealand
10
Gund Institute for Environment,
University of Vermont, Burlington, VT,
USA
*Correspondence:
hcpearson@alaska.edu (H.C. Pearson).
@
Twitter: @uasoutheast (H.C. Pearson),
@DJShearwater (M.S. Savoca), and
@sci_officer (A.J. Pershing).
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Direct and indirect carbon pathways of whales
Direct pathways
Biomass
In the pelagic ocean, animals comprise the vast majority of the biomass capable of storing
carbon from one year to the next [51]. Because whales are among the longest-lived animals,
they may constitute one of the largeststable living carbon poolsin the pelagic ocean. The singularly
large size of thegreat whales means that they interact with the carbon cycle in unique ways. While it
is obvious that whaling reduced the amount of carbon stored in populations of great whales, it is
not clear if whaling was a net source of CO
2
to the atmosphere.
Pershing et al.[7] proposed that because metabolic rates scale with individual biomass raised to a
power less than one (typically assumed to be ¾ [52]), the large size of whales makes them more
efcient at storing carbon. Using ¾ scaling, they calculated that removing one 92-ton blue whale
would provide enough uneaten krill to support seven minke whales (Balaenoptera acutorostrata)
or 1800 Adelie penguins (Pygoscelis adeliae). However, the new minke whales would have only
50% of the biomass of the missing blue whale, while the new penguins would have only 8% of
the biomass. This relationship suggests that the reduction of blue whales and other baleen
whales in the Southern Ocean has led to an ecosystem that maintains a lower total biomass of
living carbon and releases a greater amount of CO
2
for a given amount of ingested krill, than
prior to industrial whaling.
Whale falls
Large body size means that whales are particularly efcient at transferring carbon to the deep sea
via whale falls [7,53]. The overall reduction in the number of whales and, specically, the loss of
larger species and larger individuals, has altered the ux of whale carbon to the deep sea and
caused extinctions of whale-fall specialists in the North Atlantic [20].
Indirect pathways
Whale excreta released in surface waters are rich in limiting nutrients. These nutrients, released
across both vertical and horizontal gradients, may stimulate carbon fixation by phytoplankton,
enhancing ecosystem productivity and potentially stimulating carbon storage, export, and
sequestration (Figure 1)[11,40,54,55]. Two indirect pathways are discussed next: the whale
pump and the great whale conveyor belt.
Whale pump
Buoyant fecal plumes released by whales can provide limiting nutrient concentrations three to
seven orders of magnitude higher than background seawater concentrations [40,4547,56]
and can persist at the surface, allowing nutrients to leach into the surrounding seawater and
become bioavailable to phytoplankton [40,45,46,57]. Nutrient recycling and transport by whales
and other top predators may push planktonic food web structure toward systems dominated by
large-celled, quickly sinking diatoms and krill that could affect carbon export [44,58]. DVM by
whale prey (e.g., krill) and other nekton provides a particle injection pump that increases export
efciency relative to gravitational ux of particles alone [27]. In locations of high productivity and
whale abundance, zooplankton fecal material, exuviae, and carcasses provide considerable
pulses of carbon to the deep sea, thereby demonstrating the importance of whale prey to
deep carbon export [50,5961].
Whales unlock nutrients in prey through their feeding behavior and digestive processing. Speci-
cally, allochthonous nutrients promote new primary production, carbon storage, export, and
potentially, sequestration, while autochthonous nutrients promote recycled primary production
Glossary
Additionality: carbon benets,accrued
from a policy/management intervention,
exceeding the status quo.
Allochthonous nutrients: nutrients
originating outside a dened system.
Autochthon ous nutrien ts: nutrients
recycled within a dened system.
Biological carbon pump (BCP): sum
of all biological and physical processes by
which biologically derived organic carbon
is transported to the ocean depth.
Blue carbon: carbon naturally captured
by marine biota, originally used in
reference to coastal mangroves, salt
marshes, and seagrasses [83].
Capital breeder: animal that feeds and
stores energy before the breeding
season, relying on accumulated energy
stores to reproduce.
Carbon cycle: sum of processes
comprising the ux of carbon atoms
between the atmosphere, oceans, soils,
and sediments, and the organisms living
therein.
Carbon dioxide removal (CDR):
capture or retention of CO
2
,either
naturally or articially, so that it is taken
from the atmosphere for a dened
period of time.
Carbon export: ux of organic carbon,
derived from CO
2
via photosynthesis,
from the upper water column to depth
(often considered below the maximum
annual surface mixed layer depth).
Carbon fixation: conversion of
inorganic carbon (CO
2
) into organic
carbon (i.e., photosynthesis).
Carbon persistence: tendency for
carbon to remainin a reservoir and out of
atmospheric contact.
Carbon sequestration: removal of
atmospheric organic carbon, generally
via photosynthesis, for geologic
timescales 100 years.
Carbon service: natural processes
that remove organic carbon, derived
from CO
2
via photosynthesis, from the
atmosphere.
Carbon sink: process that removes
more carbon (generally derived from
CO
2
via photosynthesis) from the
atmosphere than it emits.
Carbon storage: removal of carbon,
derived from CO
2
via photosynthesis,
from the atmosphere for <100 years.
Diel vertical migration (DVM): daily
movement of zooplankton, sh, and
cephalopods (e.g., squid, octopus) to
feed nocturnally in surface waters before
returning to depth during the daytime.
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(Figure 1A[62]). Deep-diving cetaceans, such as sperm whales, feed almost exclusively on
cephalopods living well below the surface mixed layer [63,64] and thus are presumed to release
allochthonous fecal nutrients at the surface (Figure 1A) [43]. In contrast, shallow-diving cetaceans,
such as baleen whales, often feed within the surface mixed layer [8,64] and thus are presumed to
generally release autochthonous nutrients at the surface [44].
The distinction between new versus regenerated nutrients has implications for carbon cycling
and, importantly, export calculations. Nonetheless, observations that overall ecosystem produc-
tivity has declined in regions with depleted whale populations (e.g., the Southern Ocean) suggest
that the role of whales in nutrient cycling is critical to ecosystem functioning and their associated
carbon sequestration potential [8,10,11,40,44].
Great whale conveyor belt
Many great whales are capital breeders, migrating from high-latitude, nutrient-rich feeding
grounds where they spend the summer to low-latitude, nutrient-poor breeding grounds where
they spend the winter (Figure 1B) [65]. Whales usually fast on the breeding grounds, metabolizing
lipid-rich blubber for maintenance and, in females, for lactation [10]. Allochthonous nutrient inputs
via carcasses, excreta (e.g., urine, placentas), and other byproducts (e.g., sloughed skin) derived
from nutrients ingested on the feeding grounds have the potential to stimulate new productivity,
carbon export, and carbon sequestration when they are released on the breeding grounds [65].
For species such as humpback (Megaptera novaeangliae), right (Eubalaena spp.), and gray
(Eschrichtius robustus) whales, breeding grounds tend to be shallow, with lower nutrient concen-
trations and higher densities of breeding whales. The nutrient subsidies that migratory whales
bring into these systems also support sh, scavengers, and benthic macroinvertebrates [66].
Quantifying the carbon roles of whales
To date, ve studies have quantied the role of the great whales in direct or indirect carbon
pathways (Table 1). However, we caution that current estimates do not represent the total direct
and indirect carbon contributions from all great whale species across all oceans. Specically,
global biomass and whale-fall carbon values did not consider sperm whales [7], the whale
pump was examined for just ve great whale species in the Southern Ocean [8,43,44], and
examination of the great whale conveyor belt was limited to Southern Hemisphere blue whales
[10]. Thus, the values in Table 1 represent a fraction of the total contribution of the great whales
to the carbon cycle. It should be noted that some estimates report wide condence intervals
spanning multiple orders of magnitude. We have a higher level of condence in the direct carbon
pathway estimates, such as biomass and whale falls, than the indirect carbon pathway estimates.
Nonetheless, the great whale populations studied to date (Table 1) currently produce an estimated
2.0 × 10
3
Gt C storage [7], 6.2 × 10
5
Gt C year
1
sequestration, 2.2 × 10
2
Gt C year
1
xation
[8,10], and 4.0 × 10
4
Gt C year
1
export [43].
As industrial whaling devastated great whale populations and their associated carbon benets by
an order of magnitude or more (Table 1), these values should be considered a lower bound
because the full carbon values of the great whales have yet to be restored. This may provide an
opportunity to enhance CDRfrom the great whales. Still, the currently known carbon contributions
from the great whales are small compared with carbon stored in all marine animals (~1.4 Gt C [67])
or exported via the BCP [2,30,31].
Recently, an outsized role for great whales in carbon xation, storage, and sequestration was
suggested (e.g., carbon contribution of a single blue whale valued at $1.4 million [68,69]).
These estimates, based on assumptions beyond our understanding of whale ecology and
Earth system model: model that
simulates carbon movement through
atmospheric, terrestrial, and marine
environments.
Epipelagic: open ocean waters from
the surface to approximately 200 m
depth.
Export efficiency: portion of primary
productivity (PP) sinking from the upper
water column (oftentimes dened as
below the maximum annual surface
mixed layer depth), quantied as export
ux/PP.
Gravitational export pump:
downward ux of carbon in the ocean
due to gravitational sinking of biologically
derived particles.
Great whales: baleen and sperm
whales.
Limiting nutrient: nutrient required by
phytoplankton for photosynthesis but
typically present in small quantities and
which limits phytoplankton rate
processes and/or standing stocks.
Mesopelagic-migrant pump: type of
particle injection pump through which
vertically moving biota can directly, by
grazing, and indirectly, by boundary
layer drag, move particles from the
surface ocean to depth.
Nature-based solution (NBS): action
that conserves and sustainably
manages ecosystems that also
promotes biodiversity and human
well-being.
Particle injection pump: physical
processes that actively transport
biologically derived carbon to ocean
depths.
Pelagic: open ocean waters.
Photic zone: sunlit portion of the upper
water column where photosynthesis
occurs.
Surface mixed layer: uppermost,
unstratied layer of the water column.
Top-down forcing: ecosystem effects
arising from upper-trophic level
changes.
Whale fall: whale carcasses thatsink to
the deep sea.
Whale-fall sp ecialists: fauna relying
on deep-sea whale carcasses to
complete their life cycles.
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Figure 1. Great whalesdirect and indirect nutrient and carbon cycling pathways. (A) A hypothetical Southern Ocean, demonstrated by two endmembers
(deep-diving sperm whale, Physeter macrocephalus, left, vs. shallow-diving mysticete, right) that share the same direct carbon pathways (i.e., biomass carbon, whale falls) but differ
in their indirect carbon pathways (i.e., the whale pump). Sperm whales, feeding below the mixed layer, release new nutrients at the surface, stimulating new primary production,
carbon export, and carbon sequestration. Mysticetes, feeding within the mixed layer, release regenerated nutrients at the surface, stimulating recycled primary production and
carbon export. Arrow thickness indicates relative magnitude of the carbon ux. Arrow color indicates direct (orange) versus indirect (white) pathways. (B) The great whale conveyor
belt, an indirect carbon pathway depicting nutrient and carbon uxes between humpback whale (Megaptera novaeangliae) feeding and breeding grounds in the North Pacic
Ocean. Nutrients released on the breeding grounds are assumed to be new nutrients to the system, thus stimulating new primary production, carbon export, and carbon
sequestration. Nutrients released on the feeding grounds will follow a pathway similar to the mysticete in panel A. While not shown, direct carbon pathways (see panel A) will
also occur on both the feeding and breeding grounds. Illustrations by Alex Boersma.
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biological oceanography, give values for whales that do not acknowledge the complexity and
spatiotemporal variability inherent to whale ecosystem effects (Box 1). We recommend an
economic framework for quantifying the carbon values of great whales that is grounded in
whale ecology and biological oceanography (Box 2). Earth system models may aid in quantifying
indirect carbon pathways by simulating the impact of whale-derived nutrients on phytoplankton
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Figure 1 (continued).
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6Trends in Ecology & Evolution, Month 2022, Vol. xx, No. xx
and zooplankton communities and their associated carbon export. As data become available, the
economic framework can be populated to estimate whale carbon values for different whale popu-
lations and the predicted impact of conservation interventions.
Can great whale recovery be a nature-based solution to climate change?
Two-thirds of signatories to the Paris Agreement have committed to nature-based solutions (NBS)
to meet the goal of limiting global air temperature rise to 1.5°C above pre-industrial levels [70,71].
Protection of blue carbon ecosystems is a nature-based climate mitigation and adaptation solution
declared in the nationally determined contributions (NDCs) of 45 nations [72]. Whereas these NDCs
pertain to coastal blue carbon, quantication of carbon values in other marine biota has expanded
the concept to include noncoastal or oceanic systems [42,7379]. For marine vertebrates, how-
ever, based on currently available data, there remains signicant uncertainty about their carbon
sequestration potential and the ability to effectively manage their role in CDR remains debated
[80]. As more data become available, it is likely this level of uncertainty will decrease.
There have been several studies examining the impact of whales on the carbon cycle that hold
promise for including whales in blue carbon schemes (Table 1), though whether they achieve the
criteria outlined by Lovelock and Duarte remains to be seen [73]. Although it is clear that great
whales are capable of CDR through direct pathways (Table 1), the many unknowns in the indirect
pathways (Figure 1,seeOutstanding questions) preclude quantication at this time of the full
magnitude of whale-CDR required for integration into NBS calculations (e.g., sensu [80,81]) or
NDCs. For comparison, coastal blue carbon systems sequester 0.080.2 Gt C year
1
[82] and
marine vertebrates (including great whales) assessed so far sequester 0.030.05 Gt C year
1
(though this covers a limited number of species across limited regions) [54]. The global potential
for CDR via marine ecosystem recovery is likely to be <0.3 Gt C year
1
[65].
Table 1. Estimated carbon (C) fate (xed, stored, exported, sequestered) in pre-industrial whaling and modern-day great whale populations
Mechanism Species Region Pre-whaling C estimate
a
(N) Modern C estimate
(N, year)
C fate Refs
Biomass 8 Baleen whale taxa
b
Global 1.1 × 10
2
Gt C (2.9 × 10
6
) 2.0 × 10
-3
Gt C
(1.1 × 10
6
, 2016)
Stored
c
Updated
from [7]
d
Whale falls All baleen whales Global 3.6 × 10
4
Gt C/year
(2.9 × 10
6
)
6.2 × 10
-5
Gt C/year
(1.1 × 10
6
, 2016)
Sequestered Updated
from [7,95]
e
Whale pump Blue whale Southern Ocean 1.3 × 10
1
Gt C/year, range:
5.2 × 10
2
Gt C/year to
2×10
2
Gt C/year (2.4 × 10
5
)
2.8 × 10
-3
Gt C/year,
range: 1.2 × 10
-3
to
4.5 × 10
-3
Gt C/year
(5.2 × 10
3
, 2012)
Fixed [44]
Whale pump Blue, n, humpback, and
Antarctic minke whales
Southern Ocean 2.2 × 10
1
Gt C/year; range:
2.7 × 10
2
to 1.5 Gt C/year
2.2 × 10
2
Gt C/year;
range: 2.7 × 10
-3
to
1.5 10
1
Gt C/year
Fixed [8]
Whale pump Sperm whale (Physeter
macrocephalus)
Southern Ocean 2.4 × 10
3
Gt C/year
(~1.2 × 10
5
)
4×10
4
Gt C/year
(1.2 × 10
4
, 2001)
Exported [43]
Great whale
conveyor belt
Blue whale Southern
Hemisphere
1.4 × 10
4
Gt C/year
(3.3 × 10
5
)
5.1 × 10
7
Gt C/year
(1.2 × 10
3
, 2001)
Fixed [10]
a
All C values are gross and do not account for the amount of C respired by great whales.
b
Minke (Balaenoptera acutorostrata), Antarctic minke (Balaenoptera bonaerensis), sei (Balaenoptera borealis), Brydes(Balaenoptera brydei), blue (Balaenoptera musculus),
n(Balaenoptera physalus), bowhead (Balaena mysticetus), gray (Eschrichtius robustus), right (Eubalaena spp.), and humpback (Megaptera novaeangliae)whales.
c
While some great whales can live >100 years, most known lifespans of these species are <100 years; thus, the C is considered stored.
d
Values calculated per Pershing et al.[7] using updated pre-whaling and modern population estimates from Smith et al.[20].
e
Pershing et al.[7] was the rst attempt to calculate C export by whale falls. They used a conservative assumption that 50% of rorqual and gray whale carcasses are exported
while only 10% of right and bowhead whale carcasses are exported. Smith and Baco [95] suggested that a high proportion of whale deaths occur during migration. Even for
North Atlantic right whales (Eubalaena glacialis), a coastal species with a robust monitoring network, the observed number of carcasses is much lower than the estimated
number of whale deaths. For this paper, we used higher percentages: 90% for rorquals and gray whales, 50% for right and bowhead whales. This increases the export ux
estimates by a factor of 1.8 compared with the original values.
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A key component of blue carbon is the ability to manage and protect ecosystems for climate-
change mitigation [73,83]. The full CDR role of great whales (and other organisms) will only be
realized through robust conservation and management interventions that directly promote
population increases (e.g., bycatch mitigation, including entanglement reduction; vessel speed
and noise reductions in whale hotspots; creation of marine protected areas). We emphasize
that for whales to aid in CDR, additionality needs to occur [84].
Noting this, natural enhancement of the oceanic carbon sink via whale recovery could be an effec-
tive low-regret[85] CDR strategy with less risk, longer permanency, and higher efciency than
geoengineering solutions (e.g., direct carbon injection into the ocean interior [86]). While the articial
enhancement of the BCP may be leaky, with effects persisting for a few decades to approximately
150 years [86], the lifespans of great whales range from at least 50 to 200 years [87,88]. When
carbon persistence via reproduction is considered [54], whale recovery has the potential for
long-term self-sustained enhancement of the ocean carbon sink by increasing the standing
stock of stored carbon and carbon sequestered in the deep sea via direct pathways and enhancing
the BCP via indirect pathways (Figure 1). Indeed, ecosystem functions (including carbon services)
of whales have already been formally recognized by resolutions to the International Whaling
Commission and the Convention on Migratory Species [66].
Concluding remarks
Understanding the role of whales in the carbon cycle is a dynamic and emerging eld that may
benet both marine conservation and climate-change strategies. However, while whales are ca-
pable of some degree of CDR, there are numerous knowledge gaps (see Outstanding questions)
[66] that must also be addressed by policy makers seeking to incorporate whale conservation
Outstanding questions
How effective are whales at creating
and maintaining primary productivity
hotspots leading to enhanced
carbon sequestration? Quantication of
cetacean foraging depth with respect
to the annual mixed layer depth is
needed to determine the contribution
of fecal nutrients to carbon recycling
versus export. The proportion of
carbon derived from whale-stimulated
productivity that persists in the food
web versus exported, and potentially
sequestered, likely varies by location
and season.
How bioavailable are whale-derived nu-
trients? Phytoplankton uptake rate of
whale-derived nutrients depends on
numerous poorly understood factors
(e.g., whale defecation depth, persis-
tence of whale excreta in the photic
zone, nutrient leaching rate, organic
ligand presence, variation by whale
and phytoplankton species, microbial
loop remineralization). Estimates from
previous studies should be empirically
tested; incubation experiments may
help.
What is the carbon ux from cetaceans
to the atmosphere? Carbon ux from
cetaceans to the atmosphere via
respiration is needed to provide
cetaceansnet capacity for CO
2
removal. Few studies have included
estimates of CO
2
loss to the
atmosphere via respiration. Empirical
data on breathing rates, respiratory
tidal volume, and CO
2
exchange rates
in the lungs are needed.
How does cetaceansimpact on the
carbon cycle vary by species and
region? Cetaceanscarbon effects
likely have a patchy spatiotemporal
distribution and spatial variability in
sequestration timescales prohibits
a single model of whale-mediated
carbon sequestration for the global
ocean. Caution must be exercised
when extrapolating ndings across
species or regions, especially if both
direct and indirect pathways are not
included.
Box 1. Monetizing whale carbon
Whales provide numerous ecosystem services such as tourism in the form of whale watching, enhanced primary productivity,
and carbon sequestration, alongwith culturalbenets such as education, aesthetics, and existence value [89]. Recent attempts
to monetize whales have garnered attention by valuing an averagewhale at $2 million for carbon-capture and other services
[68,69]. As the authors of several foundational papers cited in these reports, we feel the scientic support for this valuation is
lacking [90,91]. Here, we present ve concerns regarding the monetary valuations of carbon dioxide removal (CDR) by whales
as proposed by Chami et al.[69].
1. Whales are proposed to increase primary productivity globally by 1%, with no supporting empirical data. Whales can
increase primary productivity, but the magnitude of this varies by species, location, and season [66]. Whales are absent
from vast areas, with no indication that they could increase phytoplankton productivity by 1%.
2. Organismslifespan inuences the duration of carbon storage. Whereas whales can store carbon for decades or lon-
ger, phytoplankton and zooplankton typically store carbon for days to months (see Carbon persistence). Such short-
term storage is generally not considered applicable for carbon-mitigation schemes, which seek storage for 100 years
[65]. Carbon must be transported into the deep ocean, or buried in sediments, to achieve centennial sequestration
times [86]. One method used to calculate export efciency assumes that 10% of net primary production is exported
and 10% of that is sequestered below 1000 m, but export and sequestration efciencies vary considerably by location
[65,86]. Regional variation in carbon-storage time and efciency could reasonably reduce the estimated CDR resulting
from whale stimulation of phytoplankton production by 100-fold.
3. Baleen whales often feed and defecate in the mixed layer, yielding primary production from nutrient regeneration [92].
Although surface-feeding whales can play an important role in nutrient recycling, they would have little effect on new
production, carbon export, or CDR, further reducing the values proposed.
4. The model relies on overly simplistic assumptions about whale population growth. Age and size structure should be
considered due to inuences on carbon-storage duration (Box 2).
5. The most advanced work on whales and carbon sequestration (see Whale falls) was not included.
Whales contribute to vital ecological functions and ecosystem services, including CDR in whale falls, yet recent valuations
go beyond our current understanding of their role in the carbon cycle [68,69]. To be viable for CDR strategies, carbon
removal by whales should be measurable and long-term, similar to the Veried Carbon Standard on land (verra.org). Field
experiments and models (e.g.,Earth system models) couldprovide the requisitedata, but recently proposed monetary values
should not be implemented in CDR or other policy strategies before such scientic documentation.
Trends in Ecology & Evolution
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8Trends in Ecology & Evolution, Month 2022, Vol. xx, No. xx
strategies as part of their climate policy. Persistence of carbon stored in whale bodies and
exported to the deep sea as whale falls via direct pathways are the best resolved and most
precise, yet the largest carbon benets may result from the indirect pathways, which are also
currently the least understood. Elucidating these key unknowns, including realizing the full poten-
tial for whales to sequester carbon, presents rich opportunities for further study. Achieving this
goal will require interdisciplinary collaboration between marine ecologists, oceanographers,
biogeochemists, carbon-cycle modelers, and economists.
Box 2. A bioeconomic framework for carbon valuation of a whale population
A simple framework to establish the carbon value of whales requires at least three components: (i) species-specic whale population dynamics, (ii) carbon dynamics, and (iii) eco-
nomic valuation. The rst component addresses how a whale and its offspring can capture carbon o ver their lifetimes. The second component addresses the processes by which
whales directly and indirectly contribute to the xation, storage, and sequestration of carbon. Both of these components will require species- or population-specic parameters.
The third component aggregates these dynamics and connects them to the social cost of carbon. This intergenerat ional sequestration potential allows us to quantify the total and
marginal carbon sequestration potential of whale populations. This framework allows for clear comparisons of different population trajectories and can also accommodate more
complex modeling approaches that tackle parameter uncertainty and complex processes. A visual representation of this framework is shown in Figure I, with a hypothetical
business-as-usual (BAU) scenario and two alternative policy interventions.
Model
We use bold symbols to denote matrices and vectors and Greek letters to denote parameters in the model. When relevant,we use subindices to denote specic times or
age classes.
Component 1: species-specic population dynamics
The discrete-time, density-dependent model of an age-structured population builds on [93] and is given by:
Ntþ1¼NtþδMIðÞNt½I
N
t
is a α× 1 vector of age-specic population sizes at time t. Parameter αdenotes the terminal age class of the population. The scalar δinduces density dependence in
the form δ¼KN
K, where Nis the total population size (or total mass) and Kis the carrying capacity. Finally, Mis an α×αLeslie matrix and Iis the identity matrix. Indexing
age class with the letter i, and letting μ
i
and σ
i
be the age-specic fecundity and survival, respectively, Mis given by:
M¼
μ1σ0μ2σ1μασα10
σ0000
0σ100
00σα10
2
6
6
4
3
7
7
5½II
Component 2: carbon dynamics
The stock of whale carbon at time tis then given by:
Ct¼a
i¼1NitMiCb
|fflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflffl}
Inbody carbon
þa
i¼1NitMiCp
|fflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflffl}
Stimulated
þa
i¼11σi
ðÞNitMiCs
|fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl}
Whalefall
½III
C
b
,C
p
,andC
s
represent the per-kilogram parameters of in-body storage of carbon, carbon storage via productivity enhancement, and sequestration due to death and
sinking, respectively. These parameters can be obtained from theoretical and eld studies [7,8,94]. Finally, M
i
is the mass-at-age.
Component 3: valuation
The present value of whale carbon over a horizon of time Tis given by:
V¼T
t¼1
θtCtψ
1þρðÞ
t½IV
θ
t
represents the social cost of carbon ($ per ton CO
2
), ρis the social discount rate, and ψis the molecular weight ratio of CO
2
to C. Using a BAU scenario, where whale populations
evolve with no additional interventions, allows us to estimate the current value of whale carbon, V
bau
. Simulating different interventions, like reducing or increasing mortality
(e.g., mitigating entanglement or increasing sheries activity in whale hotspots, respectively), one can calculate the net value of said intervention, V
pol
. The difference, V
pol
V
bau
,
is the relative value of carbon sequestered under the intervention. Figure I shows a comparison of two hypothetical alternative recovery paths and their value in relation to BAU.
Trends in Ecology & Evolution OPEN ACCESS
Trends in Ecology & Evolution, Month 2022, Vol. xx, No. xx 9
Given the high uncertainty in many of the processes described earlier, and for many natural
climate solutions in the ocean, we advocate for calculations of nature-based carbon sequestration
that explicitly tackle this uncertainty. Importantly, considering the severe and escalating nature of
the climate crisis, we feel it necessary that any consideration of whale recovery as a climate-
change mitigation strategy be carefully assessed and considered in tandem with other mitigation
strategies, particularly those that directly reduce greenhouse gas emissions. NBS grounded in a
whole-ecosystem perspective, which includes natural abundances of species in protected ecosys-
tems, will be essential to these approaches, as will better understanding of the ecological functions
provided by whales and other marine species [65]. We suggest that the precautionary principle be
applied to promote recovery of whale populations as a holistic ecosystem goal. This is likely to
garner multiple benets to combat the biodiversity and climate crises of the Anthropocene, including
enhanced ecosystem health, productivity, and resilience [8,10], with CDR being a co-benet[84].
There is power in the totality of actions taken concomitantly to meet the climate challenge.
Acknowledgments
We thank Whale and Dolphin Conservation for constructive feedback,especially Vicki James and Ed Goodall, and for funding
to support the graphics and publication fees. We also thank Kristen Krumhardt and two anonymous reviewers for helpful
feedback. M.S.S. was supported by the National Science Foundation (PRFB 1906332) and MAC3 Impact Philanthropies.
Trends
Trends
in
in
Ecology
Ecology &
Evolution
Evolution
Figure I. Stylized representation of the proposed bio-economic framework. Business-as-usual (BAU) represents the BAU scenario, while Pol1 and Pol2
represent recovery paths that are slower and faster at recovering the population, respectively. (A) Three population trajectories, (B) whale carbon associated with
each trajectory, (C) discounted value of carbon through time, (D) the difference value relative to BAU, and (E) the net present value (NPV) of each policy.
Trends in Ecology & Evolution
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10 Trends in Ecology & Evolution, Month 2022, Vol. xx, No. xx
Declaration of interests
No interests are declared.
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12 Trends in Ecology & Evolution, Month 2022, Vol. xx, No. xx
... They contribute to ecological dynamics through "top-down" and "bottom-up", "whale pump" and "whale fall" processes playing a vital role in the biochemical cycling of nutrients and carbon in the water column (Estes et al., 1998(Estes et al., , 2009Becker et al., 2021). Thus, they are considered ecosystem engineers and sentinel species crucial for ecosystem monitoring (Roman and McCarthy, 2010;Martin et al., 2021;Pearson et al., 2023). In addition, marine mammal research also contributes to the conservation and management of sensitive ecosystems and areas of interest for fisheries and marine protected areas (Bearzi and Reeves, 2022). ...
... Cetaceans, comprised of whales, dolphins, and porpoises, play a vital role in the complex structure and function of coastal ecosystems through a variety of mechanisms over both ecological and evolutionary time (Kiszka et al., 2015) and are generally considered as a crucial indicator of ocean health (Cossaboon et al., 2019;Fossi et al., 2020). Their role in top-down population control of some prey species (Williams et al., 2004;MacLeod et al., 2007), nutrient cycling (Gilbert et al., 2023), and carbon sequestration (Sheehy et al., 2022;Pearson et al., 2023) is critical for the stability of these ecosystems. Through their extensive movements, cetaceans contribute to nutrient redistribution via their excrement, fostering a dynamic environment that supports diverse marine life (Doughty et al., 2016). ...
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A comprehensive understanding of cetacean ecology is crucial for conservation and management. In 2018, Kaimana was identified as an Important Marine Mammal Area (IMMA) due to the regular presence of feeding aggregations of Australian humpback dolphins (Sousa sahulensis), Pacific bottlenose dolphins (Tursiops aduncus) and Bryde's whales (Balaenoptera edeni). Despite this, information on cetacean ecology in the Kaimana region is currently lacking. Notably, no cetacean surveys have been undertaken in Kaimana since it was officially recognized as an IMMA. We monitored food-provisioning interactions between lift-net fisheries and cetaceans from May 2021 to March 2023 to examine cetacean sightings, abundance and feeding associations. Five species were positively identified, including a new record of Killer whales (Orcinus orca). Our findings suggest a strong association between T. aduncus and lift-net fisheries, where they have been observed feeding on anchovies from outside the net in the morning. While other species were also observed, their presence was less frequent. Furthermore, year-round sightings of S. sahulensis, B. edeni, and T. aduncus during the study period indicate that these species are resident in this region. Our results suggest that Kaimana fulfills a second IMMA sub-criterion (small and resident populations of these three species) that was not previously noted in the original IMMA assessment.
... Beyond pollution, global climate change is likely to exacerbate anthropogenic impacts and physiological stress in marine mammals by intensifying habitat loss, altering ocean productivity, causing shifts in prey range and abundance, releasing pollutants stored in the sea ice, and increasing the frequency and severity of toxic algal blooms and pathogen outbreaks (Read, 2023;Davidson et al., 2012;Albouy et al., 2020;Gobler, 2020;Mahon et al., 2024). Conversely, the loss of marine mammals may exacerbate the effects of climate change on ecosystems, as large-bodied species play a role in carbon sequestration, nutrient cycling, and other vital ecosystem services (Pearson et al., 2023). Diverse stressors interact in complex ways, as is the case for mixtures of chemical pollutants, challenging the study of their combined effects (Wilson et al., 2016;Bestley et al., 2020;Romero, 2004;Wada, 2019;Tartu et al., 2017;Erbe et al., 2018). ...
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Marine mammals are integral to global biodiversity and marine health through their roles in coastal, benthic, and pelagic ecosystems. Marine mammals face escalating threats from climate change, pollution, and human activities, which perturb their oceanic environment. The diverse biology and extreme adaptations evolved by marine mammals make them important study subjects for understanding anthropogenic pressures on marine ecosystems. However, ethical and logistical constraints restrict the tractability of experimental research with live marine mammals. Additionally, studies on the effects of changing ocean environments are further complicated by intricate gene-environment interactions across populations and species. These obstacles can be overcome with a comprehensive strategy that involves a systems-level approach integrating genotype to phenotype using rigorously defined experimental conditions in vitro and ex vivo. A thorough analysis of the interactions between the genetics of marine mammals and their exposure to anthropogenic pressures will enable robust predictions about how global environmental changes will affect their health and populations. In this perspective, we discuss four challenges of implementing such non-invasive approaches across scientific fields and international borders: 1) practical and ethical limitations of in vivo experimentation with marine mammals, 2) accessibility to relevant tissue samples and cell cultures; 3) open access to harmonized methods and datasets and 4) ethical and equitable research practices. Successful implementation of the proposed approach has the potential impact to inspire new solutions and strategies for marine conservation.
... This not only reduces greenhouse gas emissions by increasing fuel efficiency, but also provides tangible benefits to climate-vulnerable whales, increasing their adaptive capacity by reducing mortality from ship strikes. Further, by restoring whale populations to pre-whaling numbers, sequestration of carbon in the deep sea from natural whale deaths could increase by nearly an order of magnitude [75,76]. Two international workshops are now being organized as a follow-up from this session, demonstrating that MPA managers are committed to collaborating and implementing innovative and cutting-edge adaptation actions. ...
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Climate change and its impacts are increasingly threatening the ability of marine protected areas (MPA) to meet their conservation goals. While integration of climate change into planning is critical, a recent global analysis found that relatively few MPAs have incorporated climate change considerations into formal management planning processes. Despite this, sessions and discussions at the Fifth International Marine Protected Areas Congress (IMPAC5) demonstrate that climate-adaptive management already permeates MPA processes, from day-today management to design and implementation. Here we review the results of an IMPAC5 knowledge exchange session that brought together a diverse group of MPA managers, Indigenous community representatives , and thought leaders to discuss improved integration of climate change into MPA management and planning. The session demonstrated the vibrancy, diversity, and engagement represented in the dynamic and fast-moving field of MPA climate change management and planning. In addition to sharing unique and diverse perspectives, the session leveraged the experience of experts to identify new and common challenges and gaps. As a result of this session, we present five recommendations, building on previous work, to guide MPA managers in the explicit and successful incorporation of climate adaptation into management planning and implementation. These recommendations hold the goal of ensuring an equitable, adaptive, and robust global MPA system. This review also provides a valuable summary of the vast repository of experience and knowledge of climate adaptive management contained within the MPA management community, and with community and Indigenous partners. Such perspectives are rarely reflected in formal scientific, policy, and management publications.
... While scientists warn that more data are needed to determine the exact role of cetaceans in carbon sequestration 102,103 , it is increasingly recognized that healthy cetacean communities are vital to the functioning of marine ecosystems 104,105 . Emerging evidence suggests that other marine mammals, such as small cetaceans 106 and sirenians 107,108 , also play important roles in maintaining Ocean Health. ...
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The current state of marine mammal populations reflects increasing anthropogenic impacts on the global Ocean. Adopting a holistic approach towards marine mammal health, incorporating healthy individuals and healthy populations, these taxa present indicators of the health of the overall Ocean system. Their present deterioration at the animal, population and ecosystem level has implications for human health and the global system. In the Anthropocene, multiple planetary boundaries have already been exceeded, and quiet tipping points in the Ocean may present further uncertainties. Long and short-term monitoring of marine mammal health in the holistic sense is urgently required to assist in evaluating and reversing the impact on Ocean Health and aid in climate change mitigation.
... Identifying foraging habitats and understanding the feeding ecology of oceanic apex predators such as odontocetes (toothed whales) is pivotal, given their important role in shaping the structure and functioning of marine ecosystems (Estes et al., 2011;Pearson et al., 2023). Hence, knowledge of their prey preferences and of the spatio-temporal variability of these prey contributes to a better understanding of their distribution patterns (Hastie et al., 2004;Friedlaender et al., 2006;Lambert et al., 2014;Mannocci et al., 2014aMannocci et al., , 2014b. ...
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Nutrient recycling by marine megafauna is a key ecosystem service that has been disturbed by anthropogenic activity. While some hypotheses attribute Southern Ocean ecosystem restructuring to disruptions in micronutrient cycling after the elimination of two million great whales, there is little knowledge of trace metal lability in whale excrement. Here we measured high concentrations of dissolved iron and copper in five baleen whale fecal samples and characterized micromolar levels of organic metal-binding ligands as a proxy for their availability. The iron-ligand pool consisted of weakly-binding ligands and intermediate-binding ligands which enhanced iron stability and potential bioavailability. In comparison, 47 novel strongly-binding metallophores dominated copper-binding, curtailing its potential toxicity. These results illustrate how marine megafauna transform prey biomass into highly-labile micronutrients that they inject directly into the surface ocean, a mechanism whaling reduced by over 90%. Thus, the rapid restructuring of pelagic ecosystems through overharvesting may cause large biogeochemical feedbacks, altering primary productivity and carbon sequestration processes in the ocean.
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Large animal conservation and rewilding are increasingly considered to be viable climate mitigation strategies. We argue that overstating animal roles in carbon capture may hinder, rather than facilitate, effective climate mitigation and conservation efforts.
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The ocean is gaining prominence in climate change policy circles as a tool for addressing the climate crisis. Blue carbon, the carbon captured and stored by marine and coastal ecosystems and species, offers potential as a “nature-based solution” to climate change. The protection and restoration of specific ocean ecosystems can form part of a climate response within climate mitigation policies such as Nationally Determined Contributions under the United Nations Framework Convention on Climate Change. For mitigation policies that seek to implement management actions that drawdown carbon, ecosystem sequestration and emissions must be measurable across temporal and spatial scales, and management must be practical leading to improved sequestration and avoided emissions. However, some blue carbon interventions may not be suitable as a climate mitigation response and better suited for other policy instruments such as those targeted toward biodiversity conservation. This paper gives context to numerous blue carbon sequestration pathways, quantifying their potential to sequester carbon from the atmosphere, and comparing these sequestration pathways to point-source emissions reductions. The applicability of blue carbon is then discussed in terms of multiple international policy frameworks, to help individuals and institutions utilize the appropriate framework to reach ocean conservation and climate mitigation goals.
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Baleen whales are subject to a myriad of natural and anthropogenic stressors, but understanding how these stressors affect physiology is difficult. Measurement of adrenal glucocorticoid (GC) hormones involved in the vertebrate stress response (cortisol and corticosterone) in baleen could help fill this data gap. Baleen analysis is a powerful tool, allowing for a retrospective re-creation of multiple years of GC hormone concentrations at approximately a monthly resolution. We hypothesized that whales that died from acute causes (e.g. ship strike) would have lower levels of baleen GCs than whales that died from extended illness or injury (e.g. long-term entanglement in fishing gear). To test this hypothesis, we extracted hormones from baleen plates of four humpback whales (Megaptera novaeangliae) with well-documented deaths including multiple and chronic entanglements (n = 1, female), ship strike (n = 2, male and female) and chronic illness with nutritional stress (n = 1, male). Over ~3 years of baleen growth and during multiple entanglements, the entangled whale had average corticosterone levels of 80–187% higher than the other three whales but cortisol levels