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Decline of coastal apex shark populations over the past half century

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Overexploitation of large apex marine predators is widespread in the world’s oceans, yet the timing and extent of declines are poorly understood. Here we reconstruct a unique fisheries-independent dataset from a shark control programme spanning 1760 km of the Australian coastline over the past 55 years. We report substantial declines (74–92%) of catch per unit effort of hammerhead (Sphyrnidae), whaler (Carcharhinidae), tiger shark (Galeocerdo cuvier) and white sharks (Carcharodon carcharias). Following onset of the program in the 1960s, catch rates in new installations in subsequent decades occurred at a substantially lower rate, indicating regional depletion of shark populations over the past half a century. Concurrent declines in body size and the probability of encountering mature individuals suggests that apex shark populations are more vulnerable to exploitation than previously thought. Ongoing declines and lack of recovery of vulnerable and protected shark species are a cause for concern.
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ARTICLE
Decline of coastal apex shark populations
over the past half century
George Roff 1, Christopher J. Brown 2, Mark A. Priest 1& Peter J. Mumby1
Overexploitation of large apex marine predators is widespread in the worlds oceans, yet the
timing and extent of declines are poorly understood. Here we reconstruct a unique sheries-
independent dataset from a shark control programme spanning 1760 km of the Australian
coastline over the past 55 years. We report substantial declines (7492%) of catch per unit
effort of hammerhead (Sphyrnidae), whaler (Carcharhinidae), tiger shark (Galeocerdo cuvier)
and white sharks (Carcharodon carcharias). Following onset of the program in the 1960s, catch
rates in new installations in subsequent decades occurred at a substantially lower rate,
indicating regional depletion of shark populations over the past half a century. Concurrent
declines in body size and the probability of encountering mature individuals suggests that
apex shark populations are more vulnerable to exploitation than previously thought. Ongoing
declines and lack of recovery of vulnerable and protected shark species are a cause for
concern.
https://doi.org/10.1038/s42003-018-0233-1 OPEN
1School of Biological Sciences, University of Queensland, Brisbane QLD 4072, Australia. 2Australian Rivers Institute, Grifth University, Nathan QLD 4111,
Australia. Correspondence and requests for materials should be addressed to G.R. (email: g.roff@uq.edu.au)
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Through hunting and widespread habitat modication, top-
level predators have been depleted throughout the worlds
continents, rivers and oceans1, driving widespread ecolo-
gical change2. While losses of apex predators and the con-
sequences for ecosystems have been well documented in
terrestrial ecosystems1, the extent and magnitude of decline in
apex predators in the marine environment is less well
understood3,4. Coastal ecosystems in particular have experienced
widespread trophic downgrading, having lost many of their top-
level predators through overshing5. Throughout the 20th cen-
tury, increasing human-shark interactions in coastal ecosystems
lead to the public perception that sharks are dangerous to people,
resulting in near extirpation of some coastal shark species
through hunting6. Despite widespread evidence for historical
exploitation of coastal sharks, historical baselines for population
sizes are largely unknown7. The absence of baselines is particu-
larly problematic for conservation of endangered and threatened
shark populations, and the extent to which targeting shark
populations reduces interaction rates with humans in coastal
ecosystems is contentious8.
Here we report on long-term changes in shark catches from the
Queensland Shark Control Program (QSCP) adjacent to the
Great Barrier Reef World Heritage Area (GBRWHA). The QSCP
has been operating since 1962 using a system of mesh nets and
baited drumlines (Supplementary Figure 1) with an aim to
minimise the threat of shark attack on humans9by reducing the
local populations of large sharks to minimise the probability of
encounters between sharks and swimmers10. The programme
started in Cairns in 1962, and has since expanded to 11 regions in
Queensland, spanning tropical and sub-tropical coastal ecosys-
tems across 1760 km of the eastern Australian coastline11 (Fig. 1a,
Supplementary Figure 2). To date, nearly 50,000 sharks have been
caught by the QSCP (Fig. 1). From the onset of the programme in
1962, increasing numbers of baited drumlines were installed in
place of nets (Fig. 1b) due to logistical constraints and issues of
bycatch (predominantly turtles and dugongs11).
Results
Species identication and taxonomic composition of shark
catch. Analysis of the QSCP catch data reveals a diverse range of
sharks (45 spp.) spanning multiple trophic levels (Supplementary
Table 1), ranging from small (~ 80 cm maximum total length
[TL
max
]) low trophic level sharks (e.g. Heterodontus portusjack-
soni) to large (> 600 cm TL
max
) apex sharks such as tiger sharks
(Galeocerdo cuvier) and white sharks (Carcharodon carcharias).
Although the QSCP has been in operation since 1962 (Fig. 1c),
records relating to species identication are considered reliable
only from ~1996 onwards following a systematic review of the
programme9. Shark catches from the long-term data set
(19622017) were therefore grouped into ve broad categories
based on reliably identiable characteristics: (1) hammerhead
sharks (Sphyrnidae, 23% of total catch, predominantly Sphyrna
mokarran &Sphyrna lewini), (2) tiger sharks (Galeocerdo cuvier,
26% of total catch), (3) whaler sharks (requiem sharks of the
family Carcharhinidae, 45% of total catch), (4) white sharks
(Carcharodon carcharias, 2% of total catch) and (5) other sharks
(4% of total catch, Supplementary Table 1).
Long-term changes in CPUE of shark populations. Bayesian
negative binomial mixed effects models revealed substantial
declines in catch per unit effort (CPUE) of large apex sharks over
the past ve decades (Supplementary Table 2). In 1962, an
average of 9.5 hammerheads were recorded per net per year,
which declined by 92% to 0.8 hammerheads in 2016 (Fig. 2a).
These declines did not appear to follow a latitudinal gradient and
were consistent among regions (Supplementary Figure 3). Ham-
merhead sharks are more vulnerable to capture in nets owing to
their unique hammer shaped cephalofoil that easily becomes
entangled11,12 (Supplementary Figure 3). Drumline catch
declined from an average of 0.25 hammerheads per drum per year
in 1962 to 0.02 in 2016 (Fig. 2a). Whaler sharks (Carcharinidae)
also exhibited large declines in CPUE: in 1962, catches averaged
18.3 individuals per net per year declining by 82% to 3.23 indi-
viduals per net per year by 2016, while catch rates of drumlines
declined from 2.3 individuals per drum per year in 1962 to 0.4 in
2016 (Fig. 2b). Declines in hammerheads and whalers were
exceptionally rapid following the deployment of nets in the early
1960s (Fig. 1b), exceeding an exponential rate of decline (Fig. 2a,
b). By the mid 1970s, average hammerhead catch rates were
4555% lower than the previous decade, and CPUE continued to
decline, reaching ~ 75% of historical baselines by the mid 1990s.
Coinciding with decreasing catch rates, the estimated annual
zero-catch probability (catching no hammerhead sharks at any
given beach within a region per year) increased by 4.8-fold
between 1962 and 2016 (Fig. 2a), while the annual zero-catch
probability of whalers increased by 6.9-fold in the same time-
period (Fig. 2b). These trends were broadly consistent across all
regions (Supplementary Figure 4). As whaler sharks encompass a
broad group of sharks within the Carcharinidae family (26 spp.,
Supplementary Table 1), the lack of species-resolution in the
long-term dataset renders it unclear as to whether the 19622016
decline in catch rates represents an even decline among whalers,
or masks long-term shifts in species composition among mem-
bers of the Carcharinidae.
In contrast to hammerheads and whalers, catch rates of tiger
sharks were relatively stable between the early 1960s and early
1990s, prior to a 74% decline in CPUE over the past 25 years
(Fig. 2b). Catches of 1.4 individuals per drum per year in 1962
declined to 0.4 individuals in 2016, while catches in nets declined
from 2.3 individuals per net per year in 1962 to 0.6 individuals in
2016 (Fig. 2c). Coinciding with ongoing declines in numbers of
tiger sharks in nets and drumlines, the annual zero-catch
probability of tiger sharks increased by 1.7-fold (Fig. 2c). The
strongest declines in tiger shark CPUE were recorded at high
latitude regions (Supplementary Figure 5), where tiger sharks
undergo seasonal migrations in warmer summer months13. Such
a result is broadly consistent with previous observations of
geographic range constriction commensurate with population
declines in pelagic predators14.
While relatively uncommon (~ 2% of total catch), white sharks
are considered high riskand are actively targeted by the QSCP11,
despite being listed as vulnerableunder the Australian Environ-
ment Protection and Biodiversity Conservation Act (EPBC) in
1999. CPUE of white sharks in the QSCP declined by 92% over
the past ve decades from 0.7 white sharks per net per year in
1962 to 0.05 individuals in 2015, and 0.1 white sharks per drum
per year in 1962 to 0.008 individuals in 2015 (Fig. 2d). These
declines are ongoing in 8 out of 9 regions (Supplementary
Figure 5) despite a complete ban of commercial and recreational
shing of white sharks since 1999 under the environment
protection and biodiversity conservation act and the enactment of
a 2002 recovery plan by the federal government15.
As is common when reconstructing historical baselines16, some
degree of uncertainty exists in the accuracy of effort records in the
early years of the QSCP. Prior to the review and standardisation
of the programme in 1992, exact setting of nets may have varied
among regions, and differences in hook types and bait on the
drumlines may have occurred among regions and through time.
Similarly, the accuracy of catch records may be questioned as
historical data has been collected by contracted commercial
sherman prior to standardised training in shark species
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identication from 1992 onwards. While minor gear variation
may have occurred, declines in CPUE were spatially consistent
among regions in the early years of the programme along nearly
1800 km of coastline (Fig. 1), and analysis of temporal trends in
bycatch revealed no clear evidence of changes in gear types in the
early years of the programme coinciding with rapid declines in
shark catches1720. Further, it is unlikely that changes in drumline
gear and bait types would have a substantial effect on CPUE as
Cairns (1962)
Townsville (1963)
Bundaberg (1973)
Capricorn Coast (1969)
Gold Coast (1962)
Mackay (1973)
Nth Stradbroke (1974)
Rainbow Beach (1974)
Sunshine Coast (1962)
Gladstone (1983)
Australia
Gold Coast, 3rd November 1963
a
Cumulative effort (nets)
Drumlines
Nets
1960 1970 1980 1990 2000 2010 2020
1960 1970 1980 1990 2000 2010 2020
0
10,000
20,000
30,000
40,000
50,000
Cumulative catch
Queensland
Cumulative effort (drumlines)
c
b
14°S
18°S
22°S
26°S
140°E 144°E 148°E
10°S
0
10
20
30
40
50
0
100
200
300
400
Fig. 1 Regional setting and historical changes in catch and effort for the Queensland Shark Control Program. a Timing of the establishment of shark
control programmes across the Queensland coastline (map created under Creative Commons Attribution 4.0 International from Geoscience Australia).
bCumulative effort for nets and drumlines and catch between 1962 and 2017. cHistorical photograph of contractors measuring sharks removed from
QSCP nets on the Gold Coast in the early years of the programme (3 November 1963), reprinted from Paterson (1990) Biological Conservation, 52(2),
147159 (ref. 17) with permission from Elsevier
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large sharks are omnivorous and opportunistic12,21,22, and likely
do not exhibit strong preference for sh or shark esh as bait. In
addition, changes in hook types during by-catch reduction trials
have no measurable effect on shark catches23, and declines were
consistent in both nets and drumlines (Supplementary Figure 3).
Notes from contractors during the early years of the programme
provide insight into changing shark dynamics and support the
observed decline in CPUE: in some regions nets were installed
shortly after the programme initiated to cope with increase(s) in
large shark catches(contractor notes), while the number of
drumlines in some regions were reduced in the 1990s due to
declining catch rates of sharks. Patterson17 notes that When net
catches were high in earlier years, reliance on lines was
unnecessary to achieve satisfactory results but as net catches
declined lines were used more assiduously in some regions, with a
consequent increase in tiger shark catches.
While some uncertainty exists with the historical data, analysis
of the detailed contemporary catch data since standardisation of
nets and drumlines throughout the region and following formal
training of contractors in shark species identication (19922017)
reveals ongoing declines over the past 25 years (Supplementary
Figure 6). CPUE of hammerheads declined by 68%, whalers by
69%, tigers by 69% and white sharks by 42%, while the probability
of catching no sharks at any given beach within a region increased
through time (Supplementary Figure 6). While ongoing declines
are a cause for concern, historical data from the long-term dataset
(19622017) suggest that the historical baselines for populations
may be substantially higher than that based on contemporary
data. This represents a classic case of shifting baseline
syndrome5,24, and implies that studies of sharks declines in the
region in recent decades12,25,26 may be predicated on a
substantially shifted baseline.
Regional depletions of shark populations. Shark control pro-
grams operate with the intent of depleting local populations of
sharks, yet the spatial scale at which these depletions occur is not
well understood. Following the initial deployment of shark nets in
Cairns and the Gold Coast in 1962 (Fig. 1), the programme
expanded along the Queensland coastline to include additional
beaches and additional regions between 1962 and 1998 (Supple-
mentary Figure 2). To quantify the spatial scale of population
declines, we explored changes in initial catch rates (calculated as
the average CPUE for the rst ve years) following the installa-
tion of gear at new beaches. Within regions, CPUE varied sub-
stantially among beaches within years, consistent with differential
habitat preferences and environmental drivers of shark distribu-
tions that operate over relatively small spatial scales27,28. Despite
such small-scale variability in CPUE, initial catch rates in newly
installed beaches were consistently similar to that of established
beaches within regions (Supplementary Figure 7). At regional
scales, initial CPUE in new net and drumline installations in
recent decades occurred at a lower rate than earlier installations
for hammerhead, whaler and white sharks (Fig. 3). Analysis of
this trend using Bayesian mixed effects models indicates a decline
in initial CPUE between 1963 and 1998 of 78% for hammerheads,
47% for whalers and 92% for white sharks (Fig. 3). Decline in
initial catch rate was considerably smaller for tiger sharks (4%,
1960 1970 1980 1990 2000 2010 2020
0
5
10
15
20
1960 1970 1980 1990 2000 2010 2020 1960 1970 1980 1990 2000 2010 2020 1960 1970 1980 1990 2000 2010 2020
0
0.25
0.50
0.75
1.00
1960 1970 1980 1990 2000 2010 2020
0
1
2
3
1960 1970 1980 1990 2000 2010 2020 1960 1970 1980 1990 2000 2010 2020 1960 1970 1980 1990 2000 2010 2020
0
0.1
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0.3
1960 1970 1980 1990 2000 2010 2020
0
0.25
0.50
0.75
1.00
1960 1970 1980 1990 2000 2010 20201960 1970 1980 1990 2000 2010 2020 1960 1970 1980 1990 2000 2010 2020
0
0.25
0.50
0.75
1.00
Carcharhinidae
–92% –82% –74% –92%
CPUE net1 year1
CPUE drumline1 year1
Annual probability of zero catch
4.8 fold increase
Carcharodon carchariasGaleocerdo cuvier
Hammerhead sharks Whaler sharks Tiger shark White shark
Sphyrnidae
6.9 fold increase 1.7 fold increase 1.3 fold increase
Fig. 2 Catch per unit effort (CPUE) in nets and drumlines with ts from Bayesian negative binomial generalised additive mixed effects models (± 95%
credibility intervals), and the annual zero-catch probability (± 95% credibility intervals). Percentages represent the % decline over the 19622017 dataset.
Photographs courtesy of Juan Oliphant (http://oneoceandiving.com/)
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Fig. 3), which is consistent with relatively stable CPUE of tiger
sharks between 1960 and ~1990 prior to region-wide declines
CPUE in the late 1990s (Supplementary Figure 6).
From a management perspective, assessing the status of stocks
through sheries data can be problematic, as CPUE may be
decoupled from abundance due to a range of behavioural and
operational factors that can affect catch rates29. The initial
declines in CPUE have been theorised to reect depletions of local
populations, with subsequent catches comprising an inux of
sharks from adjacent regions30. Such a response would result in
hyperdepletion, a phenomena by which CPUE declines more
rapidly than population abundance31. The impact of shark
control programs upon populations will vary among species, and
is likely dependant on both movement patterns, habitat use and
the degree of philopatry32. In theory, hyperdepletion would be
more likely to occur in whaler sharks that exhibit small-scale
movements and site attachment within bays on the Queensland
coastline33 than larger apex species that undergo large-scale
transoceanic migrations34 and whose populations cover entire
ocean basins35. Nevertheless, the advent of satellite tracking of
sharks presents an emerging picture that even apex species that
undergo long distance movements (>1000 km), including tiger36,
hammerhead37 and white sharks38,39, can exhibit patterns of
residency or site-attachment (see ref. 40 for a concise review),
rendering them susceptible to localised depletion in shark control
programs. Indeed, declines in the early years of the programme
and increases in the probability of annual zero catches for these
taxa may represent selective depletion of site attached or resident
individuals from the regional population. However, the aseasonal
migration of sharks to coastal nursery areas adjacent to the
QSCP12,39 would favour patterns of hyperstability (e.g. ref. 41.)
rather than hyperdepletion. The ongoing reduction in initial
CPUE as the programme expanded implies that the scale of
declines extend beyond local beaches where the shark control
programme operates, and points to serial depletion of large apex
sharks throughout the wider region over the past ve decades.
Dynamics population models should now be developed to
explore the causes of declines and policy options for reversing
them (e.g. refs. 42,43).
Long-term changes in size structure. Life-history characteristics,
such as growth, longevity and fecundity are largely correlated
with body size in sharks44,45. Size is also strongly linked to trophic
position4,45, and size-structuring in communities can be a strong
determinant of the strength of competitive and predatory inter-
actions45. Coinciding with substantial declines in CPUE over the
long-term dataset (19622015), the average size of hammerhead
sharks (Sphyrnidae) increased over the past ve decades by 5%
(210221 cm, Supplementary Table 3). As Sphyrnidaeencom-
passes both the large great hammerhead (Sphyrna mokkaran,
TL
max
=610 cm46) and the smaller scalloped hammerhead
(Sphyrna lewini,TL
max
=430 cm46), it is unclear whether the
increase in size through time reects a shift in the proportion of
scalloped vs great hammerheads, or alternatively reects selective
declines of neonate and juvenile scalloped hammerhead sharks
from adjacent coastal nursery grounds12 in the early years of the
programme. For the past two decades where species-specic data
are available (19972017), the average size of great hammerheads
declined signicantly by 22% (274215 cm) and scalloped ham-
merheads by 16% (204177 cm, Supplementary Table 4).
The average size of tiger sharks declined signicantly by 21%
(272215 cm) over the past ve decades, a pattern that was
consistent among males and females (Supplementary Figure 8).
The average size of whalers also declined signicantly by 9%
between 1962 and 2017 (193166 cm). Long-term declines in the
whaler group may reect an overall intraspecic reduction in size
over the past ve decades, and/or shifts in species composition
towards smaller species of whalers. While our results provide
insight into long-term changes in size structure of large apex
shark populations, a degree of uncertainty exists in historical
records in the early years of the programme. In the early years
prior to 1990, a bounty system was in place for large sharks over
two metres in size, which may have provided an incentive to
exaggerate the sizes of smaller sharks by contractors for monetary
gain. While the extent to which this occurred is unclear,
signicant declines in size among hammerhead, whaler and tiger
sharks have continued over the past 20 years following the
removal of bounties and review of the QSCP (19972017), and
the rate of decline in the size of tiger sharks across the long-term
Initial CPUE nets
Hammerhead sharks Whaler sharks Tiger shark White shark
Carcharhinidae (Galeocerdo cuvier)(Carcharodon carcharias)Sphyrnidae
Initial CPUE drumlines
1960 1970 1980 1990 2000 2010 2020
0
10
20
30
40
50
1960 1970 1980 1990 2000 2010 2020
0
10
20
30
50
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0
2
4
6
10
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0
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4
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5
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0.0
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YearYearYearYear
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5
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0.0
0.1
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0
5
10
15
20
25
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1.5
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2
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4
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1960 1970 1980 1990 2000 2010 2020
0.0
0.5
1.0
1.5
2.0
0.0
0.05
0.1
0.15
0.2
–92%–4%–47%–78%
Initial CPUE nets
Initial CPUE drumlines
Bundaberg
Cairns
Capricorn Coast
Gladstone
Gold Coast
Mackay
North Stradbroke Island
Rainbow Beach
Sunshine Coast North
Sunshine Coast North
Townsville
Regions
0.9
0.15
0.05
40 83
Fig. 3 Initial catch per unit effort (dened as the average CPUE of the rst ve years of operation) for each beach within regions, and model ts from
Bayesian generalised additive mixed effects models (± 95% credibility intervals) for nets and drumlines. Symbols courtesy of the Integration and
Application Network (ian.umces.edu/symbols/)
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dataset 55-year dataset (10.4 cm per decade, Fig. 4), is similar to
that occurring over the past 20 years (8.6 cm per decade).
From a demographic perspective, species with large maximum
sizes, low growth coefcients, low fecundity, and higher size at
maturity specically targeted by the QSCP (Supplementary Figure 9)
areparticularlyvulnerabletoovershing44. We used previously
published estimates of sizes at maturity of shark populations from
local studies27,47,48 to quantify changes in the probability of maturity
in hammerhead and tiger shark catches in the QSCP over the past
twodecades.Whileourestimatesassumethatsizeatmaturityis
xed over time, sharks are unlikely to exhibit rapid shifts in maturity
due to their K-selected life-history strategies44.Ourresultsindicate
signicant and substantial declines in the probability of recording
mature male and female scalloped hammerheads over the past 20
years (Fig. 3d). Most notably, the probability of recording mature
females of scalloped hammerheads declined from 54% in 1997 to
14% in 2017, while probability of mature males declined from 82 to
55% over the same time-period (Supplementary Table 5). Signicant
declines were also recorded for female great hammerheads (85% in
1997 to 59% in 2017, Fig. 3d), although no declines were observed
for male great hammerheads. Over the past 50 years, the probability
of recording sexually mature female tiger sharks declined from 43%
in 1962 to 23% in 2017, while the probability of recording mature
males declined from 34 to 9%. In contrast to other sharks caught by
the QSCP, most white sharks were juveniles and sub-adults, and
very few (<1% of females and 7% males) were of mature size.
Considering that the QSCP selectively targets larger sharks (> 1.5
m11), the ongoing decline in the probability of catching mature
hammerheads and tiger sharks is of concern, as declines in the
number of sharks reaching maturity can strongly inuence
population dynamics and inhibit recovery rates44.
Causes of declines in coastal shark populations. With wide-
spread depletions of sharks throughout the worlds oceans, con-
servation and management of shark populations is becoming
increasingly important49. Given the multi-jurisdictional nature of
apex shark movement50 and paucity of historical records, causes
of declines at regional scales can often be hard to pinpoint, and
vary substantially among species. The rate and magnitude of
declines in CPUE across multiple taxa strongly implicate shing
as the primary cause of long-term declines, and precludes alter-
native hypothesis such as environmental drivers or shifting prey
dynamics (Supplementary Table 6). To assess the potential role of
sheries in the timing of decline of coastal shark populations, we
compiled available records of local and regional commercial and
recreational sheries from the mid-20th century (Supplementary
Table 7). As nets and baited drumlines are highly efcient in
catching sharks, the QSCP is likely to have exerted localised
impacts on coastal shark populations in regions where gear has
been deployed. Indeed, shark control programmes are considered
effective because they systematically target and reduce popula-
tions of large sharks that are believed to be dangerous9.At
regional scales, considering the widespread movement patterns of
large apex sharks (Fig. 5) and genetic evidence for population
connectivity among Australian waters and throughout the Indo-
Pacic35,39,51,52, the serial declines in shark populations recorded
by the QSCP likely also reects ongoing population depletion by
recreational, and commercial sheries in Queensland and adja-
cent jurisdictions, although the absence of historical sheries data
from the 1960s and 1970s makes the early causes of declines
difcult to pinpoint. The rapid initial declines indicate that apex
sharks may be susceptible to even relatively low levels of shing
pressure3,44. While the annual regional catch of whalers and
Great hammerhead (Sphyrna mokkoran) Scalloped hammerhead (Sphyrna lewini) Tiger (Galeocerdo cuvier)
(Carcharhinidae)
ab
de
c
Whaler
1960 1970 1980 1990 2000 2010 2020
0
1
2
3
4
5
1995 2000 2005 2010 2015 2020
0
1
2
3
4
5
1960 1970 1980 1990 2000 2010 2020
0
1
2
3
4
5
YearYear
YearYear
1960 1970 1980 1990 2000 2010 2020
0
1
2
3
4
5
(Galeocerdo cuvier)
Tiger
Great hammerhead
Scalloped hammerhead
Hammerhead
(Sphyrnidae)
***
***
***
***
***
1960 1980 2000
0.2
0.4
0.6
0.8
1.0
2000 2005 2010 20151995
2000 2005 2010 20151995 2000 2005 2010 20151995
2000 2005 2010 20151995 1970 1990 2010
1960 1980 20001970 1990 2010
Freq mature Freq immature
0
80
160
160
80
0
0
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0
Freq mature Freq immature Freq mature Freq immature
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1.0
0
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0.6
0.8
1.0
0
P (maturity)
P (maturity)P (maturity)
P (maturity)P (maturity)
P (maturity)
0.2
0.4
0.6
0.8
1.0
0
0
10
20
20
10
0
0
10
20
20
10
0
Freq mature Freq immature Freq mature Freq immature
Male
***
***
Female
Male
Female
***
***
Female
Male
*
ns
Yea rYea r
Yea r
Length (m)
Length (m)
Length (m)
Freq mature Freq immature
0
90
180
180
90
0
(Sphyrna lewini)
(Sphyrna mokkoran)
Fig. 4 Long-term changes in size structure and sexual maturity. Linear regressions (± 95% condence intervals) for change in size for ahammerheads
(19622017, shading indicates period in which species specic data are available), great and scalloped hammerheads (Sphyrna mokarran and Sphyrna lewini,
19972017), bwhaler sharks (Carcharhinidae, 19622015), ctiger sharks (Galeocerdo cuvier, 19622017), and binomial probability models for sexual
maturity in male and female, dgreat and scalloped hammerheads (S. mokarran and S. lewini, 19972017) and etiger sharks (G. cuvier, 19622017). ns not
signicant. ***p< 0.001, **p< 0.01, *p< 0.05. Symbols courtesy of the Integration and Application Network (ian.umces.edu/symbols/)
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1
10
100
1000
10,000
Maximum distance (km)
c
b
Carcharhinus leucas
C.amblyrhynchos
C.sorrah
C.tilstoni
C.brevipinna
C.amboinensis
Rhizoprionodon taylori
Carcharias taurus
Galeocerdo cuvier
Carcharodon carcharias
Whaler Tiger White Other
New
Zealand
Fiji
Vanuatu
New
Caledonia
Papua New Guinea
Solomon Islands
Tasman Sea
10°S
20°S
30°S
40°S
140°E 150°E 160°E 170°E
Queensland
Coral Sea
180°E
White shark
(Carcharodon carcharias)
New
Zealand
Fiji
Vanuatu
New
Caledonia
Papua New Guinea
Solomon Islands
Tasman Sea
Pacific Ocean
Pacific Ocean
10°S
20°S
30°S
40°S
140°E 150°E 160°E 170°E
Queensland
Coral Sea
180°E
a
Tiger shark
(Galeocerdo cuvier)
Bull shark
(Carcharhinus leucas)
0500 1000 1500 2000 km
Fig. 5 Regional movement patterns among coastal and oceanic ecosystems. Representative movement tracks derived from satellite and acoustic tracking
studies of sharks adjacent to the eastern Australian coastline for atiger sharks (Galeocerdo cuvier,n=10) and bull sharks (Carcharhinus leucas,n=17),
bwhite sharks (Carcharodon carcharias,n=6), and cmaximum distance derived from movement studies for whaler sharks (Carcharhinidae), tiger sharks
(G. cuvier), and white sharks (C. carcharias) caught in the QSCP program
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hammerheads in Queensland net sheries in recent years exceeds
the annual catch in the QSCP by nearly an order of magnitude
(Supplementary Figure 10), the targeting of both neonate and
juvenile life stages and selective removal of large individuals in
recreational and commercial sheries (Supplementary Table 7)
coinciding with the expansion of shark control programs in QLD
and adjacent New South Wales53 is likely to have had a sub-
stantial impact on sharks with low population growth rates.
As tiger sharks are not generally considered a target species by
sheries within the region (Supplementary Table 7), the initial
stability in CPUE may reect either a lack of shing pressure in
the early years of the programme, or alternatively may reect
shifts in the species composition of shark assemblages in response
to overall population declines54. The ongoing declines in CPUE,
increase in probability of zero catch and reduction in size of tiger
sharks that started in the 1990s coincides with a near doubling in
the number of baited drumlines in 8 of the 11 regions
(Supplementary Figure 2). Additional increases in recreational
and commercial sheries for tiger sharks over the past 20 years
indicates that current shing pressure of these sharks may be
unsustainable25. As a consequence of local and regional
exploitation, large apex sharks that were once historically
abundant are now considered Endangeredand Vulnerable
under the IUCN Red Listing (Supplementary Table 1). Critically,
white sharks are now increasingly rare in QSCP catch, having
undergone a 92% decline over the past ve decades. The apparent
lack of recovery of protected white shark populations despite a
complete ban on commercial and recreational shing in
Queensland and neighbouring New South Wales over two
decades ago15 is a cause for concern, and implicates ongoing
catches in shark control programs on the eastern Australian
coastline and sheries bycatch as drivers of population declines.
Regional movements and connectivity of shark populations.As
top-level consumers, apex sharks exhibit widespread movements
throughout the worlds oceans4,40,50. Thus, population declines in
coastal habitats may have cascading effects in adjacent coastal and
pelagic ecosystems. To quantify the scale of shark movements and
potential for connectivity among coastal habitats, we synthesised
biotelemetry and tagging data from previous studies along
Queensland coastline (n=436 sharks, Supplementary Table 8).
Movement patterns of great and scalloped hammerhead shark
species on the Queensland coastline are currently unknown.
Genetic evidence supports connectivity of scalloped hammer-
heads along the continental shelf between Australia and Indo-
nesia52, and although speculative, analysis of population structure
suggests that adult females may migrate from Australia to
Indonesia and Papua New Guinea55. Evidence from tagging
studies in the Atlantic indicate that while great hammerheads
undergo large-scale (> 3000 km) oceanic migrations, they also
exhibit seasonal residency to coastal and coral reef ecosystems
and long-term site delity37. Most species of whaler sharks for
which data are available exhibited varying patterns of residency,
dispersal and connectivity among coastal environments on the
Queensland coastline (Fig. 5), long-range migrations and multiple
habitat use among coastal and coral reef ecosystems was observed
in bull sharks (Carcharhinus leucas56).
Satellite tracking data from tiger sharks on the Queensland
coast indicate smaller scale resident behaviour36 coupled with
widespread movement along the eastern Australian coastline (23°
S40°S, Supplementary Table 8). Tiger sharks have been reported
to migrate to higher latitudes in warmer months for foraging36,
and tracking studies provide evidence of long-distance migrations
(>1000 km) from the Queensland coast to tropical coral reef
regions of New Caledonia and Papua New Guinea (Fig. 5). Such
widespread movement patterns are consistent with recent studies
indicating population connectivity spanning among eastern and
western Australia, and Hawaii, resulting in a single large Indo-
Pacic population of tiger sharks35,51. Tracking data for white
sharks indicate that while movements are predominantly linked
to nearshore waters along the eastern Australian coastline (23°
S39°S, Supplementary Table 8), large-scale (> 3000 km) transo-
ceanic excursions were recorded from subtropical Queensland to
temperate New Zealand waters (Fig. 5). Such widespread
movement patterns of large apex sharks among coastal and
pelagic ecosystems indicates a degree of connectivity among
habitats (sandy beaches, coral reefs, seagrass beds, kelp forests)
along the eastern coastline of Australia and throughout Oceania
(Fig. 5). Depletion of shark populations recorded on the
Queensland coastline over the past 50 years may have had
cascading effects on broad-scale nutrient transfer and cross-
ecosystem linkages among adjacent food webs throughout the
region57,58.
Discussion
In terrestrial ecosystems, habitat loss and hunting have been the
primary drivers of decline in large vertebrate species1,2. The
removal of large carnivores in terrestrial systems has substantial
impacts at ecosystem scales1,2, which is often at direct odds with
conservation objectives59. Hunting to reduce conict is prevalent
in terrestrial ecosystems, yet the extent to which it occurs in
marine ecosystems is largely undocumented. While the efcacy of
shark control programs remains controversial, a general percep-
tion is that recovering shark populations are to blame for recent
increases in unprovoked shark incidents in Queensland and New
South Wales8. By providing unique insight into past coastal
ecosystem states, the QSCP data imply that increases in
humanshark interactions are occurring at a time when shark
populations are severely depleted compared to historical base-
lines. The timing of these observed declines precede previously
reported collapses of coastal and pelagic apex sharks by several
decades, and the magnitude of decline is either equal to or
exceeding rates reported in coastal oceans elsewhere in the
world3,4,60. Thus, shark populations within Australian coastlines
may be predicated on a substantially shifted baseline. Promising
signs of recovery have been reported from coastal shark popu-
lations that have undergone a history of severe exploitation in the
Atlantic61, yet ongoing serial depletions of large sharks under the
QSCP may impact upon local recovery of vulnerable and
endangered coastal shark populations.
Methods
Historical reconstructions of shing effort and shark catches. The Queensland
Shark Control Program (QSCP) employs a series of baited drumlines and mesh
nets adjacent to coastal beaches. The QSCP actively targets dangerousand
potentially dangeroussharks17, specically certain species of whalers (Carch-
arhinidae), tiger sharks (Galeocerdo cuvier), white sharks (Carcharodon carcharias)
and hammerhead sharks (Sphyrnidae). As a historical record of shark catches, the
QSCP is unique in that it represents a continuous documented long-term effort,
and that both size and identity of sharks have been recorded since the onset of the
programme. Data were accessed from the Queensland Shark Control Program
(QSCP) from the State of Queensland, Australia through the Department of
Agriculture and Fisheries (https://www.daf.qld.gov.au/sheries/shark-control-
program).
The QSCP initially used nets, although due to high levels of bycatch
(predominantly turtles and dugongs11) and declines in the number of sharks
caught in nets17, they were increasingly replaced with drumlines (Supplementary
Figure 2). Nets are considered a passive way of capturing sharks moving across
beaches, whereas baited drumlines actively target feeding sharks11. However,
evidence suggests that nets may actively target sharks, as bycatch trapped in nets
attract feeding sharks17. Previous studies indicate that different gear types select for
different sharks: hammerhead sharks and rays were particularly vulnerable to net
capture, whereas higher catch rates of tiger sharks were observed for drumlines11.
Increases in sea surface temperatures 19622016 within regions is shown in
Supplementary Figure 11.
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Since the initiation of the QSCP in 1962, contractors have recorded the total
length, sex, species and status (dead/alive) of each captured shark. Gear types have
been standardised and largely unchanged since around 199211. Nets (186 m in
length, 6 m drop and 50 cm stretched mesh size11) are predominantly set parallel,
and ~5001000 m, from shore (water depth 712 m) depending on local
bathymetric conditions (Supplementary Figure 1). Drumlines are positioned
5001000 m from the shore and hooks (single 14/0 J hook11) are baited with either
shark esh (pre-2002) or mullet (post-2002). Nets and drumlines are checked by
contractors 1520 days each month11 (Supplementary Figure 1).
Species identication is generally considered unreliable prior to 1996, while data
on species identication following a review of the QSCP in 1997 is considered more
robust9. For long-term analysis (19622017) we selected four readily identiable
groups: (i) hammerheads (Sphyrinidae, readily identiable by their attened and
laterally extended cephalofoil shaped head), (ii) requiem whaler sharks
(Carcharhinidae), (iii) tiger sharks (Galeocerdo cuvier, readily identiable by their
large vertical body stripes and blunt head shape), and (iv) white sharks
(Carcharodon carcharias, readily identiable by their robust, large, conical snout
and countershading, with a white underside and a grey dorsal area).
Effort data in the form of total number of nets and drumlines was reconstructed
using historical records from contractors logbooks between 1962 and 2017
(Supplementary Figure 2). Historical effort records account for seasonal lifting of
gear and swapping of gear between beaches during seasons to avoid bycatch of
turtles and whales, and annual effort was adjusted to reect these changes. Catch
data was standardised by effort at each site to calculate catch per unit effort
(CPUE62) for both gear types. Where catch records were unclear or uncertainty
existed regarding number of drumlines or nets, beaches were excluded from the
analysis. Similarly, with size data, individuals were excluded where contractors
appeared to have recorded measurements in imperial units, or where sizes exceeded
the maximum total length (TL
max
) for each group46. Prior to standardization of the
programme in 1996, in some minor instances, gear type was recorded as unknown
where exact records were not kept (2.41% of total catch data). In these instances,
catch from the unknowncategory was assigned to either drumline or net gear
types in proportion to the odds ratio of catch by drumlines or nets for each species
by region combination.
Statistical analysis. We modelled spatial and temporal variation in shark catches
under the QSCP between 1962 and 2017. We used Bayesian generalised linear
mixed models to model temporal change in catch as a function of time with nested
random effects of region and sites within regions. As catch rates peaked during the
warmer austral summer months (November to February), time was modelled
following nancial years (e.g. July 1962 to June 1963). Time was also treated as a
random effect and its effect on catch was modelled with an order two random walk,
which is equivalent to a cubic spline63. Gear (net, drumlines) was included in the
model as a xed effect, to account for differences in catchability between gear types.
Catch was modelled with a negative binomial distribution, which was found to
adequately account for over-dispersion in catch data. We included effort as an
offset in the GLMM, thus the models predictions were for CPUE. Each group
(hammerheads, whalers, tigers and white sharks) was modelled separately. Models
were t using the integrated nested Laplace approximations (INLA)64 in the R
package INLA65.
Prior parameters for the random walk component were specied using the
penalized complexity method which controls over-tting of the temporal trend66.
We used prior parameters of 0.1 and 0.01, though none of the modelsWAICs
changed considerably with different choices. All other parameters were given vague
(broad) priors. For each group two models were tted, the rst allowed the random
walk to vary by regions (though the random walk component for all regions shared
the same hyper-parameters), whereas the second had only an additive regional
effect and a shared global random walk. We compared the two models using the
WAIC67. We calculated annual zero-catch probability as the probability of catching
no sharks at a given site per year. Initial catch rates were dened as the average
CPUE for the rst ve years following the installation of gear at new beaches. To
quantify the spatial scale of population declines, we explored changes in initial
catch rates across all beaches between 1962 and 1998. We used Bayesian
generalised linear mixed models with a random effect of region. Effort as an offset
in the GLMM as above, and catch was modelled with a Poisson distribution.
Long-term changes in size structure. Changes in the size of sharks over the long-
term dataset (19622017) were explored for the four major groups (hammerheads,
whalers, tigers and white sharks), and for short-term data for scalloped hammer-
heads and great hammerheads (19922017) using linear mixed effects models with
gear and sex as xed effects and site and region as nested random effects. We used
previously published estimates of sizes at maturity of shark species from local
studies27,47 to quantify changes in the probability of maturity in shark catches in
the QSCP over the past two decades. While our estimates assume that size at
maturity is xed over time, we argue that this is a reasonable assumption in that
sharks are less likely to exhibit rapid shifts in maturity due to their K-selected life-
history strategies. Changes in the probability of catching mature individuals were
assessed for either sex using binomial general linear models with site and region as
nested random effects. Generalised linear models and GLMMs for maturity were t
using the package lme468 and base package in R69.
Regional movement patterns of coastal sharks. To assess potential large spatial
scale effects of Queensland shark declines, we reviewed existing literature for shark
movement data that included movements recorded within the Queensland coast-
line for shark species recorded in the QSCP catch. We then extracted a distance
metric to represent the maximum movement recorded by an individual of each
species. For both satellite and acoustic telemetry this comprised the shortest in-
water distance between the furthest points of a minimum convex polygon for the
widest-ranging tagged individual. For conventional mark-recapture (or re-sighting,
in the case of photographic identication) studies using external tags, the greatest
distance between initial capture and recapture point among individuals of each
species was used. While the greatest distance moved by an individual of each
species appears a relatively liberal representation of a species movement, we con-
sider this metric to be somewhat conservative for a number of reasons. First,
sample sizes in satellite tagging studies are generally small and deployments short.
Therefore, only the longest tracks are more likely to accurately capture any seasonal
movements undertaken by migratory species. Second, acoustic telemetry is limited
by receiver placement and any movements beyond the range of receiver arrays are
unknown. Third, mark-recapture studies are limited by spatial and temporal extent
of recapture effort, and nally, we limited our search to movement data that was
only within and/or overlapped the QSCP study areas.
Data availability
Use of the Shark Control Program data is by courtesy of the State of Queensland,
Australia through the Department of Agriculture and Fisheries (https://www.daf.
qld.gov.au/sheries/shark-control-program).
Received: 26 May 2018 Accepted: 13 November 2018
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Acknowledgements
C.J.B. was supported by a Discovery Early Career Researcher Award (DE160101207)
from the Australian Research Council, M.A.P. was supported by an Australian Gov-
ernment Research Training Program Scholarship. Use of the Shark Control Program
data is by courtesy of the State of Queensland, Australia through the Department of
Agriculture and Fisheries. We sincerely thank Wayne Sumpton and George Leigh for
invaluable discussions and critical insight into the QSCP.
Author contributions
G.R. and M.A.P. conceived the study, G.R. and C.J.B. analysed the CPUE and size data,
G.R. and M.A.P. analysed the movement data, G.R. wrote the rst draft of the manu-
script, C.J.B., M.A.P. and P.J.M. contributed to the nal draft of the manuscript.
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... Overfishing is a major anthropogenic threat in the ocean, leading the decline in stocks of various organisms and disrupting marine ecosystems (Dulvy et al., 2021;Jackson et al., 2001;Myers et al., 2007;Roff et al., 2018). Industrial fishing in the Northeast Atlantic began in the 1950s and has recorded one of the highest initial catches per unit shelf area (Ferretti et al., 2010). ...
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... Hammerhead sharks occupy higher trophic levels as meso-or apex predators and are wide-ranging warm-temperate and tropical species found in continental and insular regions. Within the family Sphyrnidae, significant regional declines have been observed, especially among the large-bodied Sphyrna zygaena, the great hammerhead shark S. mokarran, and the scalloped hammerhead shark S. lewini (Baum et al. 2003;Ferretti et al. 2008;Roff et al. 2018;Pacoureau et al. 2021), found in southern Africa (Ebert, Wintner, and Kyne 2021). These species are frequently targeted and caught as bycatch in pelagic longline and commercial line fisheries (Zeeberg, Corten, and de Graaf 2006;da Silva et al. 2015;Okes and Sant 2019;Thomas et al. 2021), supplying a significant component of the fin trade (Dent and Clarke 2015;Fields et al. 2018;Cardeñosa et al. 2022). ...
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Globally, hammerhead sharks have experienced severe declines owing to continued overexploitation and anthropogenic change. The smooth hammerhead shark Sphyrna zygaena remains understudied compared to other members of the family Sphyrnidae. Despite its vulnerable status, a comprehensive understanding of its genetic landscape remains lacking in many regions worldwide. The present study aimed to conduct a fine‐scale genomic assessment of Sphyrna zygaena within the highly dynamic marine environment of South Africa's coastline, using thousands of single nucleotide polymorphisms (SNPs) derived from restriction site‐associated DNA sequencing (3RAD). A combination of differentiation‐based outlier detection methods and genotype‐environment association (GEA) analysis was employed in Sphyrna zygaena. Subsequent assessments of putatively adaptive loci revealed a distinctive south to east genetic cline. Among these, notable correlations between adaptive variation and sea‐surface dissolved oxygen and salinity were evident. Conversely, analysis of 111,243 neutral SNP markers revealed a lack of regional population differentiation, a finding that remained consistent across various analytical approaches. These results provide evidence for the presence of differential selection pressures within a limited spatial range, despite high gene flow implied by the selectively neutral dataset. This study offers notable insights regarding the potential impacts of genomic variation in response to fluctuating environmental conditions in the circumglobally distributed Sphyrna zygaena.
... As a group, reef sharks (the species that are mostly resident on reefs) are threatened by human activities, particularly fishing, and have experienced widespread population declines (e.g. Robbins et al. 2006, Ferretti et al. 2010, Roff et al. 2018. A recent global survey of coral reefs found that reef sharks were absent on almost 20 % (69 of 371) of reefs sampled, and over half of the nations sampled (34 of 58) had abundances lower than regional expectations (MacNeil et al. 2020). ...
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... The historical ecology of elasmobranchs, and especially sharks, demonstrates that humans have engaged with and harvested taxa across the world for millennia (e.g., the Mediterranean [Giovos et al., 2021;Mojetta et al., 2018]; the North Sea [Bom et al., 2022]; the Gulf of Mexico [Martıńez-Candelas et al., 2020]; Southeast Asia [Boulanger, 2023;Boulanger et al., 2021]). Utilizing historical perspectives of shark diversity and harvest patterns, garnered through sources such as oral histories (including traditional ecological knowledge, and indigenous traditional ecological knowledge), ethnohistoric texts, art, photography, and more recent (e.g., decade-scale) survey data, make it clear that the worldwide decline in taxa has been driven by human overfishing, with particular emphasis on loss over the past century (e.g., Bradshaw et al., 2008;Juan-Jordáet al., 2022;Roff et al., 2018). This relatively recent timescale for diversity loss presents challenges for creating long-term baselines needed in conservation decisionmaking and human-wildlife conflict management, whereby historical patterns of species diversity, distribution (including habitat use), and community composition for many taxa are either understudied or unknown (e.g., Ferretti et al., 2008Ferretti et al., , 2018Heithaus et al., 2007;McClenachan et al., 2016). ...
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Introduction Elasmobranchs, such as sharks and rays, are among the world’s most endangered vertebrates, with over 70% loss in abundance over the past 50 years due to human impacts. Zooarchaeological baselines of elasmobranch diversity, distribution, and exploitation hold great promise for contributing essential historical contexts in the assessment of contemporary patterns in their taxonomic diversity and vulnerability to human-caused extinction. Yet, the historical ecology of elasmobranchs receives relatively less archaeological attention compared to that of ray-finned fishes or marine mammals, largely due to issues of taxonomic resolution across zooarchaeological identifications. Methods We explore the use of Zooarchaeology by Mass Spectrometry (ZooMS) for species identification in this unstudied group, using an archaeological case study from the marine environments of the Florida Keys, a marine biodiversity hotspot that is home to an array of elasmobranch species and conservation efforts. By comparison with 39 modern reference species, we could distinguish 12 taxa within the zooarchaeological assemblage from the Clupper archaeological site (Upper Matecumbe Key) that included nine sharks, two rays and a sawfish. Results and discussion The results indicate that, through additional complexity of the collagen peptide mass fingerprint, obtained due to the presence of the cartilaginous type II collagen, ZooMS collagen peptide mass fingerprinting provides exceptionally high taxonomic resolution in this group, yielding species-level identifications in all cases where sufficient reference material was used. This case study also highlights the added value of ZooMS for taxa that are more difficult to distinguish in zooarchaeological analyses, such as vertebrae of the Atlantic sharpnose shark (Rhizoprionodon terraenovae) and the hammerhead sharks (Sphyrna spp.) in the Florida Keys. Therefore, the application of collagen peptide mass fingerprinting to elasmobranchs offers great potential to improve our understanding of their archaeological past and historical ecology.
... especially overfishing have led to population declines across coastal, reef-associated, pelagic, and deep-sea species (Dulvy et al., 2014;Dulvy et al., 2021;Finucci et al., 2024;Pacoureau et al., 2021;Roff et al., 2018;Sherman et al., 2023;Simpfendorfer et al., 2023;Worm et al., 2024). Their low reproductive rates, slow growth, and mainly the impact of illegal, unreported, and unregulated fishing exacerbate their vulnerability (Baum et al., 2003;Bornatowski et al., 2014;Kotas et al., 2023;Pacoureau et al., 2021;Worm et al., 2013). ...
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Silky shark (Carcharhinus falciformis) populations in the South Atlantic Ocean are listed as vulnerable under the IUCN. In fact, this species is classified as critically endangered in Brazil under the Ministry of the Environment. The present study reports the first opportunistic sighting of an aggregation of 250–300 silky sharks in the Alcatrazes Archipelago Wildlife Refuge. Aggregation sites are important in the life cycle of silky sharks, and identifying these sites is essential for conservation efforts.
... Apex predator populations are rapidly changing in response to widespread environmental changes (Roff et al., 2018). Their presence or absence can lead to significant restructuring of ecosystems (e.g., Wilmers and Post, 2006;Knopff et al., 2014;Jonsen et al., 2019). ...
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Leopard seals have traditionally been considered Antarctic predators with a Southern Ocean distribution. Historically, sightings north of the Antarctic Polar Front were considered extralimital. However, recent studies suggest a significant presence of leopard seals in subantarctic regions. Here, we assess the spatial occurrence, residency status, and temporal trends of leopard seals in Chile using historical records, stranding reports, standardized monitoring data, photo-identification (photo ID) catalogs, and sightings from four research expeditions. We also characterize glaciers where sightings are concentrated, identifying glaciological and geomorphic attributes that prolong iceberg residency time, which is linked to high leopard seal concentrations. Based on these attributes, we evaluated other potential suitable glacial habitats in Patagonia. We obtained 438 sighting records of leopard seals from 1927 to 2023. Over the last 15 years, we documented a 4-18% annual increase in stranding events reported to national authorities. Most sightings (75%) were concentrated in two hotspots: National Park San Rafael Lagoon, located in Northern Patagonia, and Parry Fjord in Tierra del Fuego. Using photo ID catalogs, we identified 19 resident leopard seals, including 16 multi-year residents observed between 2010-2023 (10 in San Rafael, 6 in Tierra del Fuego) and 3 potential residents (observed multiple months in the same year in Tierra del Fuego). San Rafael monitoring data showed no inter-annual trend, but seasonal trends were observed. We also provide evidence of breeding in Chile, with records of at least 14 pups born and at least two females giving birth in multiple years. Our habitat characterization suggests that calving flux, fjord sinuosity, and fjord width variation are crucial for prolonging iceberg residency in hotspot areas. Based on these attributes, we identified 13 additional fjords in Patagonia as “very likely” suitable for leopard seals. Our study confirms that Patagonia is part of the species’ breeding distribution, shifting the paradigm that leopard seals are merely visitors north of the Antarctic Polar Front. Given the limited number of suitable glaciers in Chile and the potential impacts of climate change, our assessment highlights glacial retreat as a major threat for the ecosystem of this pagophilic marine apex predator in South America.
... Sharks are cartilaginous fish belonging to the Elasmobranchii subclass, comprising about 536 species (Dulvy et al. 2021). As these animals display numerous adaptive specializations developed throughout over 420 million years of evolution, these fish are noteworthy as one of the top food chain predators in their environments (Roff et al. 2018;Sandin et al. 2022). Most species exhibit K-strategist characteristics, such as late sexual maturity, slow growth, low fecundity, and reduced reproductive rates, and are, therefore, naturally susceptible to overfishing, with a limited capacity for population recovery (Heupel et al. 2014;Pacoureau et al. 2023). ...
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Sharks are a highly threatened group, and the threats these animals face also affect the cultural ways artisanal fisher communities interact with them. Consequently, local knowledge arising from human interactions with these animals also becomes threatened, as well as the potential of fisheries management through ethnoconservation. In this sense, this study examines the dynamics of shark use by fishers in northeastern Brazil. In 2012, data was collected through semi-structured interviews about shark uses applied to 65 traditional fishers who use sharks differently. The ways fishers use these fish are changing mainly because fishers informed an overall reduction in shark catches. Thus, the analysis of their uses comprises a historical-anthropological record, as specific interactions are quickly disappearing. Therefore, the variety of utilitarian connections and changes in shark use patterns over time reveal both ecologic and cultural extinction threats of the traditional shark fisheries in this area. We suggest implementing anthropological and biological studies that aim to contribute to the maintenance of the livelihoods of populations that interact with sharks and seek to guarantee the sustainable exploitation of these animals.
... Many of the elasmobranch species recorded in this survey, such as grey reef shark Carcharhinus ambly -rhynchos, tiger shark Galeocerdo cuvier, scalloped hammerhead shark Sphyrna lewini or whitetip reef shark Triaenodon obesus, are heavily depleted elsewhere (Osgood & Baum 2015, Roff et al. 2018. However, these sharks were frequently encountered within the MPAs sampled in this study. ...
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High-latitude coral reefs (HLCRs) are unique ecosystems with diverse biological assemblages, including many low latitude species on their distribution margins. These ecosystems are threatened by fisheries exploitation, habitat destruction and climate change; however, relative to low latitude coral ecosystems, our understanding of their structure and functioning is limited. This is particularly true for sharks and rays. In this study, we used baited remote underwater stereo-video systems to determine the effect of habitat and management on the assemblage structure of elasmobranchs on the HLCRs of southern Africa (26-28°S; iSimangaliso Marine Protected Area [MPA], South Africa, and the adjoining Ponta do Ouro Partial Marine Reserve, Mozambique). We recorded a total of 12 species of shark (142 individuals) and 9 species of ray (40 individuals) over 2 brief time frames (November 2016 and June 2017). All species were tropical with many on the southern limit of their known distributions. Sharks increased in diversity with depth and showed a preference for the reef and mosaic habitats, relative to sand. The occurrence of rays was predominantly influenced by the presence of low relief habitats. These findings highlight the need for MPAs to encompass both sand and reef habitats over broad depth ranges to effectively protect elasmobranch assemblages. We found evidence to support the high average abundance and diversity of sharks and rays within MPAs of South Africa and southern Mozambique. The results highlight the importance of marginal HLCRs, particularly those within MPAs, for the management and conservation of tropical elasmobranch species.
... However, Varghese et al. (2017) found a smaller size ranging from 70 to 269 cm TL in the Eastern Arab Sea. Interestingly, this study found that the mean size of female I. oxyrinchus tends to decrease along observation time, similar to what Roff et al. (2018) reported in Australia. A declining size in some commercial fisheries can be the impact of overfishing and low gear selectivity, as is the case for reef fishes in the Java Sea . ...
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The Caribbean is recognized as one of the regions with significant elasmobranch diversity; however, the lack of scientific data has made it difficult to conduct traditional fish stock assessments. A Productivity Susceptibility Analysis (PSA) was used to evaluate the vulnerability of the most important elasmobranch species caught by artisanal fishery carried out in the northeastern region of Venezuela. The fishing database created by the INIA and CIT was used as a starting point to obtain catch composition, and biological and fishing information. Of the 37 elasmobranch species captured in the study area, 12 were selected for the PSA. Analysis indicated that the selected species, most of them small-sized sharks, had medium and high productivity; while all species evaluated showed high susceptibility to artisanal fishing. Among the elasmobranch species with high vulnerability, seven of them comprised five sharks (C. limbatus, R. lalandii, R. porosus, C. brevipinna and C. acronotus) and two batoids (H. guttatus and A. narinari). Elasmobranchs species with medium vulnerability included two sharks (M. higmani and M. minicanis) and one batoid (H. americanus); while those with low vulnerability comprised two shark species (M canis and M. norrisi). In the study area, elasmobranch fishing has been carried out for decades without effective management strategies, and the artisanal fishery poses a potential threat to the populations of this group of fish. The species identified in this study as having high vulnerability or risk should be immediately prioritised for serious management by the national agencies responsible for fishing administration.
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Population genetic structure using nine polymorphic nuclear microsatellite loci was assessed for the tiger shark (Galeocerdo cuvier) at seven locations across the Indo-Pacific, and one location in the southern Atlantic. Genetic analyses revealed considerable genetic structuring (FST > 0.14, p < 0.001) between all Indo-Pacific locations and Brazil. By contrast, no significant genetic differences were observed between locations from within the Pacific or Indian Oceans, identifying an apparent large, single Indo-Pacific population. A lack of differentiation between tiger sharks sampled in Hawaii and other Indo-Pacific locations identified herein is in contrast to an earlier global tiger shark nDNA study. The results of our power analysis provide evidence to suggest that the larger sample sizes used here negated any weak population subdivision observed previously. These results further highlight the need for cross-jurisdictional efforts to manage the sustainable exploitation of large migratory sharks like G. cuvier.
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Conservation and management of migratory species can be complex and challenging. International agreements such as the Convention on Migratory Species (CMS) provide policy frameworks, but assessments and management can be hampered by lack of data and tractable mechanisms to integrate disparate datasets. An assessment of scalloped (Sphyrna lewini) and great (Sphyrna mokarran) hammerhead population structure and connectivity across northern Australia, Indonesia and Papua New Guinea (PNG) was conducted to inform management responses to CMS and Convention on International Trade in Endangered Species listings of these species. An Integrated Assessment Framework (IAF) was devised to systematically incorporate data across jurisdictions and create a regional synopsis, and amalgamated a suite of data from the Australasian region. Scalloped hammerhead populations are segregated by sex and size, with Australian populations dominated by juveniles and small adult males, while Indonesian and PNG populations included large adult females. The IAF process introduced genetic and tagging data to produce conceptual models of stock structure and movement. Several hypotheses were produced to explain stock structure and movement patterns, but more data are needed to identify the most likely hypothesis. This study demonstrates a process for assessing migratory species connectivity and highlights priority areas for hammerhead management and research.
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Relative abundance of many shark species in the Atlantic is assessed by compiling data from several independently conducted, but somewhat spatially limited surveys. Although these localized surveys annually sample the same populations, resulting trends in yearly indices often conflict with one another, thereby hindering interpretation of abundance patterns at broad spatial scales. We used delta-lognormal generalized linear models (GLMs) to generate indices of abundance for seven Atlantic coastal shark species from six fishery-independent surveys along the US east coast and Gulf of Mexico from 1975 to 2014. These indices were further analysed using dynamic factor analysis (DFA) to produce simplified, broad-scale common trends in relative abundance over the entire sampled distribution. Effects of drivers including the North Atlantic Oscillation index, the Atlantic Multidecadal Oscillation index, annually averaged sea surface temperature and species landings were evaluated within the DFA model. The two decadal oscillations and species landings were shown to affect shark distribution along south-east US coast. Estimated common trends of relative abundance for all large coastal shark species showed similar decreasing patterns into the early 1990s, periods of sustained low index values thereafter and recent indications of recovery. Small coastal shark species exhibited more regional variability in their estimated common trends, such that two common trends were required to adequately describe patterns in relative abundance throughout the Gulf of Mexico and Atlantic. Overall, all species’ (except the Gulf of Mexico blacknose shark) time series concluded with an increasing trend, suggestive of initial recovery from past exploitation.
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The redistribution of species has emerged as one of the most pervasive impacts of anthropogenic climate warming, and presents many societal challenges. Understanding how temperature regulates species distributions is particularly important for mobile marine fauna such as sharks given their seemingly rapid responses to warming, and the socio-political implications of human encounters with some dangerous species. The predictability of species distributions can potentially be improved by accounting for temperature’s influence on performance; an elusive relationship for most large animals. We combined multi-decadal catch data and bio-logging to show that coastal abundance and swimming performance of tiger sharks Galeocerdo cuvier are both highest at ~22°C, suggesting thermal constraints on performance may regulate this species’ distribution. Tiger sharks are responsible for a large proportion of shark bites on humans, and a focus of controversial control measures in several countries. The combination of distribution and performance data moves toward a mechanistic understanding of tiger shark’s thermal niche, and delivers a simple yet powerful indicator for predicting the location and timing of their occurrences throughout coastlines. For example, tiger sharks are mostly caught at Australia’s popular NSW beaches (i.e. near Sydney) in the warmest months, but our data suggest similar abundances will occur in winter and summer if annual sea surface temperatures increase by a further 1-2°C.
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Understanding shark habitat use is vital for informing better ecological management of coastal areas and shark populations. The Queensland Shark Control Program (QSCP) operates over ∼1800 km of Queensland coastline. Between 1996 and 2012, catch, total length and sex were recorded from most of the 1992 bull shark (Carcharhinus leucas) caught on drum lines and gill-nets as part of the QSCP (sex and length was not successfully recorded for all individuals). Gear was set at multiple sites within ten locations. Analysis of monthly catch data resulted in a zero-inflated dataset for the 17 years of records. Five models were trialled for suitability of standardising the bull shark catch per unit effort (CPUE) using available habitat and environmental data. Three separate models for presence-absence and presence-only were run and outputs combined using a delta-lognormal framework for generalized linear and generalized additive models. The delta-lognormal generalized linear model approach resulted in best fit to explain patterns in CPUE. Greater CPUE occurred on drum lines, and greater numbers of bull sharks were caught on both gear types in summer months, with tropical sites, and sites with greater adjacent wetland habitats catching consistently more bull sharks compared to sub-tropical sites. The CPUE data did not support a hypothesis of population decline indicative of coastal overfishing. However, the total length of sharks declined slightly through time for those caught in the tropics; subtropical catches were dominated by females and a large proportion of all bull sharks caught were smaller than the size-at-maturity reported for this species. These factors suggest that growth and sex overfishing of Queensland bull shark populations may be occurring but are not yet detectable in the available data. The data highlight available coastal wetlands, river size, length of coastline and distance to the 50 m depth contour are important for consideration in future whole of lifecycle bull shark management. As concerns for shark populations grow, there is an increasing requirement to collate available data from control programs, fisheries, ecological and research datasets to identify sustainable management options and enable informed stock assessments of bull shark and other threatened shark species.
Article
Sharks, rays, and chimaeras (Class Chondrichthyes; herein ‘sharks’) are the earliest extant jawed vertebrates and exhibit some of the greatest functional diversity of all vertebrates. Ecologically, they influence energy transfer vertically through trophic levels and sometimes trophic cascades via direct consumption and predation risk. Through movements and migrations, they connect horizontally and temporally across habitats and ecosystems, integrating energy flows at large spatial scales and across time. This connectivity flows from ontogenetic growth in size and spatial movements, which in turn underpins their relatively low reproductive rates compared with other exploited ocean fishes. Sharks are also ecologically and demographically diverse and are taken in a wide variety of fisheries for multiple products (e.g. meat, fins, teeth, and gills). Consequently, a range of fisheries management measures are generally preferable to ‘silver bullet’ and ‘one size fits all’ conservation actions. Some species with extremely low annual reproductive output can easily become endangered and hence require strict protections to minimize mortality. Other, more prolific species can withstand fishing over the long term if catches are subject to effective catch limits throughout the species’ range. We identify, based on the IUCN Red List status, 64 endangered species in particular need of new or stricter protections and 514 species in need of improvements to fisheries management. We designate priority countries for such actions, recognizing the widely differing fishing pressures and conservation capacity. We hope that this analysis assists efforts to ensure this group of ecologically important and evolutionarily distinct animals can support both ocean ecosystems and human activities in the future.
Article
For centuries, the primary manner in which humans have interacted with sharks has been fishing. A combination of their slow-growing nature and high use-values have resulted in population declines for many species around the world, and to date the vast majority of fisheries-related work on sharks has focused on the commercial sector. Shark recreational fishing remains an overlooked area of research despite the fact that these practices are popular globally and could present challenges to their populations. Here we provide a topical overview of shark recreational fisheries, highlighting their history and current status. While recreational fishing can provide conservation benefits under certain circumstances, we focus our discourse on the relatively understudied, potentially detrimental impacts these activities may have on shark physiology, behavior, and fitness. We took this angle given the realized but potentially underestimated significance of recreational fishing for shark conservation management plans and stock assessments, in hopes of creating a dialogue around sustainability. We also present a series of broad and focused research questions and underpin areas of future research need to assist with the development of this emergent area of research.