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Aquat. Living Resour. 28, 53–58 (2015)
c
EDP Sciences 2015
DOI: 10.1051/alr/2015006
www.alr-journal.org
Aquatic
Living
Resources
Pattern of movements within a home reef in the Chesterfield
Islands (Coral Sea) by the endangered Giant Grouper,
Epinephelus lanceolatus
Eric C
lua
1,a
, Claude Chauvet
2
, Johann Mourier
1
, Jonathan Mark Werry
3,4,5
and John E. Randall
6
1
LabEx “CORAIL” – USR 3278 CNRS-EPHE, Centre de Recherche Insulaire et Observatoire de l’Environnement (CRIOBE),
BP 1013, 98729 Papetoai, Moorea, French Polynesia
2
University of New Caledonia, BP 12814, 98802 Noumea, New Caledonia
3
Australian Rivers Institute and School of Environment, Griffith University, Gold Coast campus, Parklands Drive, Southport,
Queensland 4222, Australia
4
Griffith Centre for Coastal Management, Griffith University, Gold Coast campus, Parklands Drive, Southport, Queensland 4222, Australia
5
Ocean and Coast Research, Gold Coast, Queensland 4217, Australia
6
Bishop Museum, 1525 Bernice St., Honolulu, HI 96817-2704, USA
Received 20 May 2015; Accepted 27 August 2015
Abstract – This study determined the movements of a Giant Grouper, Epinephelus lanceolatus, in which an acoustic
tag was surgically implanted and monitored by an array of six VR2W acoustic receiver units from August 2010 to
January 2013 in the remote, uninhabited Chesterfield Islands, Coral Sea (800 km West of New Caledonia). Our data
revealed a home reef area (residency rate of 44.9%) with an increased activity revealed by movements at dawn and
dusk toward and between two adjacent reef passages, probably for foraging. The fish was absent from its resident
reef between October and December 2010 and 2012, corresponding to the time known for spawning aggregations
of this species in New Caledonia. A skipped spawning seems to have occurred in 2011. We hope these data will be
complemented in the future by locating the spawning site or sites and thus provide adequate conservation measures.
The Coral Sea links two World Heritage Sites, the Australian Great Barrier Reefs and the New Caledonian coral reefs.
It would be fitting to create a Marine Protected Area for the Chesterfield Islands between these two major conservation
areas of the sea.
Keywords: Coral reefs apex predator / serranidae / site fidelity / acoustic telemetry / spawning aggregations
1 Introduction
With a maximum total length (TL) of 2.3 m and weight
of 300 kg, the Indo-Pacific Giant Grouper, Epinephelus lance-
olatus (Bloch 1790) is the largest known reef fish. It is usually
found on coral reefs, especially where there are large caves,
but it is also known from harbours and deep estuaries, as well
as on wrecks and man-made structures, such as those for oil
exploration or wind generators. Although reported to a depth
of 100 m, it is usually found in much shallower water. It preys
mainly on spiny lobsters, small elasmobranchs, a variety of
bony fishes, large crabs, octopuses, and juvenile marine tur-
tles (Witzell 1981; Heemstra and Randall 1993). It is the most
widely distributed grouper in the world, from South Africa
and the Red Sea to the Pitcairn Islands (Randall and Heemstra
1991; Randall 1995).
Supporting information is only available in electronic form at
www.alr-journal.org.
a
Corresponding author: eric.clua@gmail.com
Information on the biology of Epinephelus la nceolatus is
incomplete compared to that of its sister species, the Atlantic
E. itajara (Randall 1967; Bullock et al. 1992;Sadovyand
Eklund 1999). Both species are particularly vulnerable to fish-
ing when they form large spawning aggregations (Coleman
et al. 1996;SadovyandDomeier2005; Saenz-Arroyo et al.
2005). E. lanceolatus does not become sexually mature un-
til it reaches 129 cm TL. It is highly valued in Asian fish
markets (Randall and Heemstra 1991; Lee and Sadovy 1998;
McGilvray and Chan 2003), resulting in special effort to catch
it at islands of the western Pacific. Based on knowledge of
fishermen and divers, local extinctions and extreme scarcity
have been reported for Western Pacific localities (Lavides et al.
2009; Zgliczynski et al. 2013). The species is classified by the
IUCN as “vulnerable”, the second highest risk of extinction
(Shuk Man and Chuen 2006).
Fishing pressure on this species has recently decreased in
some areas as a result of protective legislation. It is illegal to
spear this species in South Africa, where it is known as Brindle
Article published by EDP Sciences
54 E. Clua et al.: Aquat. Living Resour. 28, 53–58 (2015)
Fig. 1. A: The Chesterfield reefs in the Coral Sea situated midway between the east coast of Australia (AUS) and New Caledonia (NC). B:
Close up on the study site that includes Long Island (North-West of the Chesterfield reefs), which is delineated by two passages. The plain
white circle indicates the VR2W receiver called “Home” as it was standing in the vicinity of the Giant Grouper cave. The triangle indicates
the VR2W called “LP” standing for “Large Pass”, corresponding to the large passage North of Long Island. The diamond indicates the VR2W
called “SP” standing for “Small Pass”, corresponding to the Islets passage. These three receivers recorded detections from the tagged Giant
Grouper and these same symbols were also used to describe spatial and temporal distributions of detections in Figure 2A. The three other
receivers on the Eastern side of the Chesterfields reef lagoon are represented by empty circles; they did not record any detection from the giant
grouper. C: Picture of the 1.95 m TL Epinephelus lanceolatus that was caught and released in good health near receiver Home.
Bass (van der Elst 1981), and in Australia, where it is called
Queensland Grouper; such protection should be adopted else-
where. The rearing of the species for human consumption has
been successful (McGilvray and Chan 2003).
The capture of a large adult of Epinephelus lanceolatus
during a program of acoustic tagging of adult tiger sharks in
the Chesterfield Islands provided the opportunity to tag as well
as document its movements.
2 Materials and methods
The Chesterfield Islands are a series of 11 islets, of which
the largest is Long Island (length of 3.4 km). Numerous reefs
and channels, typically 30–40 m deep, lie on a north-south
oceanic ridge in the Coral Sea, isolated from Australia to the
west and New Caledonia to the east by app. 800 km in ei-
ther direction (Fig. 1A). The islands are surrounded by deep
drop-offs to over 1000 m. The islets were named for the whal-
ing ship Chesterfield that explored the Coral Sea in the 1790s.
They were used for commercial whaling in the early 1860s,
and guano was extracted in the 1870s. They have been un-
inhabited for over 40 years. Large-scale illegal fishing still
takes place for sharks and reef fishes of high value, such as the
Giant Grouper (Clua et al. 2011). Kulbicki et al. (1994) pub-
lished a checklist of the fishes of the Chesterfield Islands. They
reported 866 species, representing 34 families, noting closer
affinity to the fish fauna of New Caledonia than to the Great
Barrier Reef.
Acoustic telemetry was used for studying the movements
of large sharks in the Chesterfield Islands. An acoustic array
of six Vemco VR2W receivers were moored on concrete-filled
tyres at depths of 5–25 m in August 2010 and maintained un-
til January 2013 to track the movements of tagged Tiger Shark
(Fig. 1B, see Werry et al. 2014). One receiver was moored on a
patch reef in the lagoon opposite Long Island at a depth of ap-
prox. 5 m. For the purpose of this study, it was named “Home”.
Another receiver was moored at the edge of the large passage
north to Long Island, down the reef slope at 25 m depth and
1300 m from the receiver Home; this receiver was named LP
for “Large Pass”. A third receiver was moored 8250 m south
of the receiver Home at a depth of 22 m, at the entrance of
a smaller passage; it was named SP for “Small Pass”. The
three other receivers were moored on the Eastern side of the
lagoon and did not get any specific name during this study (see
Fig. 1B). On August 16, 2010, while fishing for large sharks
with a barbless hook and heavy line, a Giant Grouper 1.95 m
TL was caught in close vicinity of receiver “Home” (Fig. 1B).
The fish was restrained in a special harness in the sea that pro-
vided efficient protection and oxygenation of the animal, and
avoided the use of anesthetic during tagging. A small incision
was made on the central part of the abdomen, a transmitter
(Vemco V16, 69 kHz, length 54 mm, weight in the water 8.1 g,
battery life > 7 years, delay pulse 90 s) was inserted into the
peritoneal cavity, and the incision was closed with stitches.
The fish swam away vigorously on release (Fig. 1C).
Range tests were conducted to assess the distance from
which the receivers were able to detect the tagged fish. These
showed that detections significantly dropped after 400 m.
In November 2011, the acoustic data were downloaded,
batteries were replaced, and the receivers redeployed. The gear
was completely retrieved in January 2013. Following the data
download, we cleaned the raw data by deleting the rare false
detections. The valid detections were then sorted by site, date,
and time. For each receiver, we determined the number of de-
tections and a residency index, defined as the number of days
the fish was detected at a given receiver, divided by the total
number of days at liberty since tagging.
We used a generalized linear modelling framework (GLM)
to examine the effects of time of year (Month), time of day
(Hour) and site (Receiver) factors on the presence of the
grouper. The cyclicity of the “Month” and “Hour” factors were
modelled by including the variables as the cyclical function of
their sine and cosine components. The analysis used a bino-
mial error structure with a logit link function. We coded the
binomial dependent variable with 1 when the fish was detected
E. Clua et al.: Aquat. Living Resour. 28, 53–58 (2015) 55
Fig. 2. A: Scatterplot of diel detection patterns of the Giant Grouper recorded at Chesterfield reefs between August 2010 and January 2013.
Horizontal curves show daily sunrise and sunset. Shapes of the symbols on the scatterplot correspond to the receiver locations indicated in the
map in Figure 1B. B: Predicted probability of presence of the grouper at the three receivers for each calendar month and time of day inferred
from the most parsimonious GLM model.
and 0 when not detected for each hour of each day, and at each
receiver. Therefore, the fish had a total of 72 records for each
calendar day. Akaike’s Information Criterion (AIC) was used
to compare relative model support, where lower AIC values
indicate greater support for the model.
We used network analysis to investigate the spatial dy-
namics of the grouper. We implemented an Empirical derived
Markov chain (EDMC) analysis, which takes into account the
frequency of movements between receivers but also the res-
idency times and absence from the array (see Garcia et al.
2015; Stehfest et al. 2015). A Markov chain is a random pro-
cess and consists of transitions from one state to another (in
our case from receiver to receiver). The raw series of acoustic
detections was organized into an hourly detection time series.
For every hourly time step, if the fish was detected by a re-
ceiver, then the receiver ID was assigned to the state; if the fish
was not detected, it was assigned an absent state. A movement
count matrix was then computed containing movements be-
tween each receiver, as well as the movements from each state
to itself (residency periods when the fish stayed at the same
receiver) and movements to the absent state (transition periods
outside of detection range). The transition probability matrix
was constructed by dividing each number of transitions made
from one state to another or itself by the number of transitions
made from the state. We also calculated the eigenvector cen-
trality of each receiver which is a measure not only of the cen-
trality of a receiver, but also of the centrality of the receivers
it is connected to (see Stehfest et al. 2015).Thesameanalysis
was reiterated using day of year instead of hour in order to test
for differences in movement and residency timing.
3 Results
The Giant Grouper was monitored for a total of 857 days
within the study area. It was detected on 135 days (15.75%) at
receiver LP, 196 days (22.87%) at receiver Home and 161 days
(18.78%) at receiver SP, for a total of 385 days (44.92%)
on at least one receiver, 101 days (11.78%) on a minimum
of 2 receivers and 6 days (0.70%) on the three receivers
(Fig. 2A; Table 1). The best GLM model determined by the
lowest AIC was one that incorporated the factors Month, Hour
and Receiver and an interaction between Hour and Receiver
(Model 9; Tables S.1 and S.2). For all receivers, the lowest
probability of presence was in October and November and the
greatest probability of presence outside these two months was
in the morning between 6:00 and 8:00 AM (Fig. 2B). Based on
real detections, the fish was totally absent from the area dur-
ing October and November 2010 and 2012, but still present
during the same months in 2011 (Fig. 2A). Although the fish
was more present at receiver Home compared to the others, the
patterns of probabilities were consistent across all receivers
(Fig. 2B) as the fish was detected by all receivers similarly
throughout the monitoring period (Fig. 2A).
Transition probabilities between states revealed low resi-
dency at the daily scale (Fig. 3A) and relatively high residency
at the hourly scale (Fig. 3B) with the highest probability of
hourly residency being at receiver Home. Movements between
receivers occurred mostly at a daily scale (Fig. 3A) rather than
at an hourly scale (Fig. 3B). Most movements occurred be-
tween receivers Home and LP but some direct movements in
both directions did also occur between the receivers LP and SP,
56 E. Clua et al.: Aquat. Living Resour. 28, 53–58 (2015)
Tab le 1. Receiver statistics. The residency index and number of detections are given by receiver as well as for all receivers combined. Residency
index is defined as the number of days the fish was detected divided by the number of monitoring days. The eigenvector centrality values of the
matrices of movement between receivers are given for the daily and hourly time scales.
Receiver Residency Number of Eigenvector centrality Eigenvector centrality
index detections (Day) (Hour)
SP 0.157 678 0.099 0.008
Home 0.228 3587 0.145 0.020
LP 0.187 1098 0.118 0.010
All 0.449 5363
Fig. 3. Movement and residency of the grouper over two temporal scales: using A- daily and B- hourly time series of detections. For each case,
a map showing the movements is presented in which arrow size is proportional to the transition probabilities between receivers and filled circles
are proportional to the probability of residency. Number of day the fish was detected at each receiver is indicated in parentheses. Empty circles
represent the three other receivers located on the east side which did not record any detections.
probably along the outer slope of the barrier reef (Fig. 3A). The
receiver Home was the most central as suggested by the high-
est eigenvector centrality (Table 1). Probabilities of being in
an absent state or coming from/going to an absent state were
higher at the hourly scale (Fig. 3). The fish was never recorded
by the three other receivers on the opposite side of the lagoon
(Fig. 3).
4 Discussion
Movements occurred between the three receivers in
the lagoon and channels surrounding Long Island, but the
distribution of detections suggests that the usual shelter of this
grouper would be around receiver Home, as confirmed by the
sighting during a diving session of a Giant Grouper of a similar
size in a reef cave of the area in August 2010 (J.C. Toison, Pers.
Comm.). The fish used the adjacent passages (in particular the
closest Long Island passage) mainly at dawn (between 5:00
and 12:00 AM), probably for foraging purposes. The lower
detection rate recorded outside the morning period (Fig. 2B)
may be due to the fish residing in a cave, which would re-
duce its detection by the receivers. High probabilities of being
out-of-receiver range are common in reef fish (Garcia et al.
2015) which may be due to the distance between receivers or
E. Clua et al.: Aquat. Living Resour. 28, 53–58 (2015) 57
to the sheltering behaviour of the studied species. Analysis of
the movement sequences also showed that the grouper went
from one passage to another by swimming either inside the la-
goon or along the outer slope. The second hypothesis seems
more plausible as the way inside the lagoon preventing any
detection by the receiver Home would oblige the grouper to
swim and remain on a nearly bare sandy bottom area, which is
probably less attractive than the outer slope of the barrier reef
that hosts more potential prey. The differences between move-
ment matrices at hourly and daily scales (Fig. 3) indicate that
movements between receivers took more than one hour. The
absence of any detections on the three other receivers located
inside the Chesterfield lagoon indicated a strong attachment to
the passages and outer slope, with a strong residency pattern
(44.9% of days) in an area of about 12 km of diameter delin-
eated on the North and South sides by the two reef passages.
Based on the definition provided by Colin et al. (2003),
spawning aggregations of E. lanceo latus were observed along
the west coast of New Caledonia in October and November (C.
Chauvet, Pers. Comm.). The absence of the tagged E. lance-
olatus in the Chesterfields during this period and could be
explained by migration for spawning. Our observations of
E. lanceolatu s in the Chesterfields are also consistent with
a 2-month period which is dedicated to mating by its sister
species, E. itajara, in the Caribbean Sea (Mann et al. 2009).
The detection of the tagged grouper in the network of
receivers during the mating period in 2011 could be ex-
plained by its failure to build up the caloric reserve needed
for the long migration in the open sea to the spawning site.
“Skipped spawning” has been documented for several fish
species (Rideout and Tomkiewicz 2011).
A slight increase in activity, mainly at dusk, was noted for
the months May to August prior to the migration before the
end of September (see Fig. 2B), indicating an increase in for-
aging needed for the production of gametes and the upcoming
intense activity of spawning (Rideout and Tomkiewicz 2011).
Regarding the observed site fidelity of the tagged E. lance-
olatus, Elkund and Schull (2001) showed that among the 50
tagged Goliath Grouper (E. itajara) that were resighted in
the framework of their study, 64% had been tagged inshore
(outside the spawning season) and resighted on the same site
within two weeks to two months post-tagging. They could then
show evidence of site fidelity, already mentioned by Sadovy
and Elkund (1999). One fish tagged at a popular diving site
was resighted on the same wreck repeatedly for eight months.
Our findings on E. lanceolatus are consistent with such site
fidelity, except for the spawning season.
5 Conclusion
Our data suggest that Epinephelus lanceolatus moves be-
yond its Chesterfield Islands home area only for spawning. As
a large apex predator, the population of E. lanceolatus at any
site within its broad distribution is very low, even without fish-
ing. Because of its enormous size, it is the goal of fishermen,
whether by hook and line or spear, to land one of these huge
fish. Its high value in the live reef fish trade (McGilvray and
Chan 2003), makes it very susceptible to targeted fishing, es-
pecially if spawning sites are discovered. This results in the
removal of a higher percentage of the males of the population
and increases the danger of extinction. The low population of
the species, its high value, and its vulnerability should be taken
into account for conservation and management strategies. Arti-
sanal fishing does occur in the Chesterfield Islands. It primarily
targets invertebrates (such as sea cucumbers and lobsters) but
it also includes reef fishes (Clua et al. 2011).Thedataprovided
by our study will help to define the Essential Fish Habitat (see
Conover et al. 2000) of this endangered species and will aid
in creating a Marine Protected Area (MPA) for the Chester-
field Islands. The optimal MPA would be one that includes the
entire critical habitat for the species, including one or more
spawning sites. Further acoustic tagging, especially if based
on satellite tracking methodologies, is needed to identify and
protect the spawning sites.
Acknowledgements. We acknowledge the financial support of the
Agence Française de Développement through the funding of
the CRISP programme and the technical support of the Secretariat
of the Pacific Community in Noumea. We thank Thomas Vignaud for
the photo of the tagged E. lanceolatus. We also thank the IRD team
in Nouméa, in particular Armelle Renaud, for the technical support
for retrieving the VR2 receivers from the study area in January 2013.
This work was conducted under permit No. 6024-4916/DENV/SMer
(New Caledonia) and ethics ENV/17/09/AEC (Griffith University).
The contribution of two anonymous reviewers and the associate edi-
tor were highly appreciated for the improvement of this paper.
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