J-STAGE Advance published date: December 8, 2021

DIALLO, SANE, NELSON, KATSUMATA AND HAKAMADACETACEANS OFF GABON BASED ON
A 2011 SIGHTING SURVEY, WITH A PRELIMINARYDENSITY ESTIMATE OF THE HUMPBACK WHALE
MEGAPTERA NOVAEANGLIAE
Samba T. DIALLO1, Aboubacar SANE1, Thomas NELSON2,
Taiki KATSUMATA3 and Takashi HAKAMADA3*Centre National des Sciences Halieutiques de Boussoura (CNSHB),
814 Rue MA 500, Corniche Sud Madina, Boussoura Port, Conakry 224, Guinea
2
Department of Fisheries (DOF). Ministry of Agriculture, Fisheries, Physical Planning, Natural Resources
and Co-operatives (MAFPPNRC) Conway Post Oﬃce, Castries, LC04 30, Saint Lucia
3
Institute of Cetacean Research, 4–5 Toyomi-cho, Chuo-ku, Tokyo 104–0055, Japan*Corresponding author: hakamada@cetacean.jp
1

Abstract
Results of a cetacean sighting survey in coastal Gabon waters from September 4 to 9, 2011
are reported. Four whale species humpback (30 schools/191 individuals), Bryde’s (1 school/
2 individuals), sei (1 school/6 individuals), and sperm (2 schools/2 individuals), and 3 speciesof dolphin (pantropical spotted (1 school/150 individuals), Atlantic spotted (1 school/40 individuals) and bottlenose (1 school/25 individuals)) were observed over 232.1 nm of transectlines (26% of planned transects, covering 878.0 nm). Based on these survey data, using distance sampling methods, we estimate the density of humpback whales in this region to be
0.481 individuals/nm2 (CV=0.477).
Key words:

Gabon, humpback whale, distance sampling, density estimation, line transect.

Humpback whales Megaptera novaeangliae occur worldwide in all major oceans (Clapham, 2018).
The Scientiﬁc Committee of the International Whaling Commission (IWC/SC) identiﬁed seven breeding stocks (A–G) of this species in the Southern Hemisphere (IWC, 2005), with waters oﬀ Gabon
including part of stock B. From 1912–1930, 7,883 humpback whales were landed at whaling stations
in Gabon (Harmer, 1928; Best, 1994), and a further 7,080 animals were landed in 9 whaling seasons
between 1934 and 1959 (Budker and Collignon 1952; Budker 1952, 1953; Budker and Roux 1968;
Weir, 2010). While commercial humpback whaling in the Southern Hemisphere was banned by the
IWC in 1963 (IWC, 2011), after more than 50 years it is thought that stocks of this species may haverecovered. A comprehensive assessment of Southern Hemisphere humpback whales was completed in2015. An assessment model was conducted then using abundance estimates by mark-recapture methods provided in Collins et al. (2010); the depletion level (i.e., abundance per initial population size) of
the humpback whale breeding stock oﬀ Gabon was estimated to be 74% in 2015 (IWC, 2016).
Three cetacean sighting surveys have been completed in COMHAFAT (La Conférence Ministérielle
sur la Coopération Halieutique entre les Etats Africains Riverains de l’Océan Atlantique) coastal zone
waters. The ﬁrst cetacean sighting survey in Gabon coastal waters occurred in 2011; a second survey
took place in the Gulf of Guinea in 2013, covering the Côte d’Ivoire, Ghana, Togo and Benin (Diallo
and Bamy, 2013). A third survey was conducted in coastal waters of Guinea, Sierra Leone and Liberia
in 2018 (Diallo et al., 2018). A fourth proposed (2019) survey was scheduled for coastal waters oﬀ
Guinea, but it has yet to be done. During the 2019 IWC/SC68a meeting in Nairobi, Kenya, the planfor this fourth survey in the COMHAFAT coastal zone was presented. The IWC Standing Working

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CETACEANS OFF GABON BASED ON A 2011 SIGHTING SURVEY

Group on Abundance Estimates, Status of Stocks, and International Cruises (ASI) encouraged thatabundance estimates of cetacean species sighted in the COMHAFAT coastal zone be presented to the
future IWC/SC meeting to determine how they could be used for the Revised Management Procedure(RMP) or for other cetacean studies in this region (IWC, 2020). For this reason, we report hitherto unpublished estimates of humpback whale abundance data from the 2011 survey oﬀ Gabon.
The 2011 survey was conducted during the dry season in waters oﬀ Gabon (when rain is scarceand wind is weak). Because many cetacean species migrate into waters oﬀ Gabon during the austral
winter (Weir, 2010), we performed this survey during this period. The main objective was to obtain
information on the abundance and distribution of cetaceans in the coastal zone of Gabon (Diallo andBamy, 2013). The survey was performed by the Centre National des Sciences Halieutiques of Boussoura (CNSHB) under the auspices of COMHAFAT, with collaboration from some African ﬁsheries
institutions and ﬁsheries research centers, such as the Direction Générale des Pêches (DGP) in Gabon,Centre de Recherches Océanologiques (CRO) of Abidjan in Côte d’Ivoire, Institut Mauritanien deRecherche Océanographique et des Pêches (IMROP) of Nouadhibou in Mauritania, Centre de Recherche Océanographique de Dakar Thiaroye (CRODT) of Dakar in Senegal, Direction des Pêches (DP)of Cotonou in Benin, Marine Fisheries Research Division (MFRD) of Tema in Ghana, Fisheries andOceanography Research Station (SRHOL IRAD) of Limbé in Cameroon, and the Center for Applied
Fisheries Research (CIPA) of Bissau in Guinea-Bissau.
The N/O General Lansana Conté, a research vessel of Guinean nationality, 29.93 m in length,3.25 m in draught, of 1400 CV horsepower and 198 GRT (Fig. 1), was used for the survey. Further
survey details are provided in Diallo and Bamy (2013). A 10-day survey period was scheduled in September 2011, as per the itinerary shown in Table 1.
The study area was in the Gabonese EEZ including the isobaths from 200 m to 1,000 km, and excluded shallow waters and areas around oil ﬁelds. Six survey blocks were identiﬁed (left panel, Fig. 2)
comprising three oﬀshore (ON, OM, OS) and three coastal (CN, CM, CS) blocks. Within blocks, zigzag track lines of 878.0 nm total length were set (right panel, Fig. 2). Survey participants included 10
vessel crew and 9 research staﬀ; 2 Guinean researchers conducted the survey, with researchers fromeach of 6 African countries (1 from each of Mauritania, Senegal, Ghana, Benin, and Togo, and 2 from
Gabon) also participating. The cruise leader, Samba Diallo, entered data onto a computer during thesurvey; other scientists helped identify schools of cetaceans and count numbers of individuals.
The survey used a line-transect method (Burnham et al., 1980) authorized by the Scientiﬁc Com-

Fig. 1. The N/O General Lansana Conté used for COMHAFAT cetacean sighting surveys.
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DIALLO, SANE, NELSON, KATSUMATA AND HAKAMADA

Table 1.

Itinerary of the 2011 cetacean sighting survey in Gabon coastal waters.
Date
August 29
August 28–30
August 31
September 2
September 3
September 4
September 9
September 10
September 11
September 12
September 13–14
September 14

Event
Vessel left Conakry to LibrevilleParticipants arrived in LibrevillePre-survey meeting was held in Libreville
Vessel arrived at Libreville
Vessel left Libreville to survey area
Survey was started
End of the survey
Vessel arrived in Libreville
Vessel left Libreville to Conakry
Post-survey meeting was held in LibrevilleParticipants left Libreville
Vessel arrived at Conakry

Fig. 2. Gabon coastal water research areas: 2011 survey blocks conducted by COMHAFAT (left panel) andsurvey zig-zag track lines of 878.0 nm length set in survey blocks (right panel) (after Diallo and Bamy, 2013).

mittee of the IWC, following procedures and protocols in the requirement and guidelines for sighting
surveys (IWC, 2012). Surveys occurred during daylight from 30 min after sunrise to 30 min before
sunset, in good weather conditions (Beaufort scale ≤4, visibility >2 nm). A normal closing mode survey was performed for all encountered cetacean species; for further details of sighting procedures see
Diallo and Bamy (2013). Angles to sightings were measured using angle boards situated in front ofobservers. To calibrate the distances to and angle measurements of sightings, distance and angle measurement experiments were performed during the survey (to adjust measurement error if necessary).
During the survey, sighting eﬀort and weather data were recorded. All data, including photographs
belong to the COMHAFAT, with copies stored at the CNSHB in Guinea, and the Fisheries Resources
Institute, Japan Fisheries Research and Education Agency, in Japan.
To estimate humpback whale density, we used data for 13 primary (ship on-eﬀort) sightings for thisspecies, and eﬀort data. For detection function estimation, conventional distance-sampling methods
were used, including multi-covariate distance sampling, where detection on the track line is assumed
as certain (i.e. g(0)=1). The covariate to be considered for possible inclusion in the detection function

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CETACEANS OFF GABON BASED ON A 2011 SIGHTING SURVEY

is school size. Missing values for other candidate covariates (such as Beaufort scale and visibility)
precluded our considering them in detection functions.
We used the Multiple Covariate Distance Sampling (MCDS) Engine in the mrds package in R for
calculations (Thomas et al., 2010). The Hazard rate and Half normal models are ﬁtted as candidate
models for the detection function. The full model of the detection function is provided by:

{

}

−b


Hazard rate: g ( x, z ) =1 − exp  − x
a exp( Size) 



(1)

2

Half normal: g ( x, z ) = exp  − x
2a 2 exp{2( Size)}(2)

where x is the perpendicular distance from the track line to the sighting, z is a vector of covariates(i.e., Size), a (a>0) and b (b≥1) are coeﬃcients to be estimated, and Size is observed school size.
Density estimation is the second step used in the distance-sampling method. The analytical method
follows that of Buckland et al. (2015). For analysis we assumed that all schools of cetaceans on the
track line were detected. Density and its variance were estimated based on a Horvitz–Thompson likeestimator of abundance, as expressed in equations (3) and (4), respectively:
D=

1 n
si
2WL i= 1 p ( xi , zi )(3)where D is the density estimate, W is the truncation distance, L is the searching eﬀort, si is the size forith school, and p(xi, zi) is the probability of detection at a perpendicular distance xi and covariate zi.
2
2
K
r
r
1
PC PC 1 
 1  
 PCk PC +l
H jm (θ ) var( D ) = kθ j θm
L 
 2WL   L( K  1) k= 1  lk
j= 1 m= 1






(4)

where K is the number of transect, lk is searching distance in kth transect, PCk is the abundance estimate in the covered region on the kth transect, PC is the abundance estimate in the covered region, andH −1
jm (θ) is the jmth element of the inverse of the Hessian matrix of detection function with respect tothe vector of the coeﬃcients θ.
Akaike information criterion (AIC) values were compared to select the best model to estimate theprobability of a school being detected p(zi) given the covariate zi, which is expressed by the following
equation (5) using the detection function g(x, zi) (see equations (1) and (2)). The detection function
with the minimum AIC is selected as the best model among the candidate models.
p ( zi ) =

1
W

W

1

 g( x, z ) dx = W fˆ ( 0|z )
0

i

i

(5)

The 2011 survey in coastal waters of Gabon covered 232.1 nm over 6 days—only 26% of theplanned searching distance. Although 10 survey days had been initially scheduled, only the ON, CN
and CM blocks out of 6 blocks could be covered within available time (Fig. 3). During this survey, 30
schools of humpback whales (191 individuals), 2 schools of sperm whales (Physeter macrocephalus)(both with 1 ind.), and 1 school of each of Bryde’s (Balaenoptera edeni) (2 ind.) and sei (B. borealis)(6 ind.) whales, and 1 school of pantropical spotted (Stenella attenuata) (150 ind.), 1 school of Atlantic
spotted (S. frontalis) (40 ind.), and 1 school of bottlenose (Tursiops truncatus) (25 ind.) dolphins weredetected (Table 2). Sighting positions for (a) humpback whales, (b) Bryde’s, sei, and sperm whales,and (c) dolphins are shown in Fig. 3. No feeding behavior was observed during the survey.
The best model of estimated detection function ﬁtted to the relative frequency of detected schools
by perpendicular distance intervals as a function of perpendicular distance from the track line toa humpback whale sighting using 2011 survey data is depicted in Fig. 4. A density estimate of
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DIALLO, SANE, NELSON, KATSUMATA AND HAKAMADA

Fig. 3. Map of the 2011 survey area, blocks, line transects, and sighted schools of humpback whales (upper
left panel), Bryde’s, sei and sperm whales (upper right panel), and dolphins (lower panel).
Table 2.

Primary and secondary sightings during the 2011 cetacean sighting survey.
Species nameHumpback whale
Sperm whale
Sei Whale
Bryde’s whale
Pantropical spoted dolphin
Atlantic spotted dolphin
Bottlenose dolphin
Unidentiﬁed whale
Unidentiﬁed dolphin

262

Primary

Secondary

Total

Sch.

Ind.

Sch.

Ind.

Sch.

Ind.

13
1
0
1
0
1
1
2
3

50
1
0
2
0
40
25
2
15

17
1
1
0
1
0
0
6
1

141
1
6
0
150
0
0
13
10

30
2
1
1
1
1
1
8
4

191
2
6
2
150
40
25
15
25

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CETACEANS OFF GABON BASED ON A 2011 SIGHTING SURVEY

Fig. 4. Plot of the best model of estimated detection function ﬁtted to relative frequency of schools as a function of perpendicular distance (nm) from the track line to a humpback whale sighting based on 2011 survey
data. Cdf in the caption for the right panel is the abbreviation for cumulative distribution function.

0.481 ind./nm2 (CV=0.477) from this model has a low AIC of 11.437. Branch (2011) estimated humpback whale abundances in feeding grounds in Antarctic waters south of 60°S during the austral summer to be 4.36×10−5 ind./nm2 (CV=0.53) in 1992/93 between 0° and 40°E, and 2.55×10−4 ind./nm2(CV=0.75) in 1996/97 between 30°W and 0°. Branch (2011) considered that estimates of abundancein breeding stock B were greater than in the assumed corresponding feeding area (20°W–10°E) in
Antarctic waters because humpback whales in their southern migration migrated to waters north of
60°S. It is therefore likely that most humpback whales from this breeding stock do not migrate as far
south as 60°S to reach that area covered by the International Decade of Cetacean Research/Southern
Ocean Whale and Ecosystem Research Programme (IDCR/SOWER) surveys. Japanese Sighting Vessel (JSV) data identify a high density area north of 60°S (Miyashita et al., 1995), with many sightingsof this species in 2005/06 during the IWC SOWER survey (Ensor et al., 2006). Another possibility
is that animals in breeding stock B migrate to a diﬀerent longitudinal sector, supported by genetic
evidence that breeding stock B does not diﬀer signiﬁcantly in Areas II and III, in longitudinal sectors
60°W–70°E (Loo et al., 2011).
Because more humpback whales were sighted in CN and CM than in ON blocks, more survey eﬀort
could be allocated to coastal as opposed to oﬀshore blocks in future surveys (to more accurately estimate humpback whale abundance). Buckland et al. (2015) and the IWC (2012) suggested allocating
increased eﬀort to known high density strata. If more primary sightings occur in future COMHAFAT
surveys, further analyses of abundance estimates could be performed for other species from the 2011
survey data (i.e., sei, Bryde’s and sperm whales, and pantropical spotted, Atlantic spotted, and bottlenose dolphins).
Acknowledgements

The authors thank Drs. Koji Matsuoka, Megumi Takahashi and Luis A. Pastene of the Institute of Cetacean Research forkindly providing useful programs, valuable advice for data analysis, and for their kind attention to us during the workshop ofabundance estimates. Mr. Hideki Moronuki (Fisheries Agency of Japan) is also graciously thanked for providing the authorsan opportunity to estimate the abundances of cetaceans in sighting surveys in the COMHAFAT coastal zone. We hope that
this kind of collaboration will continue so that we can more accurately estimate the abundances of all cetacean species insurveys. We would also like to thank Dr. Hideyoshi Yoshida, from the Fisheries Resources Institute, Japan Fisheries Researchand Education Agency, for kindly providing scientiﬁc advice and instructions for conducting cetacean sighting surveys in
the COMHAFAT coastal zone. Thanks also to the captain, crew and researchers aboard the N/O General Lassana Conté who
participated in the survey. We would like to thank anonymous reviewers for their valuable suggestions and comments to improve this paper.

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DIALLO, SANE, NELSON, KATSUMATA AND HAKAMADA

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Received: January 15, 2021
Accepted: August 19, 2021
Published online: December 8, 2021

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