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Whale shark habitat assessments in thenortheastern Arabian Sea using satelliteremote sensingBeena Kumari a & Mini Raman aa Marine and Earth Sciences Group, Space Applications Centre(ISRO), Ahmedabad, 380 015, Gujarat, IndiaPublished online: 08 Jan 2010.

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Whale shark habitat assessments in the northeastern Arabian Sea usingsatellite remote sensing

BEENA KUMARI* and MINI RAMAN

Marine and Earth Sciences Group, Space Applications Centre (ISRO), Ahmedabad 380

015, Gujarat, India

(Received 8 March 2006; in final form 24 May 2008)

One of the major requirements for the growing whale shark tourism industry is to

identify potential areas of their aggregation for sighting. This would require baseline

information on the occurrence of whale shark and the associated environment. In

this context, the relationship between whale shark landings, phytoplankton concen-

tration and sea surface temperature (SST) in the continental shelf and offshore

regions of Gujarat coast were examined using satellite data from 1998 to 2000.

Monthly images of chlorophyll-a (chl-a) concentration, an index of phytoplankton

biomass and SST were derived for the eastern Arabian Sea from the Sea-viewing

Wide Field-of-view Sensor (SeaWiFS) and National Oceanographic and

Atmospheric Administration-Advanced Very High Resolution Radiometer

(NOAA-AVHRR), respectively. Whale sharks (Rhincodon typus) landing data

were obtained from a survey conducted by Trade Records Analysis of Flora and

Fauna In Commerce (Traffic)-India of the World Wide Fund (WWF)-India and the

Central Institute of Fisheries Technology (CIFT), India. Mean chl-a concentration

in the study area (between 20–22� N and 69–70� E) covering the continental shelf

and adjoining offshore region of coast (depth . 25 m) was observed to be signifi-

cantly higher (4.23 mg m-3 in February and 3.88 mg m-3 in March) compared to

regions seaward of the study area (mean of 1.51 mg m-3 for February and 1.16 mg

m-3 for March) and in southern latitudes of the eastern Arabian Sea (mean of 0.27

mg m-3 for February and 0.23 mg m-3 for March). The SST in the study area ranged

from 23–26�C for February and March, whereas in the southern latitudes, it ranged

from 27–29�C. The SST in regions outside the study area was marginally warmer by

0.5�C. A significant relationship between whale shark landings off Gujarat, chl-a

concentration and SST was observed. Results presented in this study contribute to

the idea that the combined use of ocean colour and SST images are an appropriate

tool to identify potential areas of whale shark aggregation for sightings.

1. Introduction

1.1 Whale shark distribution, ecology and biology

The whale shark (Rhincodon typus) is the world’s largest fish (figure 1(a)). The largest

one found to date measured 20 m and weighed 34 tonnes (Chen et al. 1997, Chen and

Phipps 2002). Despite being harmless, they are facing severe threat from humans due

to indiscriminate fishing and scientific attention (Pravin 2000). Whale sharks are

currently protected in Australia, the Maldives, Philippines, USA, Gulf of Mexico

*Corresponding author. Email: beena@sac.isro.gov.in

International Journal of Remote SensingISSN 0143-1161 print/ISSN 1366-5901 online # 2010 Taylor & Francis

http://www.tandf.co.uk/journalsDOI: 10.1080/01431160902893444

International Journal of Remote Sensing

Vol. 31, No. 2, 20 January 2010, 379–389

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and the Atlantic coast. In India, whale sharks were caught opportunistically for

decades because of high export value for their skin, meat and fins. Since the mid

1980s, whale sharks were regularly targeted on the coast of Gujarat (northeastern

Arabian Sea), mainly to supply export markets for whale shark meat and fins (Hanfee

1997). Whale shark hunting was banned on 28 May 2001 under the Indian Wildlife

(Protection) Act 1972 placing it under schedule I, the highest protection available.

Whale sharks are widely distributed in warm tropical waters (excluding the

Mediterranean) worldwide, usually between latitudes 30� N and 35� S in tropicaland warm temperate seas, both oceanic and coastal (Compagno 1984, Chen and

Phipps 2002). Records on whale shark capture and incidental landings (Trade

Records Analysis of Flora and Fauna in Commerce (Hanfee 2001)) show the

occurrence of whale sharks on the west coast of India, with reports of very few

catches on the east coast. The shelf-coastal waters of Gujarat in the northeastern

Arabian Sea is reported to be one of the favourite visiting spots for the whale

shark during the winter monsoon period, and they have been visiting the shores of

Gujarat for hundreds of years (Rao 1986, Vivekanandan and Zala 1994, Pravin2000, Hanfee 2001, Pravin et al. 2002). During December, whale sharks are

observed off the coasts of Maharashtra, Karnataka and Kerala, as well as the

east and west coasts of Sri Lanka (Silas 1986). The species is known to be

migratory, with a tagged whale shark known to have travelled a distance of

13 000 km from the Gulf of California, Mexico, to near Tonga over 37 months

(Eckert and Stewart 2001). Several studies indicate that whale sharks probably

migrate from the Sri Lankan coast along the west coast of India during

December, reaching the Gujarat coast by February to March (Silas 1986, Pravin2000, Hanfee 2001, Pravin et al. 2002). A study conducted on whale shark landing

in India during the period 1889 to 1998 suggest that Gujarat (the study area)

contributed the highest (94.3%) landing (Pravin et al. 2002).

The whale shark is a suction filter feeder and has a unique suction filter-feeding

method. As it swims with its huge mouth, which can be up to 1.22 m wide, it sucks

masses of water filled with prey into its mouth and through spongy tissue between its

five large gill arches. After closing its mouth, the shark uses gills rakers that are bristly

structures of about 10 cm long in the shark’s mouth that trap the small organisms.

India

Gujarat

Diu

Dwarka

Porbandar Mangarol

Jakhau

Kandla

Veraval

Okha

Study Area

68°E 69°E 70°E 71°E 72°E 73°E 74°E 75°E

68°E 69°E 70°E 71°E 72°E 73°E 74°E 75°E

25°N

24°N

23°N

22°N

21°N

20°N

25°N(a) (b)

24°N

23°N

22°N

21°N

20°N

Figure 1. (a) Picture of whale shark in the natural environment. (b) The study area off Gujaratin the northeastern Arabian Sea.

380 B. Kumari and M. Raman

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Anything that does not pass through the gills is eaten. Whale shark can process over

6000 litres of water each hour (Compagno 1984, Last and Stevens 1994).

1.2 Arabian Sea: physical and biological oceanography

The Arabian Sea experiences unique oceanographic features and events compared to

other world oceans. The semi-annual reversal of monsoon winds are divided into

southwest (SW) (June to September) and northeast (NE) (December to February)

monsoon phases, with two transition periods, spring inter monsoon (March to May)

and autumn inter monsoon (October to November). SW monsoon winds cause

vigorous and deep anti- cyclonic surface circulation in the Arabian Sea, inducing

both coastal and open ocean upwelling (Shetye et al. 1994). During the NE monsoon,

cold dry NE winds blow over the Arabian Sea, causing cyclonic circulation.Accordingly, waters north of 15� N experience densification and sinking of surface

waters, leading to convective mixing and deepening of the mixed layer (Prasanna

Kumar and Prasad 1996). Surface currents dissipate and hydrographic conditions in

the Arabian Sea approach those of a well-stratified and unperturbed tropical ocean

during the transition period between the two monsoon phases (Babenerd and Krey

1974). The SW and NE monsoon periods drive the biological production in the

Arabian Sea. During the SW monsoon, intense upwelling both in the coastal waters

off Somalia, Arabia and in the adjacent open ocean waters causes deepening of mixedlayers and injection of nutrients from the thermocline. This process results in very

high levels of biological production in the western Arabian Sea (Brock et al. 1991).

Similarly, wind driven upwelling and consequently high production is observed dur-

ing the SW monsoon in the southeastern part of the Arabian Sea. However, during the

rest of the season, this region is almost oligotrophic. In the NE monsoon phase,

surface cooling and densification leads to sinking and convective mixing triggering

intense biological production in the northern Arabian Sea (Prasanna Kumar and

Prasad 1996).

1.3 Whale shark ecotourism

After the enforcement of the ban (May 2001) on whale shark fishing in India, there is a

need to find out an alternate source of income for fishing communities who have been

involved in whale shark fishing for their livelihood. In this context, the potential value

of whale shark tourism is considerable, but the development of whale shark ecotour-

ism industry would require baseline information on the occurrence of this species andthe associated environment. One of the most important aspects for any such attempt

is whale shark sighting. The main purpose of this study is to find the relationship

between whale shark landings (before the ban) in Gujarat, phytoplankton biomass

and sea surface temperature (SST) using satellite data so as to identify potential

grounds for whale shark sightings based on the environment.

2. Study area

Based on the data on whale shark landings, the region between 20–22�N and69–70�E, extending from the broad continental shelf of Gujarat between Okha

and Diu to offshore regions of coast (depth . 25 m) was identified as the study

area (figure 1(b)).

Whale shark habitat assessments 381

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3. Materials and method

3.1 Whale shark landing data

Prior to the ban, the fishery for whale shark off Gujarat was known to start from

December and reach a peak in March. Whale shark landing data obtained for thisstudy is during the peak fishing period off the Gujarat coast (the study area) from

February to March 1998 to 2000 (table 1). The data were obtained from a survey

conducted by Traffic-India of the World Wide Fund (WWF)-India and the

Central Institute of Fisheries Technology (CIFT) (Hanfee 2001). In general,

whale sharks were caught by fishermen using large hooks operated from mechan-

ized wooden trawlers. Details of the fishing gear and technique have been

described by Pravin (2000) and Hanfee (2001).

3.2 Chlorophyll and SST data

Sea-viewing Wide Field-of-view Sensor (SeaWiFS) data of the Eastern Arabian Sea

(10–24� N, 66–76� E) was obtained from the Goddard Distributed Active Archive

Center (http://podaac.jpl.nasa.gov). Level 1 data (water-leaving radiances) for the

period February to March (1998 to 2000) were atmospherically corrected and pro-

cessed to level 2 (normalized water-leaving radiance nLw) using SeaDAS version 4.0

software. Chlorophyll-a (chl-a) concentration (mg m-3) was derived using theSeaWiFS Ocean Chlorophyll 4 (OC4v4.3) algorithm. The algorithm retrieved chl-a

within � 35% of in situ concentration in accordance with the goal set by SeaWiFS

mission. The images were mapped onto a uniform latitude/longitude projection.

Monthly composites were generated for all valid pixels by spatial and temporal

averaging. In these composites, pixels correspond to bins having a size of 9 � 9 km.

Monthly SST data of the same period, from NOAA-AVHRR was obtained from

the NOAA satellite archive (http://podaac.jpl.nasa.gov/sst) at the same spatial and

pixel resolution. This process makes use of the thermal infrared channels 4 and 5(10.5–11.3 mm and 11.5–12.5 mm) of NOAA-AVHRR for deriving the SST images for

the daytime pass. This region is highly sensitive to thermal variations of the Earth or

ocean. The SST is computed using the multi-channel SST (MCSST) approach, which

Table 1. Total number of whale shark caught/incidental landing in thewaters off Gujarat during February to March 1998 to 2000 (peak fishing

period).

DateNumber of whale sharks

caught off Gujarat Source

February toMarch 1998

115 Pravin et al. (2004)

February 1999 45 Traffic-India survey(1999–2000) (Hanfee 2001)

March 1999 78 Traffic-India survey(1999–2000) (Hanfee 2001)

February 2000 29 Traffic-India survey(1999–2000) (Hanfee 2001)

March 2000 43 Traffic-India survey(1999–2000) (Hanfee 2001)

382 B. Kumari and M. Raman

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essentially accounts for signal loss in atmosphere due to absorption by water vapour

and further relates brightness temperature with SST (McClain et al. 1985).

Chl-a and SST values from the images were also binned into 1��1� squares to

provide first-order estimates of the monthly means. Chl-a and SST means for the

study region (between 20–22� N and 69–70�E) were calculated. To examine the spatialvariations in the chl-a concentration and SST, latitude-wise means were computed by

taking all the non-zero values at each longitude (66–76� E), excluding the study

region. In the computations of chl-a concentration, values from waters shallower

than 20 m depth were excluded to avoid erroneous estimates of chl-a resulting from

bio-optically complex case-2 waters. In addition to the mean, standard deviation and

coefficient of variation (CV) were also calculated for the datasets to examine the

spatial homogeneity of environmental variables. Regression analysis was used to

correlate number of whale sharks caught and chl-a concentration for a particularyear and month.

4. Results

The relationship between the large number of whale shark catches, primary biomass

and SST in the study area were clearly apparent when the distribution pattern of chl-a

and SST in the southern latitudes, areas adjoining the study area and within the study

area, were examined. The distribution of the chl-a, an index of phytoplanktonbiomass concentration in the eastern Arabian Sea (10–24� N, 66–76� E) and in the

study area (between 20–22� N and 69–70�E) for the three years (1998 to 2000) during

February to March are shown in figures 2 and 3, respectively. Average chl-a concen-

tration (1998 to 2000) in the southern latitudes (10–15� N) were observed to be very

low, ranging from 0.21–0.39 mg m-3 in February (figures 2(a)–(c)) and 0.21–0.26 mg

m-3 in March (figures 3(a)–(c)). Concentrations increased northwards from 15� N,

ranging from 0.48–1.83 mg m-3 in February and from 0.29–1.53 mg m-3 in March.

The study area (between 20–22� N and 69–70� E), covering the continental shelf andadjoining offshore regions of coast (depth . 25 m) shows relatively higher concentra-

tions of chl-a ranging from 2.8–6.8 mg m-3 in February and 1.76–7.46 mg m-3 in

March. Spatially averaged chl-a concentrations (1998 to 2000) of the study area were

observed to be significantly higher (4.23 � 2.27 mg m-3 in February and 3.88 � 3.12

mg m-3 in March), compared to regions seaward and north of the study area, where

the spatial mean was observed to be 1.51 � 0.46 mg m-3 for February and 1.16 �0.38 mg m-3 for March (figures 2(d) and 3(d)). Similarly, in the southern latitudes of

eastern Arabian Sea, the spatial mean was observed to be very low (0.27 � 0.069 mgm-3 for February and 0.23� 0.043 mg m-3 for March) as compared to the study area.

The CV, which represents the index of homogeneity, is relatively less (,18–25%) in

southern latitudes, indicating a uniform distribution of chl-a. The northern latitudes

outside the study area also exhibit a uniform distribution pattern of chl-a (CV ,30 %)

compared to the heterogeneous distribution (CV , 53–80 %) in the study area.

The SST pattern also reveals similar latitudinal variation from south to north. It

depicts a decreasing trend from south to north, ranging from 29–23�C(figures 4(a)–(f)). The SST (mean of 1998 to 2000) analysis indicates that the south-ern latitudes (10–15� N) were characterized by warmer SSTs (minimum of 27.2�C,

maximum of 29.2�C) during February to March 1998 to 2000, compared to the SST

in the study area, which ranged from 23–26�C for February to March (figures 4(g)

and (h)). The SST spatially averaged over the study area (24�C) was observed to be

Whale shark habitat assessments 383

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cooler by 3�C compared to the spatially averaged SST in 10–15� N (27�C). The

regions seaward and north of the study area showed a marginal increase in SST by

0.5�C when compared with the SST in the study area. An inverse relationship was

observed between chl-a concentration and SST (figure 5). Higher concentration of

chl-a (. 1 mg m-3) was found in latitudes with lower SSTs (23–26�C).

Catch statistics of whale sharks indicate that commercial fishermen caught large

number of whale sharks (45 and 29) during February 1999 and 2000 in the study area.During March 1999, 78 whale sharks were caught and 43 were caught in March 2000.

Catch data obtained for February to March 1998 was pooled data and totalled 115

whale sharks from the study area. Figure 6(a) depicts that whale shark catches (W) are

significantly correlated with chl-a concentration (C), giving a linear equation:

W ¼ �13:978þ 14:92C; (1)

where number of data points n ¼ 25, coefficient of determination R2 ¼ 0.71 andprobability value P ¼ 0.0001. To eliminate negative catches, as indicated by the linear

fit due to no catches (shown as zero catches in figure 6(a)) for C ,¼ 2 mg m-3, an

exponential model was fitted to the data (n ¼ 25, R2 ¼ 0.93), giving the following

relation (figure 6(b)):

mgchl m–3

10°

15°

20°

65° 70° 75°

Feb 1998

(a) (b) (c)

(d)

Feb 1999 Feb 2000

65° 65° 70°70° 75° 75°

012345678

10 12 14 16 18 20 22 24

Latitude (°N)

Chl

orop

hyll-

a(m

g m

–3)

199819992000Study area 1998Study area 1999Study area 2000

0 2 4 6 8 10 12 14 16 18

Figure 2. (a)–(c) SeaWiFS images showing chl-a distribution in the eastern Arabian Seaduring February 1998 to 2000. (d) Variation in chl-a concentration in the eastern ArabianSea (10–24� N, 66–76� E) and in the study area (between 20–22� N and 69–70� E) duringFebruary 1998 to 2000.

384 B. Kumari and M. Raman

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W ¼ 1:380 e2:29C : (2)

The t-test was not performed on the data set since the number of observations was less

than 30.

5. Discussion

5.1 Relationship between whale shark catches and environmental variables(phytoplankton and SST)

A significant relationship between whale shark catches and chl-a has been revealed by

this study. Results indicate that whale shark aggregation and abundance could be a

result of the availability of high primary biomass. This is evident when the chl-a (an

index of phytoplankton biomass) distribution pattern is examined from south to

north, which reveals an increasing trend in phytoplankton biomass from southern

to northern latitudes. The increase in primary biomass in northern latitudes of the

Arabian Sea has been well documented by Banse and McClain (1986), Bhattathiri

et al. (1996) and Prasanna Kumar and Prasad (1996). This period corresponds to the

mgchl m–3

10°

15°

20°

65° 70° 75° 65° 70° 75° 65° 70° 75°

0

2

4

6

8

10

12

10 12 14 16 18 20 22 24

Latitude (°N)

Chl

orop

hyll-

a(m

g m

–3)

199819992000Study area 1998Study area 1999Study area 2000

0 2 4 6 8 10 12 14 16 18

Mar 1998 Mar 1999 Mar 2000(a) (b) (c)

(d)

Figure 3. (a)–(c) SeaWiFS images showing chl-a distribution in the eastern Arabian Seaduring March 1998 to 2000. (d) Variation in chl-a concentration in the eastern Arabian Sea(10–24� N, 66–76� E) and in the study area (between 20�–22� N and 69–70� E) during March1998 to 2000.

Whale shark habitat assessments 385

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Feb 1998

(a)

(g) (h)

(b) (c) (d) (e) ( f )

22° N

20° N

18° N

16° N

14° N

12° N

10° N

22 23 23.5 24.5 25 25.5 26.5 27 27.5 28.5 29 30

66° E 69° E 72° E 75° E 72° E 75° E69° E 72° E 75° E69° E 72° E 75° E69°E 72° E 75° E69° E 72° E 75° E

Goa

Mumbai

Goa

Mumbai

Goa

Mumbai

Goa

Mumbai

Goa

Mumbai

Goa

Mumbai

INDIAINDIAINDIAINDIAINDIAINDIA

69° E

Mar 1998 Feb 1999 Mar 1999 Feb 2000 Mar 2000

22

24

26

28

30

10 12 14 16 18 20 22 24

Latitude (°N) Latitude (°N)

SS

T (

°C)

SS

T (

°C)1998

19992000Study area 1998Study area 1999Study area 2000

22

24

26

28

30

10 12 14 16 18 20 22 24

SST(°C)

199819992000Study area 1998Study area 1999Study area 2000

Figure 4. (a)–(f) NOAA-AVHRR derived SST image of February to March 1998 to 2000,showing cooler water (blue) in the study area. (g) Latitudinal variation of SST from south tonorth during February 1998 to 2000. (h) Latitudinal variation of SST from south to northduring March 1998 to 2000.

0.1

1

10

22 23 24 25 26 27 28 29 30

Temperature (°C)

log

ch

loro

ph

yll (

mg

m–3

)

Feb-98

Mar-98

Figure 5. Relationship between logarithm of chl-a (mg m-3) and SST (�C) in the study area.

386 B. Kumari and M. Raman

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winter monsoon (December to March) in the northern Indian Ocean, characterized

by cold dry northeasterly winds leading to enhanced evaporation and lowering of

SST. Accordingly, surface waters experience densification and sinking, leading to

convective mixing and injection of nutrients into upper mixed-layer, triggering pri-mary production and increase in phytoplankton biomass. Strong correlation between

low SST and whale shark catches is implicit in the inverse relationship between SST

and chl-a, indicating the influence of physical process in growth of phytoplankton

biomass.

5.2 Possible causes of whale shark availability in the northern latitudes andaggregation in the study area

Distribution and occurrence of whale sharks was studied in detail based on the

incidental landings/capture in different landing centres along the Indian coast from

1889 to 1998 (Rao 1986, Silas 1986, Vivekanandan and Zala 1994, Hanfee 2001,

Pravin et al. 2002). Pravin et al. (2002) has reported seasonal migration of whale

shark from the south (near the Maldives) to the north along the west coast during

the winter monsoon (December to March) in the eastern Arabian Sea. In the study

area where large numbers of whale sharks were caught by commercial fishermen, the

observed increase in the average chl-a concentration is of the order of three whencompared to the regions seaward and north of the study area, and the increase is

approximately 16 times more than the southern latitudes. Study suggests that the

migration pathway of whale sharks seems to depend on the seasonal triggering of

primary production and growth of phytoplankton biomass in the northern latitudes

of the eastern Arabian Sea.

A probable explanation for the occurrence of large numbers of whale sharks in

regions of high phytoplankton concentration lies in their physiology and feeding

strategy. Being a very large animal, the whale shark may require a large quantity offorage daily to sustain an optimum physiological demand. Whale sharks feed on

planktonic organisms and are suction filter feeders. Due to their specific feeding

behaviour, they are probably dependent on dense aggregations of prey organisms.

Last and Stevens (1994) have reported that whale sharks move their heads from side

to side, vacuuming in seawater rich in plankton, or aggressively cut swathes through

schools of prey. The frequent turns may keep the whale sharks in the denser parts of

y = 14.921x –13.978

R2 = 0.8263

–200

20406080

100120140

(a) (b)

0 2 4 6 8

Chlorophyll concentration (mg m–3)

No.

of w

hale

sha

rks

caug

ht

10

Chlorophyll concentration (mg m–3)

0 2 4 6 8

No.

of w

hale

sha

rkca

ught

020406080

100120140160180

Figure 6. (a) Correlation of whale shark catch and chl-a concentration using a linear equation.(b) Correlation of whale shark catch and chl-a concentration using an exponential model.

Whale shark habitat assessments 387

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the plankton patches. The large number of whale shark catches in the study area,

where phytoplankton biomass was very high (figures 2(a)–(d), 3(a)–(d) and 6), seems

to support this view.

6. Conclusion

This study improves our understanding of the whale shark occurrence and aggrega-

tion during the winter monsoon in northern regions of the Arabian Sea. While

availability of food is a function of physical process in the region, the linear relation-

ship between whale shark catches and phytoplankton biomass provides evidence that

whale sharks adapt their movements directly to food availability, and their migration

within a region may be precisely timed to coincide with seasonal productivity events.

The seasonal visit of whale sharks to the coast of Gujarat during the winter seasonevery year generates scope for the development of an alternate source of income for

the fishing community of India by means of whale shark ecotourism industry. Remote

sensing techniques afford a new methodological approach to assess the potential areas

of whale shark sightings.

Acknowledgements

We are extremely thankful to Shri Dhiresh Joshi, Wildlife Trust of India, for provid-

ing the geolocated data on whale sharks. We also thank the anonymous reviewer for

critical and positive comments.

References

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