Whale shark habitat assessments in the northeastern Arabian Sea using satellite remote sensing

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  • This article was downloaded by: [Istanbul Universitesi Kutuphane ve Dok]On: 02 September 2013, At: 11:21Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

    International Journal of RemoteSensingPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/tres20

    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.

    To cite this article: Beena Kumari & Mini Raman (2010) Whale shark habitat assessments in thenortheastern Arabian Sea using satellite remote sensing, International Journal of Remote Sensing,31:2, 379-389, DOI: 10.1080/01431160902893444

    To link to this article: http://dx.doi.org/10.1080/01431160902893444

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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 InCommerce (Traffic)-India of theWorldWide Fund (WWF)-India and the

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

    in the study area (between 2022 N and 6970 E) covering the continental shelfand 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

    mgm-3 for February and 0.23 mgm-3 forMarch). The SST in the study area ranged

    from 2326C for February andMarch, whereas in the southern latitudes, it rangedfrom 2729C. The SST in regions outside the study area was marginally warmer by0.5C. A significant relationship between whale shark landings off Gujarat, chl-aconcentration 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 worlds 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, 379389

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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 sharks mouth that trap the small organisms.

    India

    Gujarat

    Diu

    Dwarka Porbandar

    Mangarol

    Jakhau Kandla

    Veraval

    Okha

    Study Area

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

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

    25N

    24N

    23N

    22N

    21N

    20N

    25N(a) (b)

    24N

    23N

    22N

    21N

    20N

    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 surfacewaters, 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 SWmonsoon 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 2022N and6970E, extending from the broad continental shelf of Gujarat between Okhaand Diu to offshore regions of coast (depth . 25 m) was identified as the studyarea (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

    (1024 N, 6676 E) was obtained from the Goddard Distributed Active ArchiveCenter (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 SeaWiFSmission. 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(10.511.3 mm and 11.512.5 mm) of NOAA-AVHRR for deriving the SST images forthe 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(19992000) (Hanfee 2001)

    March 1999 78 Traffic-India survey(19992000) (Hanfee 2001)

    February 2000 29 Traffic-India survey(19992000) (Hanfee 2001)

    March 2000 43 Traffic-India survey(19992000) (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 11 squares toprovide first-order estimates of the monthly means. Chl-a and SST means for the

    study region (between 2022Nand 6970E) 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 (...

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