SEABIRD AND CETACEAN OCCURRENCE IN THE BAY OF BENGAL ASSOCIATED WITH MARINE PRODUCTIVITY AND COMMERCIAL FISHING EFFORT

Abstract

Our paper reports findings on at-sea distributions of both seabirds and cetaceans in the Bay of Bengal. Our study is the first systematic attempt in creating baseline information for this data-poor region. We carried out 39 days of vessel-based observations from 2012 to 2014 over a total linear distance of 4722.3 km. We observed 2697 seabirds of 17 species and 1441 cetaceans of at least eight species. Among the seabirds, Sooty Terns Onychoprion fuscatus (n = 2282, 85% of all birds) and Wedge-tailed Shearwaters Ardenna pacifica (n = 327, 12%) predominated, whereas cetacean numbers were dominated by Spinner Dolphins Stenella longirostris (n=772, 54% of all cetaceans) and Indo-Pacific Bottlenose Dolphins Tursiops aduncus (n=533, 37%).

Full text

Mondreti et al.: Seabird and cetacean occurrence in the Bay of Bengal 91
Marine Ornithology 48: 91–101 (2020)
91
INTRODUCTION
Seabirds form only ~3% of all bird species, but their collective
biomass far outweighs that of land birds (Brooke 2004, Birdlife
International 2010). Cetaceans contribute an even smaller
proportion of mammal diversity (~2%), but their large biomass
and generally high trophic position make them ecologically
important (Katona & Whitehead 1988, Pauly et al. 1998, Schipper
et al. 2008). Therefore, mapping seabird and cetacean species’
distributions and relative abundance across the oceans provides
insight into marine food web dynamics and marine community
structure; further, mapping facilitates effective marine conservation
strategies. Many studies on seabirds (Burger & Gochfeld 2004,
Piatt et al. 2007, Mallory et al. 2010) and cetaceans (Reddy et al.
2001, Wells et al. 2004, Ainley et al. 2009) have demonstrated
their importance as ecosystem sentinels.
Most seabird and cetacean studies have focused on temperate
and high-latitude zones (Phillips et al. 2006, Karnovsky et al.
2010); much less effort has been carried out in subtropical and
tropical areas, with most attention directed to the tropical Pacific
Ocean (e.g., Bailey 1966, Bailey 1968, Ashmole 1971, Ainley
1977, Pocklington 1979, Abrams & Griffiths 1981, Au & Pitman
1986, Pitman & Ballance 1992, Ballance et al. 1997, Spear et
al. 2001, Vilchis et al. 2006). These studies revealed, in general,
that seabird species diversity is higher in warmer subtropical and
tropical seas when compared to the colder temperate and high-
latitude seas; while densities were higher in the latter (Newton
2003, Ballance 2007).
In the tropical and subtropical regions of the Indian Ocean, only
a few large-scale surveys have been conducted (e.g., Pocklington
1979, Hyrenbach et al. 2007, Thiebot & Weimerskirch 2013),
and most studies have focused on its western sector (e.g., Bailey
1968, Ballance et al. 2002, Jaquemet et al. 2005, 2014). Little
attention has been given to the Indian Ocean’s eastern sector
(Dunlop et al. 1988), and the Bay of Bengal (BOB) has received
even less attention. There is, to the best of our knowledge,
no previous systematic study of seabirds and cetaceans in the
northeastern Indian Ocean, especially for the Bay of Bengal
Large Marine Ecosystem (BOBLME). The BOB is the largest
bay in the world (2.2 million km2) and is believed to be globally
significant for seabirds (Mondreti et al. 2013, Le Corre et al.
2012, Jaeger et al. 2017) and cetaceans (Alling et al. 1986,
Kumaran 2002, Afsal et al. 2008, Smith et al. 2008, Malakar et
al. 2015).
SEABIRD AND CETACEAN OCCURRENCE IN THE
BAY OF BENGAL ASSOCIATED WITH MARINE PRODUCTIVITY
AND COMMERCIAL FISHING EFFORT
RAVICHANDRA MONDRETI1,2,3*, PRIYA DAVIDAR1,4, PETER G. RYAN5, JEAN-BAPTISTE THIEBOT6 & DAVID GREMILLET2,5
1Department of Ecology and Environmental Sciences, Pondicherry University, Kalapet, Puducherry 605014, India
(ravichandra.mondreti@gmail.com)
2Centre d’Ecologie Fonctionnelle et Evolutive, UMR 5175, CNRS - Université de Montpellier - Université Paul-Valéry Montpellier -
EPHE, Montpellier, France
3National Centre for Sustainable Coastal Management (NCSCM), Ministry of Environment, Forest and Climate Change (MOEF & CC),
Site office, Bhubaneswar, Odisha 751024, India
4Sigur Nature Trust, Chadapatti, Mavinhalla PO, Nilgiris, Tamil Nadu 643223, India
5FitzPatrick Institute of African Ornithology, NRF-DST Centre of Excellence, University of Cape Town, Rondebosch 7701, South Africa.
6National Institute of Polar Research, Midori-cho 10-3, Tachikawa, Tokyo 190-8518, Japan
Received 29 November 2018, accepted 15 January 2020
ABSTRACT
MONDRETI, R., DAVIDAR, P., RYAN, P.G., THIEBOT, J.B. & GREMILLET, D. 2020. Seabird and cetacean occurrence in the Bay of
Bengal associated with marine productivity and commercial fishing effort. Marine Ornithology 48: 91–101.
At-sea observations of seabirds and cetaceans provide essential baseline information about their biogeography and behaviour, facilitating
marine spatial planning and management. Much of the world’s oceans have been surveyed, yet some regions remain particularly data-poor
for seabirds and cetaceans, including the Bay of Bengal. We performed 39 d of vessel-based observations within the Bay of Bengal from
2012 to 2014, surveying an overall linear distance of 4722.3 km. We observed 2697 seabirds of 17 species and 1441 cetaceans of at least
eight species. Among the seabirds, Sooty Terns Onychoprion fuscatus (n=2282, 85% of all birds) and Wedge-tailed Shearwaters Ardenna
pacifica (n=327, 12%) predominated, whereas cetacean numbers were dominated by Spinner Dolphins Stenella longirostris (n=772, 54%
of all cetaceans) and Indo-Pacific Bottlenose Dolphins Tursiops aduncus (n=533, 37%). Other seabirds and cetaceans accounted for only
4% and 7%, respectively, of all sightings. The abundance and diversity of both groups was low compared to other tropical areas. We propose
that low seabird and cetacean abundance results from low productivity due to stratification in the Bay of Bengal, as well as long-lasting
disturbance, overexploitation of marine resources, possible impacts of longline fisheries, and the near absence of seabird breeding sites.
Key words: at-sea survey, biogeography, cetaceans, conservation, Bay of Bengal, seabirds,upper trophic level predators, overfishing

92 Mondreti et al.: Seabird and cetacean occurrence in the Bay of Bengal
Marine Ornithology 48: 91–101 (2020)
Some 50 species of seabirds have been recorded from the Indian
subcontinent; except for some terns, most are non-breeding
migrants (Birdlife International 2014). Of the 11 tern species
recorded from the region, nine breed on the Indian subcontinent
and most are coastal species that forage from inland estuaries
to the edge of the continental shelf (Mondreti et al. 2013). In
contrast, Sooty Terns Onychoprion fuscatus, a super-abundant
pan-tropical species, breed on some of the islands of the
Lakshadweep Archipelago (Arabian Sea) and in this region
are thought to occur only in pelagic waters of the BOB and
Arabian Sea. The BOB is potentially an important foraging site
for oceanic species such as Sooty Terns (Jaeger et al. 2017),
shearwaters (Le Corre et al. 2012) and petrels (Legrand et al.
2016). Past observations of cetaceans in the BOB, off the coast
of Bangladesh, showed that at least four cetacean families
(Platanistidae, Delphinidae, Phocinidae, and Balaenopteridae)
were present (Smith et al. 2008).
Seabirds and cetaceans are impacted by the consequences of
climate change, but effects of pelagic longline fishing are also
important for seabirds (Delord et al. 2010, Rollinson et al. 2017,
Bugoni et al. 2008, Petersen et al. 2009) and cetaceans (Werner
et al. 2015, Macías-López et al. 2012), notably through incidental
mortality (bycatch). In some cases, longlining has been driving
seabird populations to near extinction (Croxall et al. 2012), even
though mitigation measures have brought major improvements
in some areas (Rollinson et al. 2017). Therefore, it is helpful
to estimate the extent of spatial overlap between seabirds, other
upper trophic level predators, and commercial longlining fishing
to examine whether further marine spatial planning may be
needed (Cuthbert et al. 2005, Petersen et al. 2008, Copello &
Quintana 2009, Thiebot et al. 2016).
Fig. 1. The abundance of A) Sooty Terns and Wedge-tailed Shearwaters; and B) Spinner Dolphins and Indo-Pacific Bottlenose Dolphins
relative to sea surface temperature (SST). The solid line indicates the 200-m bathymetric contour. Inset shows the extent of the study area
with respect to the Indian subcontinent.
TABLE 1
Total survey effort indicating the number of days at sea,
distance covered, and hours of observation
Year TransectaMonth
Days of
observation
at sea
Sampling
effort
(hours)
Distance
covered
(km)
2012 MPB April 3 26 333.6
PBM
KPB
PBK
MD
May
May
May
May, June
3
4
3
14
32
26
26
106
370.6
270.2
273.4
2161.6
2013 MPB2
PBK2
March
March, April
3
3
23
27
279.6
326.1
2014 MPB3
PBK3
January
January
3
3
31
26
370.1
337.1
Total 39 323 4722.3
a MPB=Chennai-Port Blair, PBM=Port Blair-Chennai,
KPB=Kolkata-Port Blair, PBK=Port Blair-Kolkata,
MD=Marion Dufresne.

Mondreti et al.: Seabird and cetacean occurrence in the Bay of Bengal 93
Marine Ornithology 48: 91–101 (2020)
Broadly, our main research objective was to provide important
baseline information on seabird and cetacean sightings for the
data-poor BOB (Narvekar & Prasanna Kumar 2006). The specific
objectives of our study were to: (i) provide information about the
at-sea distributions and abundances of seabirds and cetaceans
within the BOB; (ii) understand broad links between these
occurrences and abiotic (sea surface temperature, bathymetry)
as well as biotic (chlorophyll a) parameters; and (iii) illustrate
potential overlaps between seabird and cetacean occurrences and
pelagic longlining activities.
MATERIALS AND METHODS
Study area
The BOB (Fig. 1) is designated as one of 64 global Large Marine
Ecosystems (LMEs; Sherman & Hempel 2009, Heileman et al.
2009, Hossain 2004). It spans an area equivalent to 6% of the Indian
Ocean, although the International Hydrographic Organization
(IHO) publication, Limits of Oceans and Seas, excludes the
BOB and Arabian Sea from the Indian Ocean (International
Hydrographic Organization 1953).The BOB forms a semi-closed
tropical basin that receives freshwater output from the Ganges,
Brahmaputra, and Godavari rivers. Nutrient-rich runoff from these
rivers is the primary reason for the high primary production of the
region’s coastal waters (Sarma et al. 2016). By comparison, the
central parts of the BOB are less productive due to the absence of
upwelling and mixing (Dwivedi 1993). The continental shelf along
the east coast of India is < 45 km wide, except for the northern
portion, where the continental shelf is > 200 km wide (Varkey
et al. 1996).The BOB is considered to have lower biological
production than the neighbouring Arabian Sea (Prasanna Kumar et
al. 2002), but, nonetheless, is a class I highly productive ecosystem
(> 300 g·cm-2·y-1; Sherman & Hempel 2009). Annual primary
productivity in BOB is at least twice that of the Benguela upwelling
ecosystem and the North Sea (Sherman & Hempel 2009, Mondreti
et al. 2013), from which one might presume that the BOBLME
possesses numerous seabird and cetacean species.
At-sea surveys
We surveyed seabirds and cetaceans, either from the flying bridge
or the bow, onboard passenger and research vessels during nine
cruises in April–May 2012, February–March 2013, and January
2014, along two major shipping routes: Chennai to Port Blair (CPB)
and Kolkata to Port Blair (KPB) (Table1, Fig.1). RM performed
most observations during 3–5d cruises on board passenger vessels
of the shipping corporation of India: M/V Nancowry (157 m),
M/V Akbar (149.5 m) and M/V Harshavardhana (132.5m). PGR
performed additional observations during a 23-d research cruise of
R/V Marion Dufresne (120 m). Due to very low bird and cetacean
densities, we used Method II described by Tasker et al. (1984) to
perform continuous surveys of seabirds and cetaceans (Ballance
& Pitman 1998).
Due to low seabird and cetacean numbers observed during all the
cruises, we recorded them out to the limit of detection (~500m).
All observations were made continuously during steaming from
Fig. 2. The abundance of A) Sooty Terns and Wedge-tailed Shearwaters; and B) Spinner Dolphins and Indo-Pacific Bottlenose Dolphins in
relation to chlorophyll a (ChlA) levels. The solid line indicates the 200-m bathymetric contour.

94 Mondreti et al.: Seabird and cetacean occurrence in the Bay of Bengal
Marine Ornithology 48: 91–101 (2020)
sunrise to sunset, counting birds on one side of the vessel in a 90°
quadrant (usually the side with best visibility, i.e., least sun glare).
All cetaceans seen were counted, irrespective of the side of the
vessel on which they were first detected. Observations were not
made during unfavourable weather conditions (visibility < 3km and
rainy days) and stopped when the coasts were visible at a distance
of 10–15 km. We used 10× binoculars from an approximate eye
height of 25 m (ship’s bridge) or 8 m (bow), when the ship was
cruising at 12–26 km·h-1 (6.5–14 knots). Because the counts
were not quantified according to the area searched, and given the
possible confounding effects of ship avoidance or attraction, we
focused on the relative number of individuals sighted instead of
absolute abundance. Counts were made following international
Seabirds at Sea (SAS) standards (Camphuysen et al. 2004,
Camphuysen & Garthe 2004, Tasker et al. 1984, Johansen et
al. 2015), incorporating sightings of cetaceans. The geographic
position of the ship at the beginning of each transect was recorded
from a hand-held Geographic Positioning System (GPS; Garmin
eTrex10) and was later checked with the ship’s GPS navigation
system for accuracy. During each transect, the time and exact
coordinates of each seabird or mammal observation was recorded.
Subsequently, for analysis, transects were divided into one-hour
bins, with sightings allocated accordingly into the appropriate
bins. There were no ship-following seabirds except for coastal
species, observations of which were restricted to the continental
shelf (<100km from shore).
Oceanographic variables
We retrieved oceanographic variables, including bathymetry
(ETOPO1, spatial resolution 0.01667 degrees), sea surface
temperature (SST; NOAA POES AVHRR, GAC, 0.1 °C), and
chlorophylla concentration (ChlA; Aqua MODIS, NPP, 0.05degrees)
from the Bloomwatch website (https://coastwatch.pfeg.noaa.gov/
coastwatch/CWBrowser.jsp). We selected these variables based on
their ecological and functional relevance for seabirds and cetaceans
(Garthe et al. 2009). Remotely sensed SST and ChlA data are
proxies of surface water mass distributions and primary productivity,
respectively. Similarly, water depth or bathymetry can also influence
seabird occurrence (Schneider 1997) and cetacean distributions (Yen
et al. 2004). Frequent cloud cover prevented the use of daily or
weekly satellite-derived datasets, so we used monthly environmental
datasets. For some variables, remote sensing data were not available
for the survey month in the year of the survey, so we used the data
from previous years. Given the irregular spatial resolution of the
oceanographic variables, we aggregated all data of the survey bins
into 0.2° x 0.2° grid cells and recalculated the abundance of seabirds
and cetaceans for these grid cells.
Fisheries data
Longline fishing catch and effort data were retrieved and compiled
from publicly available sources: the Indian Ocean Tuna Commission
(http://www.iotc.org) and the Regional Fisheries Management
Organizations for the period 1952–2013. All sectors in the BOB
were mapped into 5° × 5° grid cells. Wherever the spatial resolution
of fishing data was not uniform, the larger cells (10° × 10° and
20° × 20°) were parsed and the smaller cells (1° × 1°) were
aggregated into 5° × 5° standard grid cells. The data were integrated
and analysed in ArcGIS 10.1 (ESRI, Inc. Redlands, CA, USA) to
produce maps of fishing effort (number of hooks per 5° × 5° cell),
following the methodology described in Lewison et al. (2004).
Fishing effort is expressed as the mean number of longline fishing
hooks per year deployed in a standard 5° × 5° cell.
Data processing
We overlaid bins of seabird and cetacean abundances over SST
and ChlA raster layers (kriged) to examine the spatial distributions.
All spatial analyses were performed using ESRI ArcGIS, version
10.1 (ESRI 2012). Because each of the environmental variables
were not normally distributed, wherever necessary, we applied
data transformation and performed our analysis using the mean.
There were some temporal mismatches between sightings and
environmental data that could confound identification of patterns in
the data. Therefore, we did not attempt to model the possible factors
explaining species distributions. Due to varying cruise speeds, we
expressed abundance as the number of individuals sighted per
hour. We then extracted the oceanographic variables (SST, ChlA,
bathymetric characteristics) for each grid cell using ArcGIS 10.1
software. At-sea bird abundance data were converted to the same
resolution (5° × 5°) as that of fishing effort maps. We calculated
a single value mean for each of these 5° × 5° cells. Each point,
which represents numbers of seabirds and cetaceans seen per hour
of observation, was overlaid on the longline fishing effort raster.
Additionally, we calculated mean encounter rates per 100 linear km
for each species for all of the survey months.
RESULTS
In accordance with our research objectives, we collected seabird
and cetacean observations in some areas of the BOB, thereby
providing important baseline information for this data-poor region.
Sightings were scarce, and low animal occurrences precluded
detailed analyses of links with patterns of primary productivity,
SST, bathymetry, and fisheries. Despite these limitations, we were
able to draw interesting general conclusions from our data.
TABLE 2
Monthly survey effort of seabirds and cetaceans, including the numbers sighted and encounter rate of each species
Month Effort
(hrs)
No. of individuals sightedaMean encounter rate (sightings/100 km)
SOTE WTSH BNDO SPDO SOTE WTSH BNDO SPDO
January
March
April
May
June
65
35
41
120
62
1
775
652
768
86
3
10
6
113
195
156
163
102
112
0
12
461
50
114
45
0.08
0.57
1.34
0.66
0.80
0.15
0.57
0.60
0.87
1.93
0.30
0.28
0.24
0.08
0.00
0.08
1.00
0.12
0.29
0.16
a SOTE = Sooty Tern, WTSH = Wedge-tailed Shearwater, BNDO = Indo-Pacific Bottlenose Dolphin, SPDO = Spinner Dolphin.

Mondreti et al.: Seabird and cetacean occurrence in the Bay of Bengal 95
Marine Ornithology 48: 91–101 (2020)
Total numbers and species sightings
During at-sea transects, we covered a total distance of 4722.3 km,
at an average speed of 20km·h-1, for a total number of 39 d, and
with a linear survey effort of 323 observation hours (Table1). We
observed 2697 seabirds of 17 species and 1441 cetaceans of at least
eight species (Table2).
Sooty Terns Onychoprion fuscatus (n = 2282 individuals) and
Wedge-tailed Shearwaters Ardenna pacifica (n= 327 individuals)
accounted for 97% of all birds observed, while 91% of all individual
cetaceans observed were Indo-Pacific Bottlenose Dolphins Tursiops
aduncus (n = 533 individuals) and Spinner Dolphins Stenella
longirostris (n = 772 individuals). Although we sighted cetaceans
along all transects, they were most abundant close to the Andaman
and Nicobar islands (Figs. 1B, 2B). Overall, throughout the study
area, cetaceans were sighted sporadically, usually in low numbers.
Seasonality in seabird and cetacean sightings
Our at-sea observations revealed noticeable monthly (seasonal)
differences in both seabird and cetacean numbers (Table 2). The
only exception was the Indo-Pacific Bottlenose Dolphins, which
were encountered frequently throughout all months of the study.
Peak Sooty Tern numbers were observed during spring (March–
May), whereas maximum numbers of Wedge-tailed Shearwaters
were seen during the commencement of the southwest monsoon
(June; Table 2). Maximum encounter rates for Sooty Terns (average
1.34 sightings·km-1) occurred during spring (April), whereas
maximum encounter rates for Wedge-tailed Shearwaters (average
1.93 sightings·km-1) occurred in June. Maximum encounter rates
for Spinner Dolphins (average 1.0 sighting·km-1)occurred during
March, whereas bottlenose dolphins (average 0.3 sightings·km-1)
were sighted most frequently in winter (January).
Physical and biological habitat
SST and ChlA values varied little across the BOB (Figs. 1, 2).
However, SST increased significantly from North to South—the
northern waters were cooler than the south (Figs. 1A, B). High
ChlA values occurred at the head of the Bay and along the east
coast. Overall, we observed low SST and high ChlA values at
the head of the bay; high SST and low ChlA values occurred
in the central and southern areas of the bay (Figs. 1A, 2A). By
comparison, water depth (bathymetry, BATH) increased gradually
from north to south (Fig. 3A). The floor of the central BOB is flat,
with depths ranging from 2000–3000m; depths of up to 4700m
occur towards the mouth of the BOB.
We observed seabirds throughout the study area, independent of
changes in SST, ChlA, or bathymetric characteristics (Figs.1A, 2A,
TABLE 3
Seabird and cetacean species observed during the present study in Bay of Bengal
Group Species name Number of individuals
Seabirds Sooty Tern (Onychoprion fuscatus) 2282
Wedge-tailed Shearwater (Ardenna pacifica) 327
Greater Crested Tern (Thalasseus bergii) 29
Lesser Crested Tern (Thalasseus bengalensis) 15
Sandwich Tern (Thalasseus sandvicensis) 11
Caspian Tern (Hydroprogne caspia) 10
Flesh-footed Shearwater (Puffinus carneipes) 5
Streaked Shearwater (Calonectris leucomelas) 5
Jouanin’s Petrel (Bulweria fallax) 3
Barau’s Petrel (Pterodroma baraui) 2
Black-naped Tern (Sterna sumatrana) 2
Wilson’s Storm Petrel (Oceanites oceanicus) 1
Masked Booby (Sula dactylatra) 1
Red-tailed Tropicbird (Phaethon rubricauda) 1
South Polar Skua (Catharacta maccormicki) 1
Pomarine Skua (Stercorarius pomarinus) 1
Roseate Tern (Sterna dougallii) 1
Cetaceans Spinner Dolphin (Stenella longirostris) 772
Indo-Pacific Bottlenose Dolphin (Tursiops aduncus) 533
Striped Dolphin (Stenella coeruleoalba) 77
Pantropical Spotted Dolphin (Stenella attenuata) 50
Dwarf Sperm Whale (Kogia sima) 5
Pygmy Sperm Whale (Kogia breviceps) 3
Short-finned Pilot Whale (Globicephala macrorhynchus) 1

96 Mondreti et al.: Seabird and cetacean occurrence in the Bay of Bengal
Marine Ornithology 48: 91–101 (2020)
3A). However, the highest numbers of Sooty Terns were observed
in the shelf break waters of Andaman and Nicobar islands. Most
cetaceans were observed close to the shelf break (~200 m deep,
Fig.3B) in the coastal waters of the Andaman and Nicobar islands,
which are characterised by high productivity and low surface
temperatures (Figs.1B, 2B).
Overlap with longline fisheries
While most intense longline fishing zones were situated in the
centre of the bay, some effort was situated in the continental shelf
areas of the Andaman and Nicobar islands (Figs. 4A, 4B). The
highest numbers of both seabirds and cetaceans were observed in
these intense fishing zones.
Longline fishing fleets in the BOB are operated by different
countries, but most fishing effort is undertaken by Taiwan and
Japan (Table 4). Taiwan’s and Japan’s share of the total fishing
effort in BOB between 1953 and 2014 was 59.6% and 30.2 %,
respectively. In BOB waters, Taiwan has been operating its
longline fleet since 1967, whereas Japan has been operating since
1953 (http://www.iotc.org). In comparison to Taiwan and Japan,
Indian longliners constituted a mere 0.36% of the total longline
fishing effort in BOB from 1953–2014.
DISCUSSION
Low occurrences of seabirds and cetaceans
Our seabird encounter rate (8.3 birds·h-1) was much less than that
recorded in the Mozambique Channel (89.4 birds·h-1; Jaquemet
et al. 2005, 2014). One possible explanation for the low density
of seabirds in BOB is the lack of breeding colonies in the region
(Feare et al. 2007, Le Corre et al. 2012, Mondreti et al. 2013).
Only the pan-tropical Sooty Tern was fairly common in BOB.
This species has impressive dispersal ability thanks to its very
low flight costs, which allow it to exploit patchy prey in the vast
tropical ocean (Ballance et al. 1997). It often occurs in large flocks
associated with cetaceans and tropical tunas, which enhances
foraging efficiency (Au & Pitman 1986, Jaquemet et al. 2005,
Thiebot & Weimerskirch 2013). Indeed, recent tracking studies of
Sooty Terns from the western Indian Ocean confirmed that birds
breeding in remote islands off the Indian Ocean visit BOB during
the non-breeding season (Jaeger et al. 2017). The largest breeding
colonies of Sooty Terns in the Indian Ocean occur on three island
groups: the Europa and Juan de Nova islands in the Mozambique
Channel, and the Glorieuses Islands in Seychelles (Jaquemet et
al. 2008). The Wedge-tailed Shearwater is another pan-tropical
seabird species that breeds in the Seychelles and other remote
islands of the Indian Ocean and is also known to forage in BOB
(Le Corre et al. 2012).
The encounter rate of cetaceans (4.5 dolphin·h-1) was also
much lower than recorded in the western tropical Indian Ocean
(77.1dolphin·h-1; Ballance & Pitman 1998). One possible reason
for the low sightings of cetaceans in the BOB is low habitat
quality (Qasim 1977, Gomes et al. 2000, Prasanna Kumar et
al. 2002, 2010), which impacts prey abundance and availability.
This low habitat quality might result from high freshwater
discharge from the Ganges River (especially during the Southwest
Monsoon), resulting in stratification, wherein deeper layers are not
mixed with the surface layers, thus reducing primary production
Fig. 3. The abundance of A) Sooty Terns and Wedge-tailed Shearwaters; and B) Spinner Dolphins and Indo-Pacific Bottlenose Dolphins in
relation to bathymetry (BATH).

Mondreti et al.: Seabird and cetacean occurrence in the Bay of Bengal 97
Marine Ornithology 48: 91–101 (2020)
compared to the Arabian Sea (Qasim 1977, Gomes et al. 2000,
Prasanna Kumar et al. 2002, 2010; Olsen et al. 2013). In addition,
the BOB, as a marine environment, is exposed to substantial
levels of a wide range of pollutants (Holmgren 1994, Kaly 2004),
and these stressors may also contribute to both low seabird and
cetacean occurrences. Another stressor is the low concentration of
dissolved oxygen due to the presence of Oxygen Minimum Zones
(OMZs) in BOB (Bristow et al. 2017, Sarma et al. 2016). OMZs
have acute effects on diving animals by limiting prey distribution
(Nasby-Lucus et al. 2009).
Drivers of predator distributions
The inter-monthly variations in seabird abundances in the BOB
were possibly influenced by breeding phenology, but recent
studies at the Lakshadweep Islands, India, revealed breeding
asynchrony of Sooty Terns (Mondreti et al. 2018). In Seychelles,
Sooty Terns also lack a fixed breeding season (Jaquemet et al.
2007), and thus, seasonal changes in this species might reflect
seasonal conditions within the BOB. Seasonal variation also
occurred in dolphins, which are not constrained by the need
to return to land to breed, and thus, environmental variables
probably play a role in their seasonal occurrence in the BOB.
However, it is unclear whether these drivers occur within the
BOB or in adjacent waters.
Seabird and cetacean distributions probably follow changes in
surface circulation patterns in the BOB, which are marked by
seasonal fluctuations (Gomes et al. 2000, Prasanna Kumar et al.
2010). Surface circulation in the BOB is driven by river runoff
and wind, and is strongly influenced by the Equatorial Current
system (Potemra et al. 1991, Shetye et al. 1996, Varkey et al.
1996). Surface circulation can enhance productivity through
upwelling. In the BOB, upwelling is confined to the narrow
continental shelf region (Shetye et al. 1991); central parts of the
bay are characterized by the absence of upwelling and mixing
(Dwivedi 1993). Some oceanic seabirds forage away from the
coast where they are largely independent of upwelling zones.
Variations in seabird numbers could be associated with changes
in oceanographic variables, particularly SST and ChlA, but their
values were largely uniform throughout the BOB (Figs. 1A, 2A).
Our limited data and the temporal mismatch between species and
environmental data precluded detailed spatial analysis of fine-
scale linkages between predators and oceanographic variables. As
a result, we did not find correlations between environmental and
species data. Additional investigations with greater survey effort
are required to associate seabird and cetacean occurrences with
environmental parameters in this region.
The BOB is deep, with weak bathymetric gradients (Sarma et al.
2000). Therefore, bathymetric characteristics appeared to have little
impact on seabird distribution, although indirect effects through
water circulation and stratification may exist. Therefore, facilitative
association with subsurface predators, such as tuna Thunnus spp.
and dolphins (Ashmole & Ashmole 1967, Au & Pitman 1986), and
local enhancement (Poysa 1992, Buckley 1997, Grünbaum & Veit
2003, Silverman et al. 2004, Fauchald et al. 2011), could play a role
in the distribution of seabirds in the BOB.
Major fishing zones in the BOB were limited to inshore waters,
usually within 10km of the coast (Devaraj & Vivekanandan 1999).
However, we observed considerable deep-sea fishing in BOB
waters, mainly longlining for tuna. We found substantial overlap
between seabird occurrences and longline fishing areas in the BOB
(Figs. 4A, 4B), with seabirds using these fishing zones for foraging.
Conservation Implications
Globally, seabirds are one of the most threatened groups of
birds, and their conservation status has deteriorated rapidly over
the last few decades (Croxall et al. 2012, Spatz et al. 2014).
Cetaceans are equally challenged (Reynolds et al. 2009), with
three species becoming extinct during the last 60 years; several
TABLE 4
Percentage share of country-wise fishing fleet employing pelagic longlines in the Bay of Bengal
Country
Total fishing effort,
1953–2014
(in thousands of hooks)
Percentage
fishing effort GearaFishing effort period
Taiwan 1 561 646 59.6 ELL 1967–2013
Japan 790 648 30.2 FLL, LL 1953–2013
Korea 135 125 5.2 ELL, LLEX 1975–1987, 1992–1997, 1999–2005, 2007–2013
China 92 952 3.6 ELL 1999–2013
Seychelles 15 446 0.6 LLEX 2000–2001, 2003–2013
India 9327 0.36 LL 1991, 1994–1997, 2005–2012
1999, 2002–2012
Spain 8357 0.33 LL 1999, 2002–2012
Australia 929 0.04 LL 1999, 2002–2007, 2009, 2012–2013
Mauritius 692 0.04 ELL, LL 2002–2004, 2006–2008
Thailand 344 0.01 LL 2011
Portugal 303 0.01 LL 2008–2012
Maldives 150 0.01 LL, FLL 2012–2013
a LL=Longline, FLL=longline fresh, ELL=longline targeting swordfish, LLEX=exploratory longline (compiled from Longline
fishing dataset of Indian Ocean Tuna Commission (IOTC), 1953–2014).

98 Mondreti et al.: Seabird and cetacean occurrence in the Bay of Bengal
Marine Ornithology 48: 91–101 (2020)
other species are on the brink of extinction (Turvey et al. 2007).
Humans colonized the coast of BOB some 65 000 years ago,
and their presence and pressure upon regional natural resources
since has been substantial. Notably, in the first century AD,
33% of the world’s GDP was generated by 75 million Indian
people, far more than the whole of the Roman Empire (Maddison
2006). Today, the coastline of the BOB is more or less entirely
occupied by people, with many fishing communities living
permanently on the beach and participating in foraging activities
(e.g., foraging for seabird eggs in virtually any accessible area,
including supposedly protected zones and bird sanctuaries;
Mondreti et al. 2018). The basin countries of the BOB are home
to 25% of the world’s human population (Kaly 2004, Preston
2004), with some 400 million people living along the BOB coast.
Surveillance and enforcement of environmental regulations are
weak (Mondreti et al. 2018). One of the central challenges to
understanding the impact of fisheries on seabirds is the lack of
at-sea monitoring. Continued at-sea and colony observations
in this region over the next several decades would be desirable
to monitor any improvement, or decline, in the situation for
seabirds and cetaceans in the region.
ACKNOWLEDGEMENTS
RM was funded by a MAHEVA (MAn Health Environment
biodiVersity in Asia) Fellowship from Montpellier University,
an ERASMUS MUNDUS Action 2 programme. We are grateful
to the Shipping Corporation of India and the crew on board
the M/V Nancowry, M/V Akbar, and M/V Harshavardhana for
granting permission to RM to carry out onboard observations
of seabirds and cetaceans on different cruises. PGR thanks the
researchers and crew of the R/V Marion Dufresne for their
hospitality. We thank Dr. Nina Karnovsky for her suggestions in
carrying out data analysis, and Dr. Clara Péron for suggestions
on remote-sensing, data sourcing, formatting, and for all GIS and
statistical aspects related to the study. We thank Dr. Vivekanandan
and Dr. Satya Narayana Sethi of CMFRI, Chennai for helping to
source fisheries data, and Rathish Viswanathan for invaluable help
in the field. Lisa Ballance and an anonymous reviewer provided
valuable input which improved the manuscript.
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