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Stomach contents of small cetaceans stranded along the Sea of Oman and Arabian Sea coasts of the Sultanate of Oman

Authors:
  • The MareCet Research Organization
  • Independent consultant, Megaptera Marine Conservation
  • JPI Oceans and Konsortium Deutsche Meeresforschung

Abstract and Figures

This study examined the stomach contents of 11 bottlenose dolphins (Tursiops sp.), five Indo-Pacific humpback dolphins (Sousa chinensis) and two spinner dolphins (Stenella longirostris) that were found stranded along the Omani coastline. Across the three species examined, a total of 4796 fish otoliths and 214 cephalopod beaks were found, representing at least 33 species in 22 families. Prey item importance was calculated using the percentage by number and percentage by frequency of occurrence methods, and a modified index of relative importance. The fish families Apogonidae, Carangidae and Scombridae were the most numerically important prey of the bottlenose dolphins. Sciaenidae was the most numerically important fish family for the Indo-Pacific humpback dolphins. The myctophid Benthosema pterotum formed the majority of the prey items of spinner dolphins. Cephalopod remains found in the stomach samples were represented by the families Sepiidae, Loliginidae and Onychoteuthidae. The known depth distribution of prey items of bottlenose dolphins indicated that the animals fed in a wide variety of habitats. Indo-Pacific humpback dolphin prey items indicated feeding in shallow coastal areas. Spinner dolphins appear to have exploited the upper 200 m of the water column for food, where their vertically migrating mesopelagic prey are found at night. Most prey species found in the stomach contents do not appear to be of current commercial importance in Oman. However, the findings here indicated that all three species of dolphins were feeding in areas where artisanal and/or commercial fishing occurs and has conservation implications.
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StomachcontentsofsmallcetaceansstrandedalongtheSeaofOman
andArabianSeacoastsoftheSultanateofOman
LouisaS.Ponnampalam,TimJ.Q.Collins,GiannaMinton,IsabelleSchulz,HowardGray,RupertF.G.OrmondandRobert
M.Baldwin
JournaloftheMarineBiologicalAssociationoftheUnitedKingdom/Volume92/SpecialIssue08/December2012,pp16991710
DOI:10.1017/S0025315411002104,Publishedonline:06February2012
Linktothisarticle:http://journals.cambridge.org/abstract_S0025315411002104
Howtocitethisarticle:
LouisaS.Ponnampalam,TimJ.Q.Collins,GiannaMinton,IsabelleSchulz,HowardGray,RupertF.G.OrmondandRobert
M.Baldwin(2012).StomachcontentsofsmallcetaceansstrandedalongtheSeaofOmanandArabianSeacoastsofthe
SultanateofOman.JournaloftheMarineBiologicalAssociationoftheUnitedKingdom,92,pp16991710doi:10.1017/
S0025315411002104
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Stomach contents of small cetaceans
stranded along the Sea of Oman and Arabian
Sea coasts of the Sultanate of Oman
louisa s. ponnampalam
1,2,3
, tim j.q. collins
1,4
, gianna minton
1,5
, isabelle schulz
6
,
howard gray
8
, rupert f. g. ormond
2,7
and robert m. baldwin
1
1
Environment Society of Oman, PO Box 3955, P.C. 112, Ruwi, Sultanate of Oman,
2
University Marine Biological Station Millport,
Isle of Cumbrae, Scotland KA28 0EG,
3
Institute of Ocean and Earth Sciences, C308, IPS Building, University of Malaya, 50603 Kuala
Lumpur, Malaysia,
4
Ocean Giants Program, Wildlife Conservation Society, 2300 Southern Boulevard, Bronx, New York 10460, USA,
5
Institute of Biodiversity and Environmental Conservation, Universiti Malaysia Sarawak, 94300 Kota Samarahan, Sarawak, Malaysia,
6
Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, D-27570 Bremerhaven, Germany,
7
Marine
Conservation International, 5/6 Lang Rigg South Queensferry, Edinburgh, Scotland EH30 9WN,
8
School of Biomedical Sciences,
University of Durham, South Road DH1 3LE
This study examined the stomach contents of 11 bottlenose dolphins (Tursiops sp.), five Indo-Pacific humpback dolphins
(Sousa chinensis) and two spinner dolphins (Stenella longirostris) that were found stranded along the Omani coastline.
Across the three species examined, a total of 4796 fish otoliths and 214 cephalopod beaks were found, representing at least
33 species in 22 families. Prey item importance was calculated using the percentage by number and percentage by frequency
of occurrence methods, and a modified index of relative importance. The fish families Apogonidae, Carangidae and
Scombridae were the most numerically important prey of the bottlenose dolphins. Sciaenidae was the most numerically
important fish family for the Indo-Pacific humpback dolphins. The myctophid Benthosema pterotum formed the majority
of the prey items of spinner dolphins. Cephalopod remains found in the stomach samples were represented by the families
Sepiidae, Loliginidae and Onychoteuthidae. The known depth distribution of prey items of bottlenose dolphins indicated
that the animals fed in a wide variety of habitats. Indo-Pacific humpback dolphin prey items indicated feeding in shallow
coastal areas. Spinner dolphins appear to have exploited the upper 200 m of the water column for food, where their vertically
migrating mesopelagic prey are found at night. Most prey species found in the stomach contents do not appear to be of current
commercial importance in Oman. However, the findings here indicated that all three species of dolphins were feeding in areas
where artisanal and/or commercial fishing occurs and has conservation implications.
Keywords: cetaceans, stomach contents, diets, feeding, habitat, Sea of Oman, Gulf of Masirah, Oman
Submitted 20 October 2011; accepted 24 November 2011; first published online 6 February 2012
INTRODUCTION
Dietary analysis is an important aspect of research into the
feeding ecology of animals. It enables the quantification of a
species’ nutritional needs (Benoit-Bird, 2004), and of the
amount of food consumed by an individual or population
(Hyslop, 1980). It also enables the estimation of predation
pressure on the prey fauna (Robison & Craddock, 1983) and
in the case of cetaceans, assists in the exploration of the
hypothesis that distribution is closely linked to the distri-
bution and abundance of prey (Hui, 1985; Selzer & Payne,
1988; Bowen & Siniff, 1999). Methods of studying the
feeding ecology of cetaceans include stable-isotope and fatty
acids composition analysis (Nin
˜o-Torres et al., 2006), trace
metal measurements (Lahaye et al., 2005), fecal analysis
(Smith & Whitehead, 2000) and the examination of stomach
contents (Di Beneditto & Siciliano, 2007).
Overall, relatively little study has been conducted on the
diets of cetaceans in the northern Indian Ocean. Baldwin
(2003) briefly mentioned the stomach contents of bottlenose
dolphins (Tursiops truncatus) and Indo-Pacific humpback
dolphins (Sousa chinensis) found on Merawah and Bu Tina
Islands in Abu Dhabi. Robineau & Fiquet (1994) examined
the stomach contents of a single juvenile male bottlenose
dolphin found along the Arabian Gulf coast of Saudi Arabia.
There is also some published information on the diets of
marine cetaceans in India, comprising studies on the
stomach contents of eight cetacean species collectively, includ-
ing the three species examined here (Natarajan & Rajaguru,
1983, 1985; Karbhari et al., 1985; Silas et al., 1985;
Krishnapillai & Kasinathan, 1987; Krishnan et al., 2007). A
thorough search of the literature on marine cetacean dietary
studies revealed no results for Pakistan, Iran and Sri Lanka.
To date, knowledge about the diets of cetaceans in the
Sultanate of Oman is very limited, though there have been
basic observations of feeding behaviour (Salm, 1991;
Baldwin et al., 1998, 2004). This study analyses the stomach
contents of stranded, beach-cast bottlenose dolphins
Corresponding author:
L.S. Ponnampalam
Email: louisa.ponnampalam@gmail.com
1699
Journal of the Marine Biological Association of the United Kingdom, 2012, 92(8), 16991710. #Marine Biological Association of the United Kingdom, 2012
doi:10.1017/S0025315411002104
(Tursiops sp.), Indo-Pacific humpback dolphins (Sousa chi-
nensis) and spinner dolphins (Stenella longirostris) with the
aims of better understanding the distribution and patterns
of habitat use of these dolphin species in Oman, of assessing
whether the dolphins could be competing with local fisheries
for resources, and of considering whether there is evidence of
niche partitioning between dolphin species with overlapping
distributions.
MATERIALS AND METHODS
Sample collection sites
Stomach contents samples were collected from cetaceans
found during dedicated and opportunistic beach surveys
along the Oman coastline, and opportunistically during any
stranding events that were reported. Dedicated surveys were
conducted from 2000 to 2003, covering 534 km of beaches
from Muscat in the north, to Dhofar, and from 2005 to
2006, covering over 565 km of beaches from As Sahm on
the Al Batinah coast, south to the island of Masirah. These
surveys yielded a total of 18 individual carcasses representing
three species; the bottlenose dolphin, Indo-Pacific humpback
dolphin and spinner dolphin and provided the samples exam-
ined in this study (Figure 1). The carcasses were found in
varying states of decomposition and were therefore categor-
ized according to ‘stranding states’, using definitions provided
by Geraci & Lounsbury (2005) (Table 1).
collection of stomach contents samples
Stranded beach-cast cetaceans were dissected in situ upon dis-
covery, to locate any food remains in the stomach and oeso-
phagus following established methods for stomach contents
collection (e.g. Sekiguchi, 1995; Dolar et al., 2003). The
stomach and oesophagus were first removed from the body
cavity. To ensure that prey items were not lost upon
removal, the stomach and oesophagus were turned inside
out, placed over a series of simple kitchen sieves with a
maximum mesh size of 2 mm and rinsed thoroughly using a
hose. If available, otoliths were extracted directly from the
intact skulls of fish in the sample while length measurements
of whole or semi-digested prey that was still relatively intact
were taken. Stomach contents with whole or semi-digested
prey were left soaking in water for a few days, and then
rinsed to separate the soft tissues of the prey items from the
bony parts. The samples were then sieved multiple times
over a collecting tray using sieves of decreasing mesh size,
reducing the stomach contents to cephalopod beaks, fish oto-
liths, fish scales and bones. The otoliths, fish scales and bones
were then left to dry, while cephalopod beaks were stored in
70% ethanol. Eight of the stomachs were full, seven were
less than half-full, and no information on stomach fullness
was available for the remaining two stomach samples that
were collected by associate researchers (Table 1).
Identification and enumeration of prey items
Each dried stomach contents sample was sorted into fish oto-
liths, fish dentary bones, other fish bones and scales, and
cephalopod beaks. The otoliths and dentary bones were
sorted into left and right, while the beaks were sorted into
upper and lower, and these were itemized. Otoliths were
further sorted according to type (i.e. shape, size and
pattern), and matched into pairs when both left and right
sides of equal sizes were available (i.e. one right and left
otolith that matched in pattern and size equated to one
otolith pair). The number of fish eaten was estimated by
counting the number of otolith pairs plus any other unpaired
otoliths (i.e. each otolith pair was considered to be one fish,
while remaining unpaired otoliths that were different from
each other in pattern and size were counted as one fish
each). Due to their very small size and high abundance, the
otoliths in each spinner dolphin sample were tallied and
divided by two to estimate the total number of fish consumed
(Dolar et al., 2003). The higher count of either upper or lower
cephalopod beaks in each sample was used to estimate the
number of cephalopods consumed (Pinkas et al., 1971;
Dolar et al., 2003; Amir et al., 2005). Fish dentary bones
were used to estimate the number of fish consumed when a
sample did not contain any otoliths. In samples where both
otoliths and fish dentary bones were available, the dentary
bones were identified and included in the analysis only if
they did not match with any species that had already been
identified by otoliths.
Otoliths were identified to the lowest taxonomic level poss-
ible using Smale et al. (1995) while cephalopod beaks were
identified to the lowest taxonomic level possible using
Clarke (1986) and Kubodera (2005). Reference collections
specific to Oman and the Middle East are not available. The
identification of fish dentary bones was made by K.
Longenecker (Bishop Museum, Honolulu, Hawaii, USA).
Cephalopod beaks that could not be identified were sent to
M. Begon
˜a Santos (Centro Oceanogra
´fico de Vigo, Spain)
and S. Plo
¨n (South African Institute for Aquatic
Biodiversity), but remained unidentified due to the absence
of matching samples from their respective work regions.
Fish classification and information on fish distribution and
habitat followed Al-Abdessalam (1995) and Randall (1995)
while similar information for cephalopods followed
Al-Abdessalam (1995) and Jereb & Roper (2005). An
attempt was made to use fish otolith lengths, and rostral
and hood lengths of cephalopod beaks to back-calculate the
masses and standard lengths of the fish and cephalopods
that were consumed. However, due to the lack of available
regression equations and allometric coefficients for most of
the fish species and various cephalopod species in Oman,
these measurements were not utilized in this study.
Calculation of prey importance
Following the equations used in Sekiguchi (1995) and Amir
et al. (2005), three different indices were used to quantify
the occurrence and relative importance of prey items in the
stomachs: (1) percentage by number (%N); (2) percentage fre-
quency of occurrence (%FO); and (3) the modified index of
relative importance (IRI).
Percentage by number (%N) is the calculation of the
numerical abundance (%N
i
) of each prey species present in
the diet. This was calculated as:
%N=(Nij/Nj)100
where N
ij
is the number of prey species ipresent in stomach j,
and N
j
is the total number of prey present in stomach j.
Percentage frequency of occurrence (%FO) is the calcu-
lation of the frequency of occurrence of each prey species
1700 louisa s. ponnampalam et al.
(%FO
i
) within the stomach samples. This was calculated as:
%FO =(FOij/Fj)100
where FO
ij
is the frequency of stomachs i containing prey j,
and F
j
is the total number of stomachs containing prey
remains.
The index of relative importance is a rank measure of the
importance of different prey species in the diet. This was cal-
culated as a modified version of Pinkas et al. (1971)
(in Sekiguchi (1995)):
IRI =%N%FO
Percentage similarity (PS) was calculated to investigate
dietary overlap between stomachs of the same species, as
well as interspecific dietary overlap, using the equation in
Silver (1975). This was calculated as:
PSab =Smin(%Nia,%Nib )
Fig. 1. Maps showing (inset) the location of Oman, and the locations at which stomach contents of stranded bottlenose, Indo-Pacific humpback and spinner
dolphins were collected during dedicated and incidental beach surveys in (A) Muscat/Sea of Oman and (B) Gulf of Masirah regions. Numbers in parentheses
in the legends indicate the sample size for each dolphin species. Depth contours are in metres.
stomach contents of cetaceans in oman 1701
where a and b represent different stomachs and i represents
the prey species.
RESULTS
A total of 4796 fish otoliths and 214 cephalopod beaks (upper
and lower) were found in the 18 stomachs (across all three
species) of cetaceans examined. No crustacean or other
remains were found. Eleven stomachs of bottlenose dolphins
were examined, of which 18.0% (2) contained only fish
remains, 27.0% (3) contained only cephalopod remains, and
55.0% (6) contained both fish and cephalopod remains. In
terms of number, 71.0% of the diet of bottlenose dolphins com-
prised fish, while the remainder 29.0% comprised cephalopods.
A total of five stomachs of Indo-Pacific humpback dolphins
were examined, of which 20.0% (1) contained both fish and
cephalopod remains, while 80.0% (4) of stomachs contained
only fish remains. In terms of number, 87.0% of the diet of
humpback dolphins comprised fish, while the remaining
13.0% comprised cephalopods. Two stomachs of spinner dol-
phins were examined, and both fish and cephalopod remains
were found in each. Combining data from the two specimens,
99.0% of the total number of prey remains found in the
spinner dolphin stomachs comprised fish, while the remaining
1.0% comprised cephalopods. There was not any prey species
overlap between bottlenose dolphins and spinner dolphins or
between Indo-Pacific humpback dolphins and spinner dolphins.
Only one prey species overlapped between bottlenose dolphins
and Indo-Pacific humpback dolphins (i.e. Otolithes ruber)and
one genus overlapped between bottlenose dolphins and
spinnerdolphins(i.e.Apogon). However, the total percentage
number of Apogon fish found in the stomachs of bottlenose dol-
phins was much higher than in spinner dolphins (56.1% versus
0.05%).
Bottlenose dolphins
A total of 416 whole sagittal otoliths from an estimated 228
individual fish were found in the stomachs of eight of the 11
bottlenose dolphins sampled (Table 2). These represented 22
species of teleost fish distributed across 20 genera and 14
families. Numerically, the most important prey family was
the Apogonidae (cardinalfish), in which two of three species
present, Apogon cyanosoma and A. multitaeniatus, accounted
for 55.2% of all fish prey consumed (Table 2). Apogonidae,
Scombridae and Sciaenidae were the most frequently occur-
ring families (36.4% each) (Table 2). Of the otoliths found
in the bottlenose dolphins’ stomachs, 1.4% were broken or
degraded severely, preventing identification and were thus
not included in analysis.
In terms of relative importance, the family Apogonidae had
the highest modified IRI (2042.0) (Table 2). Apogon multitae-
niatus ranked first in terms of number (32.0%) and had the
highest modified IRI (582.4), although Otolithes ruber,
Scomber japonicus and Terapon jarbua occurred more fre-
quently (27.3% each). Apogon cyanosoma ranked second in
terms of number and modified IRI (23.2%, 422.2), but also
did not occur as frequently as the former three species.
Carangoides malabaricus ranked third in terms of number
(11.8%), but only occurred in one stomach (%FO ¼9.1),
whereas O. ruber,S. japonicus and T. jarbua each made up
only approximately 6.0% in terms of number, though the
three ranked first in frequency of occurrence. Otolithes ruber
also had a higher modified IRI than C. malabaricus (166.5
versus 107.4). One prey species (Epinephelus sp.) only
accounted for ,5.0% in terms of number while all other
prey species found in the stomachs accounted for ,1.0%
each in terms of number (Table 2). The average number of
fish prey taxa per stomach was 3.6 (+SD 1.6). The number
of fish prey items found per stomach ranged from 0 127
(mean ¼21.1, +SD 37.2, N ¼11). Percentage similarity
(PS) calculations across dolphin samples revealed low levels
of overlap in fish prey species found in the diet. Only one
pair of stomachs, both from individuals from the Gulf of
Masirah, had PS .50.0%, while 19 of the 28 stomach pairs
(67.9%) had a PS of 0.0. The stomach contents of the single
dolphin from Muscat (Sea of Oman) had a PS of 0.0 compared
with all the other stomach contents of the Gulf of Masirah dol-
phins (Table 3) and its mainly pelagic prey species are
Table 1. Summary of stomach samples analysed. Stranding state (SS) definition (i.e. code for condition of animal/carcass) is based on Geraci &
Lounsbury (2005). SS: 1 ¼live animal; 5 ¼skeletal remains. Stomach fullness (SF): 8/8 ¼full.
Sample number Species Sex Body length (cm) Location SS (15) SF ( 8/8)
26-05-01-03 Tursiops sp. U 196 Barr al Hikman 5 N/A
29-09-01-05 Tursiops sp. U N/A Barr al Hikman 4 8
29-09-01-06 Tursiops sp. M 260 Barr al Hikman 4 8
04-10-01-01 Tursiops sp. M 253 Masirah Island 3 1
09-10-01-01 Tursiops sp. U 246 Masirah Island 5 N/A
22-10-01-09 Tursiops sp. U 216 Masirah Island 4 8
01-11-01-01 BaH Tursiops sp. U N/A Barr al Hikman 5 8
01-11-01-03 BaH Tursiops sp. M 220 Barr al Hikman 5 8
05-11-01-01 Tursiops sp. M 210 Khaluf 5 8
15-02-06-01 Tursiops sp. M 226 Masirah Island 3 8
02-05-06-01 Tursiops sp. U 275 Muscat 3 8
16-12-01-01A Sousa chinensis F 268 Khaluf 2 3
16-12-01-05A Sousa chinensis F 257 Khaluf 3 3
29-12-01-05 Sousa chinensis M 250 Khaluf 3 1
29-12-01-06 Sousa chinensis F 245 Khaluf 4 2
24-02-06-02 Sousa chinensis U 226 Masirah Island 5 ,1
27-08-05-01 Stenella longirostris M 171 Al Batinah 4 8
27-08-05-02 Stenella longirostris M 130Al Batinah 3 3
, length measured to caudal peduncle, as tail flukes had been severed; U, unidentified; M, male; F, female.
1702 louisa s. ponnampalam et al.
indicated in Table 2. A total of 174 cephalopod beaks from 95
cephalopods (squids and cuttlefish) were found in nine of the
11 bottlenose dolphin stomachs examined, all of which were
from the Gulf of Masirah. These beaks represented at least
four species in two genera and two families, namely the
Sepiidae and Loliginidae (Table 2). There were three other
unidentified species that accounted for 60.0% (48) of the
lower beaks that were analysed.
indo-pacific humpback dolphins
A total of 21 otoliths from 13 teleost fish were found in the
stomachs of the five Indo-Pacific humpback dolphins
sampled (Table 4). These represented five species in four
genera and three families that occur predominantly in
turbid, shallow water habitats. Numerically, the most impor-
tant prey family was Sciaenidae (53.9%), followed by Ariidae
(30.8%) and Synodontidae (15.4%). The most frequently
occurring prey family was Sciaenidae (100.0%). Arius sp.
was the most important prey species in terms of number
(30.8%) though it only had a frequency of occurrence of
20.0%. The sciaenids Johnius sp. and Otolithes ruber each
ranked second in terms of number (23.1%). However, O.
ruber occurred more frequently than Johnius sp. (60.0%
versus 40.0%) (Table 4). Otolithes ruber had the highest modi-
fied IRI (1384.8), followed by Johnius sp. (923.2) and Arius sp.
(615.4) (Table 4). The average number of fish prey taxa per
stomach was 1.4 (+SD 0.9). The number of fish prey items
found per stomach ranged from 1 7 (mean ¼2.6, +SD
2.6, N ¼5). PS calculations across samples revealed minimal
overlap in fish prey species found in the stomachs. Only one
pair of stomachs had PS .50.0%, while 6 of the 10 stomach
pairs had a PS of 0.0 (Table 3). Only one of the five
Indo-Pacific humpback dolphin stomachs that were examined
contained cephalopod beaks. These comprised one upper and
two lower beaks and represented two squids in total, but were
not successfully identified due to a lack of comparable beaks
from the region.
Spinner dolphins
A total of 4355 sagittal otoliths from 2179 fishes were found in
the stomachs of the two spinner dolphins sampled (Table 5).
These represented five species in at least four genera and four
families of teleost fish that occurred almost entirely in meso-
pelagic water depths. The Myctophidae was the most impor-
tant prey family in terms of number and frequency of
occurrence (99.4% and 100.0% respectively). Due to the
small sample size of two dolphins, the modified IRI for
spinner dolphins was not calculated. A total of 23 upper
beaks and 14 lower beaks were retrieved from both spinner
dolphins that were sampled. These all represented
Onychoteuthis banksi from the family Onychoteuthidae
(Table 5).
DISCUSSION
Bottlenose dolphins
The results show that the bottlenose dolphins examined in this
present study consumed a relatively large variety of prey
(Table 2). However, it was evident that the diet of dolphins
from the Gulf of Masirah and of the single animal from
Muscat differed in terms of preferred habitat of prey species.
The depth distributions of the majority of prey species of bot-
tlenose dolphins from the Gulf of Masirah suggest that the
dolphins there fed primarily in shallow, coastal inshore
waters and over the continental shelf (Al-Abdessalaam,
1995; Randall, 1995). Conversely, the bottlenose dolphin
that stranded in the Muscat capital area had primarily eaten
Table 2. Percentage number and percentage frequency of occurrence of
fish and cephalopod families and species found in stomachs of bottlenose
dolphins. IRI, index of relative importance; n
s
, total number of stomachs
containing prey items; , prey items of sole bottlenose dolphin found in
Muscat; ∗∗, lower beak count.
Prey species N %
number
F%
frequency
Modified
IRI
Fish
Family Apogonidae 128 56.1 4 36.4 2042.0
Apogon cookii 2 0.9 2 18.2 16.4
Apogon cyanosoma 53 23.2 2 18.2 422.2
Apogon multitaeniatus 73 32.0 2 18.2 582.4
Family Carangidae 29 12.7 2 18.2 231.1
Carangoides
malabaricus
27 11.8 1 9.1 107.4
Selar crumenophthalmus 2 0.9 1 9.1 8.2
Family Engraulidae 2 0.9 2 18.2 16.4
Thryssa setirostris 2 0.9 2 18.2 16.4
Family Gerreidae 1 0.4 1 9.1 3.6
Gerres oyena 1 0.4 1 9.1 3.6
Family Kyphosidae 1 0.4 1 9.1 3.6
Kyphosid 1 1 0.4 1 9.1 3.6
Family Mullidae 2 0.9 2 18.2 16.4
Parupeneus rubescens 1 0.4 1 9.1 3.6
Upeneus tragula1 0.4 1 9.1 3.6
Family Nemipteridae 1 0.4 1 9.1 3.6
Scolopsis vosmeri 1 0.4 1 9.1 3.6
Family Sciaenidae 16 7.0 4 36.4 254.8
Otolithes ruber 14 6.1 3 27.3 166.5
Umbrina canariensis 2 0.9 2 18.2 16.4
Family Scombridae 19 8.3 4 36.4 302.1
Gymnosarda unicolor1 0.4 1 9.1 3.6
Katsuwonus pelamis 3 1.3 1 9.1 11.8
Scomber japonicus 14 6.1 3 27.3 166.5
Thunnus albacares1 0.4 1 9.1 3.6
Family Serranidae 9 3.9 1 9.1 35.5
Epinephelus sp. 9 3.9 1 9.1 35.5
Family Sparidae 2 0.9 1 9.1 8.2
Crenidens crenidens 2 0.9 1 9.1 8.2
Family Sphyraenidae 1 0.4 1 9.1 3.6
Sphyraena jello 1 0.4 1 9.1 3.64
Family Teraponidae 15 6.6 3 27.3 180.2
Terapon jarbua 15 6.6 3 27.3 180.2
Family Trichiuridae 2 0.9 2 18.2 16.4
Trichiurus lepturus 2 0.9 2 18.2 16.4
Fish totals 228 100.0
Cephalopods
Family Sepiidae 30 37.5 8 72.7 2727.3
Sepia pharaonis 23 28.8 7 63.6 1831.7
Sepia latimanus 5 6.3 2 18.2 114.7
Sepia sp. 2 2.5 2 18.2 45.5
Family Loliginidae 2 2.5 1 11.1 22.7
Loligo sp. 2 2.5 1 9.1 22.7
Unidentified sp. 1 2 2.5 1 9.1 27.8
Unidentified sp. 2 38 47.5 4 36.4 1727.3
Unidentified sp. 3 8 10.0 3 27.3 272.7
Cephalopod totals 80∗∗ 100.0
Total 308 100.0 n
s
¼11
stomach contents of cetaceans in oman 1703
scombrids, which is a family of pelagic and epipelagic fish
(Nelson, 1994). Both cephalopod families found in the
stomachs of the Gulf of Masirah bottlenose dolphins are typi-
cally found nearshore and over continental shelf waters.
However the Sepiidae are primarily demersal while
Loliginidae are primarily neritic (Jereb & Roper, 2005).
The findings in the stomach contents of bottlenose dol-
phins suggest that this species tends to exploit the upper
200 m for food in a wide variety of habitats. The relatively
high number of apogonids and the cuttlefish Sepia pharaonis,
and high variety of other reef-associated fish species such as
Terapon jarbua and Scomber japonicus in the diet of Gulf of
Masirah dolphins suggest that those dolphins may have
favoured areas around coral reefs (Al-Abdessalaam, 1995).
Additionally, their diet also comprised species such as the
croaker Otolithes ruber, that occur on sandy and muddy sub-
strates (Al-Abdessalaam, 1995), which is consistent with the
environment in the northern portion of the Gulf of Masirah
where most of the bottlenose dolphin specimens were
found. This shallow sheltered area and the adjacent central
part of the Gulf of Masirah, is highly productive and food is
unlikely to be scarce (Banzon et al., 2004). Most fish species
consumed by the dolphins that stranded in the Gulf of
Masirah were diurnal (Al Abdessalaam, 1995; Randall,
1995). However, the occurrence of several nocturnal and
benthopelagic species in those stomachs suggest that the dol-
phins hunted during both day and night, perhaps in concert
with circadian migrations of fish to and from the surface.
The purely piscivorous diet of mostly larger, pelagic scom-
brids, all of which are diurnal species (Al-Abdessalaam,
1995), suggests that the dolphin that stranded in Muscat
had fed during the day probably in deeper waters.
Demersal, soniferous fish families, such as Sparidae and
Sciaenidae form the dietary basis for bottlenose dolphins in
the Pacific region (Walker, 1981; Barros & Wells, 1998).
Similarly, sciaenids appeared to be important in the diet of
bottlenose dolphins found stranded in the Gulf of Masirah,
while sparids were also found in their food items. Five of
the nine fish families consumed by offshore bottlenose dol-
phins in Hong Kong were represented in the stomach contents
of the Gulf of Masirah bottlenose dolphins (i.e. Apogonidae,
Carangidae, Trichiuridae, Sparidae and Serranidae).
Additionally, there were three genera (i.e. Apogon,Trichiurus
Table 3. Percentage similarity (PS) values for fish prey species between bottlenose dolphin stomachs that were analysed and between Indo-Pacific
humpback dolphin stomachs that were analysed.
PS values for bottlenose dolphin samples
Sample number 26-05-01-03 29-09-01-05 29-09-01-06 04-10-01-01 01-11-01-01 05-11-01-01 15-02-06-01
26-05-01-03
29-09-01-05 0.0
29-09-01-06 0.0 15.4
04-10-01-01 0.0 15.5 0.0
01-11-01-01 36.4 24.5 0.0 0.0
05-11-01-01 20.0 46.2 0.0 4.0 61.1
15-02-06-01 0.0 0.0 2.5 0.0 0.0 0.0
02-05-06-01 0.0 0.0 0.0 0.0 0.0 0.0 0.0
PS values for indo-pacific humpback dolphin samples
Sample number 16-12-01-01A 16-12-01-05A 29-12-01-05 29-12-01-06
16-12-01-01A
16-12-01-05A 14.3
29-12-01-05 0.0 0.0
29-12-01-06 33.3 14.3 66.7
24-02-06-02 0.0 0.0 0.0 0.0
Table 4. Percentage number and percentage frequency of occurrence of
fish families and species found in stomachs of Indo-Pacific humpback dol-
phins. IRI, index of relative importance; n
s
, total number of stomachs con-
taining prey items.
Prey species N %
number
F%
frequency
Modified
IRI
Family Ariidae 4 30.8 1 20.0 615.4
Arius sp. 4 30.8 1 20.0 615.4
Family Sciaenidae 7 53.8 5 100.0 5385.0
Johnius sp. 3 23.0 2 40.0 923.2
Otolithes ruber 3 23.0 3 60.0 1384.8
Unidentified
sciaenid
1 7.7 1 20.0 153.8
Family
Synodontidae
2 15.4 1 20.0 307.6
Saurida
undosquamis
2 15.4 1 20.0 307.6
Total 13 100.0 n
s
¼5
Table 5. Percentage number and percentage frequency of occurrence of
fish and cephalopod families and species found in stomachs of spinner
dolphins. n
s
, total number of stomachs containing prey items. Modified
index of relative importance not calculated due to small sample size.
Prey species N % number F % frequency
Fish
Family Apogonidae 1 0.05 1 50.0
Apogon semiornatus 1 0.05 1 50.0
Family Gempylidae 8 0.4 1 50.0
Neoepinnula orientalis 8 0.4 1 50.0
Family Myctophidae 2165 99.4 2 100.0
Benthosema pterotum 2165 99.4 2 100.0
Family Nomeidae 1 0.05 1 50.0
Cubiceps pauciradiatus 1 0.05 1 50.0
Unidentified sp. 1 4 0.2 1 50.0
Fish totals 2179 100.0
Cephalopods
Family Onychoteuthidae 23 100.0 2 100.0
Onychoteuthis banksi 23 100.0 2 100.0
Total 2202 100.0 n
s
¼2
1704 louisa s. ponnampalam et al.
and Epinephelus) and one species of fish (i.e. Trichiurus lepturus)
that offshore bottlenose dolphins in both Hong Kong and the
Gulf of Masirah had consumed in common (see Barros et al.,
2000). Of the 34 fish families found in the stomach contents
of Indo-Pacific bottlenose dolphins in Zanzibar, eight were
also represented in the stomach contents of the Gulf of
Masirah dolphins, while there were four genera (i.e. Apogon,
Carangidae,Gerres and Saurida) in common between the two
populations. Additionally, only one (Apogonidae) of those
eight families was important for dolphins from both areas. In
India, the stomach of a bottlenose dolphin also contained predo-
minantly sciaenids (Natarajan & Rajaguru, 1985).
The lack of similarity in the stomach contents of all the
Gulf of Masirah specimens and the Muscat specimen may
be a reflection of the presence of two species of bottlenose dol-
phins (T. aduncus and T. truncatus) in Oman occupying two
different habitats. Although tissue samples for genetic analysis
and cranial material for taxonomic measurements were routi-
nely collected when available and feasible, other factors,
including sample degradation and the absence of cranial
remains, precluded identification of carcasses to species
level. Bottlenose dolphin specimens used here were typically
in either advanced stages of decomposition or located on
very remote beaches with little opportunity for collection of
skeletal remnants. Nonetheless photographs and observations
of live sightings of bottlenose dolphins during small-boat
surveys in the Gulf of Masirah between 2000 and 2006
suggest that it is most likely that T. aduncus inhabits the
shallow waters (mostly in depths of less than 20 m) of this
large bay. Individuals had estimated body lengths of less
than 3 m, had slender rostra, prominent ventral spotting
and physically resembled T. aduncus as described in Wang
et al. (2000) (Ponnampalam, 2009; Minton et al., 2010).
Those sighting depths were consistent with the reported pre-
ferred depth of T. aduncus (Findlay et al., 1992; Wells & Scott,
2002). The dolphin found stranded in Muscat was also less
than 3 m long. However, it was the largest of all bottlenose
dolphins examined in this study and at 2.75 m length
(Table 1), exceeded the maximum recorded length of T.
aduncus as reported by Wang et al. (2000). We consider it
likely that this specimen, which was more robust, stocky
and blunt-nosed, was a T. truncatus. Its diet, consisting
mainly of a variety of pelagic tuna, reflects that it was
almost certainly feeding off the Muscat coast. There the con-
tinental shelf is narrow, depths greater than 200 m can be
found within 10 km from shore (Figure 1) and fishermen
are often seen hand-lining for yellowfin and longtail tuna
within 5 km of the coast. Examination of its mouth showed
that all teeth were erupted, though not worn, a sign that the
dolphin was a sub-adult, and could possibly attain more
than 3 m in length when fully grown. Ponnampalam (2009)
and Minton et al. (2010) have previously reported that bottle-
nose dolphins sighted in the Muscat region are stocky, blunt-
nosed and heavily scarred, some of which had estimated body
lengths exceeding 3 m and resembled T. truncatus.
Skin samples taken from these dolphins await genetic ana-
lyses for confirmation of species-level identification. Until
then, no definitive conclusions can be made on their taxo-
nomic identity.
indo-pacific humpback dolphins
The prey spectrum of Indo-Pacific humpback dolphins exam-
ined here appeared to be small in comparison to bottlenose
dolphins and to consist mainly of coastal, inshore species that
inhabit murky waters with muddy substratum. The croaker
Otolithes ruber is a benthopelagic sciaenid species found in rela-
tively shallow and turbid waters (Al-Abdessalaam, 1995). The
other genus of sciaenid represented in the stomach contents
was Johnius. Randall (1995) reports four species of Johnius to
occur in Omani waters, all of which are demersal, coastal
species (Sasaki, 2001). The otoliths of sea catfish (Ariidae)
that were found in one of the stomachs were too degraded to
enable species-level identification. Randall (1995) reported
that sea catfish in Oman are represented solely by the genus
Arius in four different species. All four species are demersal,
and mostly occur in shallow inshore, rocky and muddy
waters. Dead ariids floating at the surface, likely to be fishing
discards, were often encountered during small-boat surveys in
the Gulf of Masirah (Ponnampalam, 2009).
Baldwin et al. (2004) showed a clustering of live sightings of
Indo-Pacific humpback dolphins in the Ghubhat Hashish, a
semi-enclosed and shallow bay located in the northern
reaches of the Gulf of Masirah, particularly in the areas
around Mahawt Island and Khaluf, where groups of up to
50 animals were observed. Four of the five Indo-Pacific hump-
back dolphins examined in this study were found on the beach
of Khaluf (see Figure 1). Acoustic watches conducted for ceta-
ceans during small-boat surveys in the Gulf of Masirah, par-
ticularly at stations situated between Khaluf and Barr al
Hikman, also yielded a variety of grunts and croaks typical
of soniferous fish species such as O. ruber and Johnius sp.
(Al-Abdessalaam, 1995). The sciaenids found in the Indo-
Pacific humpback dolphin stomachs had the two highest
values of modified IRI of all the prey species found. It is
thus likely that the Gulf of Masirah, which is characterized
by very shallow (10 m), murky waters, particularly in the
northern portion where Indo-Pacific humpback dolphins are
frequently sighted and occur in large groups, is a rich source
of food and provides important feeding grounds for this
species—direct observations of feeding activity in the area
further support this notion (Baldwin et al., 2004).
In this study, although the sample size was small and prey
species diversity was low, the Oman Indo-Pacific humpback
dolphins showed some similarity in their mainly piscivorous
diet to other conspecifics. One study of stomach contents
from eight dolphins in Hong Kong revealed that 81% of the
almost entirely piscivorous diet was based on the families
Sciaenidae, Engraulidae and Clupeidae (Jefferson, 2000).
Another Hong Kong study reported that the dolphins con-
sumed trichiurids and ariids (Barros et al., 2004). In southern
China, Indo-Pacific humpback dolphins were also found to be
preying mainly upon estuarine fish species (Wang, 1995).
Sciaenid fish were important to the Indo-Pacific humpback
dolphins in both Hong Kong and Oman. There were
two prey genera in common between the dolphins in this
study and those from Hong Kong, one of which (Johnius)
was numerically important for both populations. The
Indo-Pacific humpback dolphins in Xiamen Harbor, southern
China, had also consumed fish of the genus Johnius (Wang,
1995). Conversely, in India, an Indo-Pacific humpback
dolphin incidentally caught in gill nets was found to have con-
sumed only teleost fish, although none of these were sciaenids
or ariids (Krishnan et al., 2007). Finding marine catfish in the
stomachs of humpback dolphins in this study is interesting to
note. Barros et al. (2004) pointed out that the catfish are a
‘bottom-dwelling species possessing dangerous spines
stomach contents of cetaceans in oman 1705
covered by a venomous mucus, capable of inflicting lacerating
wounds’ (Smith & Heemstra, 1986). Given the apparent abun-
dance of ariids in Oman, the dolphins may have adapted a way
to digest or avoid their venomous, spiny prey without harming
their stomachs and internal organs, taking advantage of a
possibly abundant food resource.
Spinner dolphins
Myctophids, the primary prey items of both spinner dolphin
specimens examined here, are common and occur throughout
most of the world’s oceans (Dalpadado & Gjøsaeter, 1988;
McClatchie & Dunford, 2003). The Myctophidae is a family
of fish known to undertake daily vertical migrations from
bathypelagic (1000 – 4000 m) and mesopelagic (200
1000 m) depths during the day, to depths of 200 m up to
the surface during the night (Clarke, 1973; Paxton & Hulley,
1999). A survey in 1990 in the Sea of Oman by the RV
‘Rastrelliger’ found that myctophids were most abundant off
the Al Batinah coast (Johannesson, 1995), where the two
spinner dolphins examined in this study had been found
stranded.
Gjøsaeter (1984) and Johannesson (1995) reported the
myctophid Benthosema pterotum to be the most abundant
and dominant mesopelagic species in the Sea of Oman.
Benthosema pterotum was also the only mesopelagic species
found in the vicinity of a fixed station in the Sea of Oman
(Sætersdal et al., 1999). Another species found in the stomachs
of the spinner dolphins was the bathypelagic nomeid Cubiceps
pauciradiatus. It inhabits water depths between 58 and
1000 m but is found near the surface at night (Butler, 1979).
This species only contributed 0.4% in number among the
stomach contents, and may have been dispersed in the
dense schools of B. pterotum near the surface. The gempylid
Neoepinnula orientalis is found between 200 and 570 m
depth (Nakamura & Parin, 1993). It is not known if N. orien-
talis undertakes vertical migration towards the shallower
depths at night although some species of Gempylidae do
(Nakamura & Parin, 1993). Apogon semiornatus is an
inshore species that occurs on rocky or rubble reef, and by
day has a preference for caves. It is found at 5 30 m depth
(Heemstra, 1995). Though not typically reported in the diet
of spinner dolphins, apogonids have been found in their
stomach contents by studies elsewhere (Perrin & Gilpatrick,
1994).
Fitch & Brownell (1968) suggested that spinner dolphins
feed at depths greater than 250 m. Dolar et al. (2003) hypoth-
esized that this species may feed as deep as 400 m. However,
spinner dolphins in the Philippines were observed feeding
near the surface at night (Dolar et al., 2003). The high
density of myctophids near the surface at night in the Sea of
Oman (Gjøsaeter, 1984; Valinassab et al., 2007) coupled
with their high abundance in the dolphins’ stomachs suggest
that the spinner dolphins examined here are likely to have
fed mainly in the epipelagic layer at night. Although spinner
dolphins were observed frequently in association with
schools of feeding yellowfin tuna (Thunnus albacares) in the
day during detailed behavioural studies off Muscat, feeding
was rarely observed (only 3% of total observation time)
(Ponnampalam, 2009).
The diet of spinner dolphins has been studied extensively
in the Eastern Tropical Pacific (ETP) and in the Hawaiian
archipelago. Fitch & Brownell (1968) examined the stomachs
of five spinner dolphins caught in the ETP and found a mainly
piscivorous diet comprising 6 15 species in eight families,
whereby the myctophids B. panamense and Lampanyctus par-
vicauda constituted 50% of the dolphins’ diet. Squid predomi-
nated in the stomach contents of a single dolphin caught in
Hawai’i, although there were also fish from the families
Gempylidae and Myctophidae (Shomura & Hida, 1965).
Dolar et al. (2003) examined the stomachs of 45 spinner dol-
phins that had been caught in the tuna driftnet fishery in the
Sulu Sea and found that myctophids formed the basis of the
dolphins’ diet (32 species, 94% in occurrence), with three
species dominating. Furthermore, 10 families of mesopelagic
cephalopods, dominated by the family Enoploteuthidae, and
crustaceans from three families (Oplophoridae, Penaeidae
and Sergestidae) were also found in the stomachs (Dolar
et al., 2003).
The stomach contents of the Oman spinner dolphins had a
much lower diversity (five fish and one cephalopod species)
than those in the ETP and the Philippines. However, as in
the Sulu Sea, Hawai’i and ETP, mesopelagic fish, particularly
myctophids dominated the stomach contents. The genus
Benthosema was important for both ETP dolphins and the
ones in this study, while the squid O. banksi was a prey
item of both Sulu Sea spinner dolphins and the spinner dol-
phins from this study. In contrast, in two separate studies in
India, examination of the stomach contents of two spinner
dolphins caught incidentally in gill nets revealed that one
animal had mainly consumed the epipelagic scad mackerel
Megalaspis cordyla (Karbhari et al., 1985), while the other
had mostly taken shallow water pandalid prawns and the
squid Loligo duvaucelli (Natarajan & Rajaguru, 1985), while
in yet another study in India, a spinner dolphin caught inci-
dentally was found to have consumed mainly the shrimp
Solenocera crassicornis, also found in shallow water
(Krishnan et al., 2007). Two possible reasons for the low diver-
sity in the stomach contents examined here are the extremely
small sample size, lacking representation from what could be a
larger prey spectrum and/or over-representation of a single
species of mesopelagic fish (B. pterotum) in the Sea of
Oman (Gjøsaeter 1984; Valinassab et al., 2007).
Commercial importance of prey species
Of the 22 prey species found in the stomachs of bottlenose
dolphins, only a few are of major commercial importance in
Oman, being targeted either by traditional or industrial
fishers, or both (Al-Abdessalaam, 1995). Apogonidae, the
most important prey family numerically for bottlenose dol-
phins has no commercial value in Oman (Al-Abdessalaam,
1995). Otolithes ruber, important to both bottlenose and
Indo-Pacific humpback dolphins, contributes significantly to
the overall croaker fishery in Oman (Al-Abdessalaam, 1995;
Ministry of National Economy, 2007). In 1995, trichiurids
ranked among the top five families in the Omani demersal
fish catch, along with sciaenids, siganids, serranids and lethri-
nids (Siddeek et al., 1999). Arius spp., consumed by
Indo-Pacific humpback dolphins, is caught commercially in
bottom trawlers, although not highly valued as a food fish in
Oman, and thus are mostly discarded due to the lack of
local demand (Al-Abdessalaam, 1995). Mesopelagic species
in the Sea of Oman, particularly the dominant Benthosema
pterotum, remain a largely unexploited fisheries resource,
thus providing the potential for developing a fishery for fish-
meal (Gjøsaeter, 1984; Johannesson, 1995). Al-Marzooqi
1706 louisa s. ponnampalam et al.
(2001) reported that the abundance of myctophids in the Gulf
of Oman and Arabian Sea has sparked interest in developing a
fishmeal fishery in the area. However, Valinassab et al. (2007)
reported that maximum yields of B. pterotum in the Sea of
Oman, which undergo seasonal fluctuations, would not be
adequate for sustaining the fishery. More research is needed
on the consumption biomass of spinner dolphins in Oman
to be able to discern whether a myctophid fishery would
have impacts on their feeding ecology.
limitationsofthisstudy
As discussed above, small sample sizes may lead to biases
associated with lack of the full range of prey species or over-
representation of species comprising the last few meals of
the few animals examined. Further limitations pertaining to
this dataset include the lack of availability of relevant allo-
metric coefficients to conduct length and weight regressions.
This prevented percentage by weight (%W) and passage
time analyses. Additionally, the identification of cephalopod
beaks may not be entirely accurate for several reasons.
Firstly, all published references for Oman, including those of
the Ministry of Agriculture and Fisheries (currently known
as Ministry of Agriculture and Fisheries Wealth) indicate
that Sepia pharaonis and Octopus aegina are the only cephalo-
pod species to occur in Oman. Published information on
cephalopod diversity for the wider region is also scant and
there was no readily available reference collection of beaks
or a suitable beak identification key for Oman or surrounding
waters.
Possible sources of bias in the data
There are biases associated with studying diet based on
stomach contents of beach-cast cetaceans. For example, the
dolphins in this study were found in varying states of
decomposition and with varying states of stomach fullness.
Their causes of mortality could not always be determined,
and in some cases might have been due to illness or
by-catch. If the samples had come from sick individuals,
samples used here may not be completely reflective of a
normal diet in a healthy dolphin. In the case of by-catch, dol-
phins are sometimes known to regurgitate some or all of their
stomach contents upon capture or entanglement in the fishing
gear, perhaps due to stress (e.g. Sekiguchi et al., 1992;
Krishnan et al., 2007). If some of the stomachs studied here
were from by-caught dolphins, then the results may also not
be truly representative of their actual diet, as they may have
been feeding opportunistically near fishing vessels or off
fishing gear prior to incidental capture. There are also biases
associated with the digestibility of prey items. For instance,
larger and denser fish otoliths tend to be retained intact for
longer than smaller otoliths (Murie & Lavigne, 1985), due to
their differing rates of erosion (Jobling & Breiby, 1986). It is
thus possible that the results here over-represent fish species
with larger otoliths and under-represent fish species with
smaller and more fragile otoliths. It also does not necessarily
indicate that all prey species found in each stomach were
taken in a single meal. Another bias resulting from stomach
contents analysis is the longer retention of cephalopod beaks
within the stomach compared with fish otoliths (Heezik &
Seddon, 1989). However, this bias is likely not to have affected
the results here as most samples examined in this study con-
tained more otoliths than beaks. Some of the prey remains
found could have been secondary prey items (i.e. prey
consumed by the dolphin’s prey), thus potentially biasing
results. Additionally, due to small sample size and lack of
life history data for examined specimens, it was not possible
to examine the diet and prey selection of each dolphin
based on sex, maturity or seasonality. Several studies have
found dietary differences at varying stages of maturity and
life history due to demographic differences in habitat selection
and/or ontogenetic changes in prey choice, between sexes and
between seasons (Robertson & Chivers, 1997; Amir et al.,
2005; Santos et al., 2007).
CONCLUSIONS
While the stomach contents data indicate that bottlenose dol-
phins and Indo-Pacific humpback dolphins in the Gulf of
Masirah are feeding in overlapping habitats, the results also
indicated that the two species have different prey spectra
with only minor overlaps in species consumed. Spinner dol-
phins were more specific in their prey choice and were able
to exploit what appears to be an abundant food resource
from the vertically migrating mesopelagic deep-scattering
layer. Only a small percentage of the three dolphin species’
prey items are of current commercial importance, indicating
that there is little direct competition with local fisheries.
However, a number of animals examined in this study
showed signs of mortality due to fisheries interaction, indicat-
ing that these dolphins still face a significant risk of incidental
capture from feeding in the same highly productive areas
where fishing occurs. Despite the limitations of this study,
the data presented here remain important in furthering our
understanding of the ecology of small cetaceans in this
region and for designing future studies on this subject.
ACKNOWLEDGEMENTS
This work was supported by an Association of
Commonwealth Universities (ACU) doctoral scholarship
granted to L.S.P. We are very grateful to the Government of
Oman, in particular, the former Ministry of Regional
Municipalities, Environment and Water Resources (now
known as the Ministry of Environment and Climate
Affairs), the former Ministry of Agriculture and Fisheries
(now known as the Ministry of Agriculture and Fisheries
Wealth), and the Oman Natural History Museum of the
Ministry of Heritage and Culture for granting us permission
to conduct our research. Thanks are also owed to the Board
Members of the Environment Society of Oman and personnel
at Five Oceans Environmental Services in Oman for their
support. Our gratitude is also extended to the many volunteers
who gave assistance during beach surveys, and in the proces-
sing of the stomach contents samples—Vic Cockcroft, Anna
Hywel-Davies, Fergus Kennedy, Andy Willson, Simon
Wilson, Kris Vallancey, Richard Willing, Natalie Little,
Robyn Manley, Eli Verges-Vidal, Louise Waters, Ross
Rosenquist, Cathy Little, Rebecca Lovegrove, Victoria Hill
and Rebekah Oxley. We would like to express our appreci-
ation to Dr Begon
˜a Santos, Dr Stephanie Plo
¨n and Dr
Kenneth Longenecker for their kind assistance in identifying
cephalopod beaks and fish dentary bones. Surveys would
not have been possible without the funding provided by
Shell Oman Marketing, Ford Environmental Grants,
stomach contents of cetaceans in oman 1707
Petroleum Development Oman, the UK Foreign and
Commonwealth Office, Veritas Geophysical, the Peter Scott
Trust for Education and Research in Conservation, Salalah
Port Services, DHL Worldwide Express, Truck Oman,
Oman Air, the Wildlife Conservation Society, Muscat
Pharmacy, KPMG, Han-Padron and Associates, Five Oceans
Environmental Services and the Marina Bandar Al Rowdah.
Last but not least, we would like to thank Anouk
Ilangakoon and the four anonymous referees for reviewing
and improving an earlier version of this manuscript.
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Correspondence should be addressed to:
L.S. Ponnampalam
Environment Society of Oman
PO Box 3955, P.C. 112, Ruwi, Sultanate of Oman
email: louisa.ponnampalam@gmail.com
1710 louisa s. ponnampalam et al.
... The prey spectrum presented in our study, which is considerably wider than those previously reported for humpback dolphins (Barros & Cockcroft 1991;Barros et al. 2004;Krishnan et al. 2008;Plön et al. 2011;Ponnampalam et al. 2012;Parra & Jedensjö 2014), suggests a generalist foraging strategy. Based on previous studies, it was thought that dolphins of the genus Sousa feed primarily on demersal prey (Jefferson & Curry 2015). ...
... Perhaps the most unexpected finding was the presence of gobies in the current samples of dolphin diet, which represents 13.9% of the prey items recovered from dolphin stomachs in the present study, but have not previously been reported for the genus Sousa (e.g. Wang 1965; Barros & Cockcroft 1991;Barros et al. 2004;Krishnan et al. 2008;Plön et al. 2011;Ponnampalam et al. 2012;Parra & Jedensjö 2014) nor for other coastal delphinids the size of humpback dolphins (e.g. Gannon & Waples 2010;Di Beneditto et al. 2011;Milmann et al. 2016). ...
... Although the feeding spectrum can be covered mostly, if not entirely, by increasing the number of examined stomachs, the contribution of each prey species may still be biased due to varying digestion rates. For example, predation on some species, such as bummalo can be commonly observed in the field, but this prey is rarely found in dolphin stomachs (Barros et al. 2004;Krishnan et al. 2008;Ponnampalam et al. 2012;Parra & Jedensjö 2014). Due to the small size and fragile structure of their otoliths, the dietary importance of these species may be underestimated. ...
Article
The Pearl River Delta (PRD) region on the southeast coast of China has long been known as a highly productive fishing ground. Since the late 1980s, fishing pressure in the PRD has been intense, which warrants concerns of potential fishery-related impacts on the food resources and foraging ecology of apex marine predators in this region, such as the Indo-Pacific humpback dolphin (Sousa chinensis). In this study, we examined 54 stomachs with food remains, collected from beached carcasses of humpback dolphins recovered during fifteen years between 2003 and 2017. The 6043 identified prey items represent 62 teleost taxa, primarily small estuarine fish, but also larger reef fish. The dolphins appear to be opportunistic foragers, hunting across the water-column, with preference for shoaling and meaty fishes (e.g. Chrysochir aureus IRI% = 38.6%, Johnius belangerii IRI% = 23.1%, Mugil cephalus IRI% = 14.0%). Our findings suggest a dietary shift in recent years, from primarily demersal (as previously reported) to greater intake of neritic and pelagic fish. Dolphin foraging group size has decreased in recent years, which corresponds with declining size and numbers of prey items retrieved from dolphin stomachs. We suggest that these are indicators of declining food resources. Faced with a shortage of preferred prey, humpback dolphins may have broadened their dietary spectrum to maintain their daily energy intake, while their foraging group size decreased in response to the altered tradeoff between the costs and benefits of group foraging. This article is protected by copyright. All rights reserved
... LWPTF & GLPTF; see Results) to be important characters for discriminating between T. truncatus and T. aduncus. Investigations into stomach contents of dolphins in the waters of Oman reveal bottlenose dolphins to be feeding in either inshore/coastal habitats or offshore (Ponnampalam et al., 2012). Although not identified to species in the paper, those feeding nearshore were from the Gulf of Masirah, a region known to be dominated by T. aduncus, while one dolphin apparently feeding offshore was from a region (off Muscat) dominated by T. truncatus. ...
... Although not identified to species in the paper, those feeding nearshore were from the Gulf of Masirah, a region known to be dominated by T. aduncus, while one dolphin apparently feeding offshore was from a region (off Muscat) dominated by T. truncatus. Furthermore, those dolphins feeding inshore showed a diet that includes species such as the croaker, Otolithes ruber, that occur on sandy and muddy substrates (Ponnampalam et al., 2012). ...
Article
Local adaptation and adaptive radiations are typically associated with phenotypic variation suited to alternative environments. In the marine environment, the nature of relevant ecological or environmental transitions is poorly understood, especially for highly mobile species. Here we compare three genetic lineages in the genus Tursiops (bottlenose dolphins), using linear measurements and geometric morphometric techniques, in the context of environmental variation in the northwest Indian Ocean. Cranial morphology was clearly differentiated comparing Tursiops truncatus and Tursiops aduncus, while a recently discovered genetic lineage, found in the Arabian Sea, was morphologically most similar to T. aduncus from the same region, but distinct for various measures, particularly metrics associated with the lateral dimension of the skull. The extent of divergence between T. truncatus and T. aduncus compared to differences between the T. aduncus lineages is consistent with the recent phylogeny for these species. Therefore, with the corroboration of genetic and morphological inference, we propose two conservation units of T. aduncus be recognized in the region at a sub-specific level so that their conservation can be managed effectively. We consider possible evolutionary mechanisms associated with regional habitat characteristics and the exploitation of distinct prey resources.
... LWPTF & GLPTF; see Results) to be important characters for discriminating between T. truncatus and T. aduncus. Investigations into stomach contents of dolphins in the waters of Oman reveal bottlenose dolphins to be feeding in either inshore/coastal habitats or offshore (Ponnampalam et al., 2012). Although not identified to species in the paper, those feeding nearshore were from the Gulf of Masirah, a region known to be dominated by T. aduncus, while one dolphin apparently feeding offshore was from a region (off Muscat) dominated by T. truncatus. ...
... Although not identified to species in the paper, those feeding nearshore were from the Gulf of Masirah, a region known to be dominated by T. aduncus, while one dolphin apparently feeding offshore was from a region (off Muscat) dominated by T. truncatus. Furthermore, those dolphins feeding inshore showed a diet that includes species such as the croaker, Otolithes ruber, that occur on sandy and muddy substrates (Ponnampalam et al., 2012). ...
... Previous studies indicated that B. pterotum was a zooplankton feeder that fed mainly on copepods, ostracods, and euphausiids (Dalpadado and Gjøsaeter 1988;Ishihara and Kubota 1997;Dypvik and Kaartvedt 2013). They also showed that it was an important prey resource for predatory species, including fishery-targeted species (Ishihara and Kubota 1997;Chiou and Lee 2004;Chiou et al. 2006;Ponnampalam et al. 2012;Weng et al. 2015). The high abundance of this species suggests that it is probably an important contributor to the biological pump and a key mediator of the food web dynamics in Kagoshima Bay. ...
... So far, there have been no studies examining what species prey on B. pterotum in Kagoshima Bay. However, previous studies have reported that B. pterotum is frequently utilized as prey by predatory species, including fishery-targeted species, in other areas (Ishihara and Kubota 1997;Chiou and Lee 2004;Chiou et al. 2006;Ponnampalam et al. 2012;Weng et al. 2015). The results from this study strongly suggest that the high abundance of B. pterotum in Kagoshima Bay means that it probably plays an important role as a biological pump contributor and may mediate the link between secondary production and higher trophic levels. ...
Article
We examined the feeding habits of Benthosema pterotum in Kagoshima Bay. Benthosema pterotum undertook diel vertical migration and preyed in shallow waters at night. The most frequently consumed prey were copepods, and B. pterotum selectively foraged on poecilostomatoids. Poecilostomatoids were more predominant in the Kagoshima Bay zooplankton community than in neighboring sites, which suggested that the B. pterotum preference for poecilostomatoids may be an adaptation by the local population to prey availability. Benthosema pterotum probably plays an important role as a biological pump contributor and as a mediator that links secondary production and higher trophic levels.
... Members of this family are caught as bycatch in large numbers by long-liners and tuna purse seiners [2][3][4]. Species of the genus Cubiceps have been found in the stomachs of top predators such as tunas, swordfish, seabirds, fur seals [5][6][7][8][9][10][11][12], dolphins [13][14][15], and balaenopterid whales [3]. Despite their wide distributions and immense ecological significance, little has been done to elucidate different aspects of the population biology of any species of this family that are important for their sustainable management and conservation. ...
Article
Full-text available
Fish of the Nomeidae family are a significant part of marine food webs and are an emerging fisheries resource. The formulation of sustainable management and conservation strategies is constrained by the lack of comprehensive research on the population biology of any of these species. We, therefore, studied reproductive aspects of Cubiceps whiteleggii as a representative of Nomeidae. We used a biological index (gonadosomatic index), as well as macroscopic (visual observations of the surface and transverse ovary sections) and microscopic analyses, in search of proxies for estimating the ovarian maturity status. Samples were collected from April 2014 to December 2017 in Kagoshima Bay, Japan. Asynchronous ovary development in C. whiteleggii indicated multiple spawning during a single reproductive season. Ovarian maturity stages were divided into six groups based on the appearance of the most advanced oocytes, post-ovulatory follicles, and atretic oocytes: immature, maturing, mature, spawned, spent, and resting. Females with ovaries at mature or spawned stages were defined as sexually mature individuals. The size at 50% sexual maturity was estimated to be 126 mm standard length. The spawning season was estimated to last from July to November. Both biological and macroscopic indices were found unsuitable to be used as independent indices in estimating reproductive aspects requiring scientists and managers to search for better alternatives. The conceptualization of appropriate management protocols for the fishery should be aided by these findings, which are the sole knowledge available on the reproductive features of this species. This will ensure its sustainable harvest and make it easier to conduct future research on fish reproduction.
... Moreover, stomach contents only represent the last food before death (probably several days), which might also produce individual variations in short-term feeding habits. The prey species identified here revealed that the PRE humpback dolphins feed on both pelagic and demersal fish ( dolphins preyed almost exclusively on fish and rarely on cephalopods and crustaceans (Barros et al. 2004;Ponnampalam et al. 2012;Parra and Jedensjö 2014;Jefferson and Smith 2015). ...
Article
Full-text available
Accurate diet identification of top predators is crucial to fully understand their ecological roles. Compared to terrestrial animals, gathering dietary information from cetaceans is notoriously difficult. Here, we applied a multilocus metabarcoding approach to investigate the diet of vulnerable Indo‐Pacific humpback dolphins and Indo‐Pacific finless porpoises from the Pearl River Estuary (PRE), China. Our analyses identified 21 prey fish species from the 42 humpback dolphin stomachs, as well as 10 species of fish and one species of cephalopod from the 13 finless porpoise stomachs. All of the taxa were assigned to the species level, highlighting that the multimarker approach could facilitate species identification. Most of the prey species were small and medium‐sized fishes that primarily fed on zooplankton. The calculated similarity index revealed a moderated dietary overlap between the two cetaceans, presumably due to the feeding of the two predators in association with fishing vessels in the PRE. A more diverse diet was observed in humpback dolphins in the closed fishing season compared to the fishing season, implying the influence on the dolphin diet due to the availability of commercial fishery resources. However, according to the results of species rarefaction curves, our findings on the feeding habits of the two cetaceans are still limited by insufficient sample size and therefore should be interpreted with caution. This study represents a first attempt to apply the multilocus DNA metabarcoding technique in the diet analysis of small cetaceans, although more efforts are needed to improve this type of analysis. This article is protected by copyright. All rights reserved
... Given that ecological niche models for the two species are otherwise very similar, it is possible that in this area, both species have similar trophic niches but segregate by space use. Stomach content analysis of these two species across the region, often identify common prey items (although with different index of importance), further supporting a trophic niche overlap between these two species (Amir et al., 2005;Barros & Cockcroft, 1991;Ponnampalam et al., 2012). Furthermore, isotopic trophic niche analysis of four small cetaceans in South Africa waters identified a 40% overlap between humpback and Indo-Pacific bottlenose dolphins and suggested temporary variation, diet compositions and habitat utilization as possible strategies for reducing competitive pressure (Browning et al., 2014). ...
Article
Citizen science data are becoming increasingly relevant in wildlife studies, especially when obtaining data requires costly logistics. In the Arabian/Persian Gulf, baseline information about cetaceans is scarce despite their regular presence. From 2012 to 2019, a citizen science campaign conducted in the United Arab Emirates (UAE) obtained information on 1,292 cetacean sightings. These were methodically validated by experts, resulting in 1,103 records where species was confidently assigned. From the 12 species reported, we analyzed the spatial patterns of occurrence and drivers of habitat preference for the three most reported species: Tursiops aduncus (Ta 53%), Sousa plumbea (Sp 45%), and Neophocaena phocaenoides (Np 2%). Ecological niche modelling, considering seven environmental variables (physicochemical: temperature, silicate, calcite, pH; physiographic: distance from coast, bathymetry; biological: chlorophyll) showed subtle niche partitioning among the three species. Our results suggest different diets and energy requirements for Np, with Ta and Sp, segregating mostly by different patterns of space use. Specifically, Sp prefers close proximity to shore, particularly areas with complex networks of inland canals. This information provides an estimate of habitat preferences, that can be used in formulating effective conservation measures. Sp shows the narrowest area of suitable habitat and is at particular risk from habitat degradation.
... Similarly, the Apogonidae was one of the three families recorded in the highest abundance in bottlenose dolphin (T. trunctatus) stomachs in the Sea of Oman (Ponnampalam et al., 2012). And off Zanzibar, Apogon apogonides was one of the five most important prey species of fish identified in stomachs of T. aduncus (Amir et al., 2005). ...
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Insights into the food habits of predators are essential for maintaining healthy predator populations and the functioning of ecosystems. Stomach content and stable isotope analyses were used to investigate the foraging habits of an apex predator, the Indo-Pacific bottlenose dolphin (Tursiops aduncus) in south-western Australia. A total of 2,594 prey items from 26 families were identified from the stomachs of 10 deceased stranded dolphins. Fish otoliths from stomach contents were used to identify fish to family or species level. Ninety-three percent of identified stomach contents were perciforme fishes, however, perciformes comprised only 30% of the catch during prey sampling. Gobiidae species, small fish generally < 100 mm in total length, were the most prevalent family identified in dolphin stomachs, accounting for 82% of identified prey, yet Gobiidae accounted for 12.7% of the catch during prey sampling. For stable isotope analyses, tissue samples from 14 free-ranging dolphins were analyzed for nitrogen (δ¹⁵N) and carbon (δ¹³C) ratios. From stable isotope analyses and boat-based dolphin photo-identification surveys (n = 339, 2007-2011), results indicated niche differentiation between coastal and inshore (bay and estuarine habitat) dolphins. Carbon signatures showed that coastal dolphins had a more pelagic diet compared to a benthic diet observed in the inshore dolphins. Whereas, nitrogen signatures of coastal dolphins showed higher nitrogen levels than offshore dolphins, likely attributed to feeding on enriched prey typical of estuarian environments. Overall, these results indicated that bottlenose dolphins in the study area were selective foragers and that their foraging is specialized by the habitats most frequently used.
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