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International Journal of Marine Science 2014, Vol.4, No.46, 1-12
http://ijms.biopublisher.ca
7
fishing gears (trawlers, purse seines, long line and gill
net) operating off the Goan coast.
Analysis of temporal variations using ANOVA did not
reveal any significant differences between the seasons.
It appears that all the three common elasmobranchs (
S.
laticaudus, C. griseum
and
H. walga
) were residents
of this region. Further, the lack of seasonal differences
in their occurrence could be attributed to minor
fluctuations in the water temperature (µ= 28.86 ±
1.14
℃
). On the other hand, the observations made on
the elasmobranch abundance between the two
sampling sites showed marked differences. The
northern part of the present study area is characterized
by the presence of large estuaries, whereas the
absence of a large estuary in the southern part
highlights the role of fresh water intrusion in adjacent
coastal region. This implies that due to the presence of
a large estuarine system and the transient nature of
elasmobranch population during most of the year, they
occurred in less abundance in the northern region. On
the other hand, absence of a large estuary in the
southern region resulted in less freshwater discharge
to the inshore shelf waters, which might be the
reason for higher catches from this region.
Published reports (Froese and Pauly, 2011) suggest
that most of the presently recorded elasmobranchs
undertake amphidromous migration.
Analysis of size class among the observed
elasmobranchs indicated that juveniles dominated the
population during most of the sampling duration,
except December-January, suggesting that the inshore
region functions as perennial nurseries for
young-of-the-year and juveniles (Ansari et al., 1995).
However, the occasional capture of adults along the
estuarine channel during the post-monsoon season
(December–January) suggested that they undertook
amphidromous migration.
Elasmobranchs are voracious predators that frequent
the inshore coastal waters primarily due to availability
of abundant prey resources. Analysis of degree of
fullness of gut content revealed low proportion of
empty guts in
S. laticaudus
suggesting high feeding
intensity as compared to
C. griseum
and
H. walga
.
Relatively high
I
RI
values for teleosts in the
elasmobranch diet were influenced by
I
RI
data of
S.
laticaudus
and
C. griseum
. The dominance of natantian
decapods in the diet of
S. laticaudus
along with
comparable proportion of teleosts suggested that it is a
non-selective predator. It is essential to note that the
inference drawn here could be biased as most of the
analyzed samples were juveniles. Devadoss (1989)
suggested that during post-parturition, when the fishes
are unable to move fast owing to limited strength, they
feed on epibenthic fishes and invertebrates. On the
other hand, high proportions of a single prey group
(clupeoid fishes and cephalopod molluscs)
observed
in the case of
C. griseum
and
H. walga,
respectively
suggested that these species are specialized feeders
.
Further, high proportions of crustaceans and pelagic
teleosts in the diet of
S. laticaudus
and
C. griseum
suggested a bentho-pelagic mode of foraging, whereas
the dominance of decapods and squids in the ray diet
suggested that it is an epibenthic feeder.
Elasmobranchs are known to adapt to various feeding
strategies depending upon the type and degree of
specialization in respect of habitat, prey items and
morphology (Wilga et al., 2007).
Gut content analysis of different size groups suggested
that juveniles of
S. laticaudus
preferably fed on small
crustaceans (Mysis and small sized
Miyakea nepa
),
medium sized individuals fed on a mixed diet of large
crustaceans (prawns and
M. nepa
) and variety of
teleosts, whereas adults preferred teleosts. A similar
observation made by Abdurahiman et al. (2010)
indicated that juveniles mostly feed on epibenthic
crustacean and slow moving organisms whereas adults
feed on benthic and pelagic teleosts
.
It is known that
carnivorous fishes become more ichthyophagous with
size and age (Renones et al., 2002). This could be
attributed to the differences in locomotion and ability
to catch prey at different life stages (Devadoss, 1989)
suggesting a tendency for an ontogenic shift in the diet.
Diet breadth (
B
) analysis for the above three species
suggested that they were generalist feeders, which
consumed a wide array of prey items including
penaeid prawns, stomatopods, clupeoid fishes and
cephalopods. Published literature (Ellis et al., 1996;
Raje, 2003; Navia et al., 2007; Gutteridge et al., 2011)
suggested that epibenthic crustaceans, cephalopod
molluscs and teleosts were the common prey items of
elasmobranchs worldwide. Diet overlap between
species is related to the degree of competition among
organisms under conditions of limited resource