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International Journal of Marine Science 2014, Vol.4, No.46, 1-12
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10
species. Body parameters (length, weight) of the
specimens were measured, followed by removal of the
gut, which was subsequently weighed and preserved
in 70% alcohol. Degree of fullness of stomach was
categorized as empty, partially filled or full following
Hyslop (1980) and the contents were examined under
microscope. Stomach contents were identified up to
the lowest possible taxon following FAO species
identification sheets (Fischer and Bianchi, 1984;
Carpenter and Niem, 1998).
Further, only
S. laticaudus
was selected to assess
ontogenic shift in the diet as both juvenile and adult
specimens were available. These samples were
categorized into three different size groups namely
small (10–25 cm, juveniles), medium (26–40 cm, sub
adults), large (41–57 cm, adults) and the percent Index
of Relative Importance (
I
RI
) for each prey item for the
all size groups was calculated.
3.6 Data analysis
Species abundance and weight data from all the five
sub-samples of each trawl haul were summed up to
represent a single sample and standardized to 60-min
trawl in view of the variability in trawling duration.
Subsequently, spatio-temporal trends of elasmobranch
abundance and weight were computed and
comparisons by season and site were analyzed by
ANOVA (Sokal and Rohlf, 1987) using the Microsoft
Excel 2007 program
.
Stomach content data was compiled and Index of
Relative Importance “
I
RI
” (Pinkas et al., 1971) was
computed to evaluate the importance of each prey
item.
I
RI
= (
% N
i
+
% W
i
) ×
% FO
i
where
N
i
,
W
i
and
FO
i
represent percentage of number,
weight and frequency of occurrence of prey “i”,
respectively.
Index of diet breadth “
B
” of a species (Levins, 1968)
was computed to establish the level of specialization
of each examined species and to identify whether it is
a generalist or specialist feeder.
B
i
= [(Σ
j
p
ij
2
)
-1
-1] (n-1)
-1
where,
B
i
is Levins’ standardized index for predator
“i”,
p
ij
is the proportion of the diet of predator “i” that
is made up of prey “j”, n is the number of prey items.
Dietary similarity index “
S
” (Linton et al.,
1981) was
computed to evaluate the extent of diet overlap
between the commonly observed species.
S
= 100 (1 – ½ Σ׀
P
xi
-
P
y
i
׀)
where,
P
xi
and
P
yi
are the proportions of the diets of
the species examined ‘x’ and ‘y’
respectively, of prey ‘i’.
Trophic level of the species was computed to assess
the position of the examined species in the food web
(Christensen and Pauly, 1992).
n
TL
i
= 1+ Σ (
DC
ij
.
TL
j
)
j=1
where,
TL
i
is the trophic level of the species “i”,
DC
ij
is the proportion of the prey species “j” in the diet of
“i”,
TL
j
is the trophic level of the prey species.
3.7 Collection and analysis of fishery data
Marine fish landing data of Goa for the periods
1969–2004 (CMFRI) and 2006–2010 (Directorate of
Fisheries, Government of Goa, 2011) were obtained
and the trends in overall elasmobranch fisheries of
Goa as well as assorted data for sharks, rays and
skates were compared with fishing effort (number of
fishing vessels) for the above period. Subsequently,
the elasmobranch trends during 1969–2004 were
compared with trends of other higher trophic level
(HTL) and lower trophic level (LTL) species during
the same period
.
Authors’ contributions
MRH and VPP carried out sampling for the study along the
potential fishing grounds off Goa, followed by taxonomic
identification of elasmobranchs and their seasonal distribution.
MRH also carried out the dietary analysis of the biological
samples and interpreted the same. VPP interpreted the overall
results of the current work and discussed the same in the
manuscript. CUR planned, designed and participated in the
field work and guided in interpreting the manuscript.
Acknowledgements
The authors are grateful to the Ballast Water Management
Programme, India executed by the National Institute of
Oceanography (CSIR), India for Directorate General of
Shipping, Ministry of Shipping, Government of India. The first
author was supported by University Grants Commission (UGC),
Government of India. We thank the crew of the trawlers for
allowing us to collect samples for the study. The regional data
on fish landings (2006-2010) was made available by the