Basic HTML Version
International Journal of Marine Science 2014, Vol.4, No.46, 1-12
http://ijms.biopublisher.ca
8
availability (Odum, 1971). High percentage of diet
overlap between
S. laticaudus
and
H. walga
suggested
that both the species were generalist feeders of
epibenthic crustaceans. However, despite occupying
similar trophic levels, the low degree of diet overlap
between
S. laticaudus
and
C. griseum
suggested that
availability of wide array of prey items could have
resulted in resource partitioning among these species
(Burrell, 1992). Secondly, differences in mouth
morphology, dentition and or feeding behaviour may
influence degree of competition towards resource
utilization (Scrimgeour and Winterbourn, 1987).
Despite being the top predators in the marine
ecosystem, elasmobranch populations are dwindling.
The meagre contribution of elasmobranchs to the total
marine fish landings of Goa during 1969–2004 despite
the increased fishing effort suggests the absence of an
organized elasmobranch fishery off Goa. The reported
figures could be inaccurate as elasmobranchs form
incidental by-catch of bottom trawlers, purse seines
and gill nets, and depending on their size, they are
either utilized or discarded. Moreover, Illegal,
Unreported and Unregulated (IUU) fishing of
demersal species is known to occur during the
monsoon ban period, when fishing vessels from
neighbouring states harvest catches in the coastal
waters off Goa and land them in their respective home
states (Pramod, 2010). However, such catches are not
evaluated. An inverse relationship between the total
marine fish landings and elasmobranch landings
suggested that much of the fishing effort was
concentrated towards the exploitation of LTL (sardines,
mackerels, prawns) and other HTL fishes (tuna,
perch-like fishes, flatfishes). The fishing effort in
terms of mechanized vessels operating along the Goan
shelf waters has increased from only four vessels in
1963 to 1152 in 2004 (Directorate of Fisheries,
Government of Goa) suggesting that the expansion of
marine fishery resulted in exploration of deeper waters
(> 40 m depth), which support substantial stocks of
HTL and LTL organisms (Rodionov, 2005). Among
the three groups listed by the CMFRI – sharks, skates
and rays, the former dominated the elasmobranch
landings during the post-mechanization period. This is
mainly due to the contribution of trawlers, purse
seines and gill nets those harvested large quantities of
S. laticaudus
and
S. zygaena
at 30-50 m depths off
Goa. On the other hand, relatively meagre catches of
batoid fishes during the above period may be
attributed to discarding of under-sized juveniles back
into the sea, owing to lack of commercial value
(personal observation). Secondly, there is no record of
the discarded fraction of the elasmobranch by-catch
that resulted in poorly documented batoid fish catches.
Bonfil (1994) opined that lack of authentic
information on biological aspects of elasmobranch
by-catch across the globe stemmed out of inadequate
recording of elasmobranch catches.
Decreasing trends of elasmobranchs and LTL fishes
with a simultaneous increase in other HTL fishes
during 1969–2004 suggested shift in the fishing effort.
A significant variations in trends of these resources
during 1969–1979 suggested that although the
quantum of fishing with traditional crafts and gears
did not vary substantially, further expansion of the
mechanized fishery during this period led to increased
harvesting of HTL fishes from deeper waters (up to 80
m depth). The recent catch trends (1979–2004)
displaying decrease in elasmobranchs and other HTL
fishes with a simultaneous increase in LTL fishes
resulted in decrease in the Marine Trophic Index along
this region.
Despite the increase in fishing effort in the shelf
waters (30–80 m depths) that supported the bulk of the
elasmobranch landings (personal observation), there
was no increase in their catches corresponding to
increase in HTL fishes (Figure 6) suggesting the role
of the biological attributes of these species. The 38
elasmobranch species listed by the CMFRI as bulk
contributors towards the elasmobranch fishery off the
Southwest coast of India are known to be long-lived
with slow growth rate, late maturity and low fecundity
(Raje et al., 2007). All the above attributes make these
species highly vulnerable to the ever-increasing
fishing effort. Most of the individuals caught in
demersal fishing gear are young-of-the-year and
juveniles, which are discarded back into the sea and
whose recruitment to the coastal ecosystem depends
on their survival rate. However, there is no
documentation of survival of these under-sized
individuals. Further, there is no assorted data on
species and gear available for landings of large sharks.
3 Materials and Methods
3.1 Study site
The 105 km long coastline of Goa is aligned
NNW-SSE, faces the Arabian Sea with diverse