Abstract:
After the war, the export oriented, live blue swimmer crab fishery in
northern Sri Lanka has emerged as an increasingly important fisheries sector because
of its economic value. Based on passive gill nets set from small boats in shallow
waters, local fishing communities have adapted fishing methods to harvest this
valuable resource. However, the open access to the waters off Jaffna has triggered a
dramatic expansion of fishing activities and the use of some destructive fishing
methods. The fishery catch and effort statistics from 2007 to 2011 were obtained from
Department of Fisheries and grouped into two categories: before and after (the war
ended in 2009-July). These statistics show that in the postwar period, there has been
an increase in fishing effort and the proportion of blue swimmer crabs in the total
fishery production has increased considerably. However, a large proportion of the
catch is often rejected by exporters due to high mortality of damaged crabs.
Consequently, monofilament gillnets have been banned and although illegal
monofilament nets are still common in the live-crab fishery, Wool type gillnets are
now increasingly being used as an alternative. This study examines the differences in
the crab catches between the monofilament and Wool type gillnets. Catch statistics
and population parameters were compared for each type of fishing gear from samples
collected between January to April 2012 off the Karampan coast in Jaffna. The data
that was compiled included: Catch per Unit Effort (CPUE) (under four weight
categories and bycatch); length-frequency distributions; proportion of damaged crab
in catches; the proportion of undersize (<50g) individuals; and the length at 50%
maturity (L50). Higher catch rates of larger (>250g) Portunus pelagicus were reported in
Monofilament gillnets than Wool type gillnets. But considerable proportions of
captured crabs were damaged due to both types of gillnet operations and the level of
damaged crabs from both gillnets were similar (P>0.05). Gear selectivity of Portunus
pelagicus is shown by the significantly higher proportions of small crabs (<50g) in the
Monofilament gillnets. Both types of gillnet caught a high proportion of immature
individuals (<L50). There are concerns about the state of the resource following the
increase in fishing effort with gill nets and the expansion of the blue swimmer crab
fishery. The wastage of damaged crabs in monofilament nets and the high proportion
of small immature crabs in the catch could be avoided by use of Wool type nets and
larger mesh sizes. Alternative types of gear type for catching crabs could be
considered for fisheries management in the future. The introduction of crab traps
with escape gaps for undersized juveniles may help to conserve the resource while
decreasing the proportion of damaged and undersized juveniles in the catch.