The use of observers aboard commercial vessels is a useful method of obtaining data
on catch composition (Allen et al., 2001). In this study, fishing ground has much influence
on the catch of collapsible pot targeting blue swimming crab in the upper Gulf of Thailand.
Different fishing ground induces the different of fishing operation, catch composition, crab
size catch, and discard species. Because of the many larger of pot numbers onboard in
commercial scale, the discard problems were bigger than small scale both in quantity and
quality (numbers and species). However the fishing ground of commercial scale showed
positively for catching the larger crab size. Hence, select the fishing ground can induce the
blue swimming crab size selectivity