Discharge, water level and current
Water level is the environmental parameter most often cited in the literature as a migration
trigger. This parameter is however also correlated to discharge and current speed, as will be
demonstrated in Chapter 4. But the latter variables are less easy to observe and quantify than
water level, which might explain the higher occurrence of water-level in records of migration
triggers. The correlation between these environmental cues leads Poulsen et al. (2004) to
simply say, for a number of Mekong species, that ‘the arrival of the monsoon season triggers
the fish to migrate’.