Because there is precious little that can be done about rooting out a "foreigner" once it has established itself in rivers or large lakes.
Si kastaba ha noqotee, waa wax iska fudud in laga fogaado xad-dhaafka fiisada iyadoo la sameynayo "fiiso orod" dal deris ah oo dhinaca dhulka ah ama duulimaad jaban.