The Fire Department no longer will be able to have firefighters fill in for colleagues who call in sick, a move that City Manager Sharon A. Addison admits violates the union contract. Ms. Addison directed Fire Chief Dale C. Herman on Wednesday morning that firefighters cannot be called into work to fill in for their colleagues who are absent because of illness. The new practice of “not backfilling” sick firefighters begins Sunday, Ms. Addison said. The City Council was notified about the change during an executive session Monday night, a day before two new council members were elected on platforms of taking a softer stance against the fire department’s union. ... The city and Watertown Professional Fire Fighters Association Local 191 are in the midst of a bitter three-year contract dispute. Chief Herman said the approach will only drive a bigger schism between the union and the city.