Being non-lytic, filamentous phages can replicate at high frequencies and often carry virulence factors, which are important in the evolution and emergence of novel pathogens. However, their net-effect on bacterial fitness remains unknown. To understand the ecology and evolution between filamentous phages and their hosts, it is important to understand (1) fitness effects of a filamentous phages on their hosts, and (2) how these effects depend on the environment.To determine how the net-effect on bacterial fitness by filamentous phages changes across environments, we constructed phage-bacteria infection networks at ambient (15 PSU) and two stressful salinities (11 & 7 PSU) using the marine bacterium, Vibrio alginolyticus and its derived filamentous phages as model system.We observed no significant difference in network structure at 15 PSU and 11 PSU. However, at 7 PSU phages significantly reduced bacterial growth changing network structure. This pattern was mainly driven by a significant increase in bacterial susceptibility.Our findings suggest that filamentous phages decrease bacterial fitness at stress-full environmental conditions, which may impact bacterial communities, alter horizontal gene transfer events and favour the evolution of novel pathogens in environmental Vibrios.