Wrasse species, and primarily ballan (Labrus bergylta), corkwing (Symphodus melops) and goldsinny wrasses (Ctenolabrus rupestris), have frequently been used as “cleaner fish” to control salmon lice (Lepeophtheirus salmonis) infestations in salmonid fish farms. Here, we analysed the efficacy of wrasses in controlling lice infestations by combining estimates of the occurrence of salmon lice in wrasse gut contents and digestion time with a salmon louse population dynamics model. We conducted scenario simulations of salmon louse dynamics under conditions representative for salmonid farms in southern and central Norway with different amounts of wrasses deployed. In the simulations, wrasses were deployed from 1st of June to 1st of December either in the first calendar year of production, for production cycles starting January–May, or in the second year, for production cycles starting in June–December. Delousing treatments were performed if salmon louse numbers exceeded regulatory limits. Results suggested that deployment of wrasses with a 1:20 wrasses per salmon ratio postponed the first salmon louse treatment after wrasses deployment by on average 37 days and reduced the number of delousing treatments per salmon by 10%. The longest postponement was at intermediate to low external infestation pressure and use of wrasses gave the largest benefit when deployed during the first calendar year of production. Our results quantify the efficacy of using wrasses to control salmon lice and reduce the likelihood of sea louse levels exceeding regulatory threshold levels.