In this field study, the broods of male smallmouth bass (Micropterus dolomieui) were manipulated to determine if males adjust their level of defense in response to changes in brood size. Conspecific models of equal size were used to elicit defense before and after manipulations, prior to swim-up of offspring from the nest. Males with augmented broods increased their defense and males with reduced broods decreased their defense, both relative to a control. These results were in the predicted directions; however, the risky tactic, biting, was more independent of brood size changes relative to a less risky threat behaviour, the jaw display. These results demonstrate that parents adjust their level of investment in response to changes in brood size relative to their own expected future reproduction, and that risks of injury or mortality associated with the parent's different defense tactics can explain why some measures of brood defense are relatively independent of brood size.