In fish with paternal care, males often eat their offspring (i.e. filial cannibalism). This is regarded as a male's adaptive strategy to maximize lifetime reproductive success by enhancing his own survivorship or the survivorship of the remaining offspring at the cost of current reproductive success. Although the parental energy reserve has been considered a primary factor influencing filial cannibalism, the possibility that mate availability may also be an important factor has been overlooked. When many mates are available, males may receive a surplus of eggs, which can be treated as an energy reserve and reallocated to future breeding attempts. We present a game theoretical model for the evolution of filial cannibalism, incorporating intrinsic parental condition and extrinsic breeding system components which determine mate availability. The model predicts that filial cannibalism is favoured under the following conditions: (1) the male's energy reserve is low; (2) mate search efficiency is high; (3) the population density is high; (4) the sex ratio is female biased; (5) the male care period is long; and (6) the female's refractory period is short. Conditions 2-6 facilitate filial cannibalism through an increase in mate availability.