In the fish Aidablennius sphynx, in which males continuously care for up to 7000 eggs throughout the breeding season, females prefer to mate with males that already guard eggs. The present study shows that this preference appears to be adaptive because the probability of eggs being cannibalized decreased with brood size. In the field, on average 36 eggs disappeared from nests per day, where the main egg predator seemed to be the guarding male. Experiments showed that males selectively consumed dead eggs, probably to prevent the spread of infections. However, only with large broods did the numbers of eggs cannibalized in the field correspond to the egg mortality rate, that was determined to be 0.8%. When guarding small broods, males have probably also eaten healthy eggs. Breeding males suffered an average weight loss of 19.4%. This suggests that breeding males are restricted in foraging opportunities. When a male was experimentally fed, he cannibalized fewer eggs only when guarding small broods, not when guarding large broods. Therefore, it seems that caring males daily harvest eggs to remain in sufficient condition. In large broods they use eggs that recently died. When the male is guarding small broods the low numbers of dead eggs do not suffice, and healthy eggs are also eaten.