Gape size allometry in pike reflects variation between lakes in prey availability and relative body depth

1. The gape size of the predator and the body depth of its prey are the main factors deciding whether a gape-limited piscivore can ingest a potential prey fish. The connection between morphological variation in pike and its prey was studied in five lakes with different proportions of pike, perch and roach. 2. At low prey abundance the prey fish are larger than in lakes with a higher prey density. In contrast, the pike are smaller but have relatively high gapes in the lake with the lowest prey availability, and larger but with smaller gapes at the highest prey density. There is a positive correlation with relative gape size of pike and relative body depth of prey among the five lakes. 3. Using the body depth in perch and roach and the gape size in pike. the proportion of the prey populations that could be eaten by the pike was calculated. In the low prey density lakes the pike could ingest a smaller proportion of the prey than in the high prey density lakes where most pike can eat all available prey sizes. 4. Competition for food in a situation where prey are big may cause a selection for a higher gape size. Our results indicate that this mechanism could be the reason that pike from different lakes vary in relative gape size.