We did a seasonal study of northern pike diets in Lake Thompson, a large natural lake about 40 miles west of Brookings. It was under the ice in the winter that we really saw the northern pike eating the biggest prey items -- black crappies, that year's common carp (~10 inchers), etc. During the open water season, they ate a lot more smaller fishes and even a lot of invertebrates.
http://pubstorage.sdstate.edu/wfs/thesis/Sammons-Steven-M-M-S-1993.pdf 
