Ontogenetic Diet Comparison of Atlantic Cod and White Hake Occurring at Historically Low Population Abundances

Abundances of Atlantic cod Gadus morhua and white hake Urophycis tenuis have dropped to historically low levels in the southern Gulf of St. Lawrence. Stomach samples were collected during late summer 2001 and 2002 to compare ontogenetic diet patterns at a time when food presumably was not limiting. White hake had a narrow diet and showed an abrupt ontogenetic diet change: fish less than 35 cm total length (TL) consumed benthic invertebrates (sevenspine bay shrimp Crangon septemspinosa, gammarid amphipods, or polychaetes), while those at least 35 cm TL preyed mainly on Atlantic herring Clupea harengus. Atlantic cod had a broad diet initially dominated by benthic and pelagic invertebrates (Arctic argid Argis dentata, Aesop shrimp Pandalus montagui, Eualus spp., mysids, northern propeller clams Cyrtodaria siliqua, and polychaetes). Fish (mainly Atlantic herring, capelin Mallotus villosus, and small demersal species) were gradually incorporated into the Atlantic cod diet but did not comprise more than 50% of prey biomass until the cod reached a length of 50-60 cm TL. The incidence of cannibalism was low for both fishes. Diet overlap was low between all sizes of Atlantic cod and white hake but was high between adjacent size-classes of Atlantic cod and for all size-classes of white hake greater than 35 cm TL (where Atlantic herring comprised more than 70% of prey biomass).