Ontogenetic niche changes in haddock Melanogrammus aeglefinus reflected by stable isotope signatures, δ13C and δ15N

We investigated stable nitrogen (delta N-15) and carbon (delta C-13) isotope signatures in haddock Melanogrammus aeglefinus in Ullsfjord, Norway, and found evidence of an ontogenetic shift as a result of growth but in the absence of other morphological changes. Stable isotope signatures and trophic level (TL) changed significantly from a pelagic signature and a low TL to a more benthic associated signature and higher TL when the fish were ca. 20 cm in length. The stable isotope signatures of haddock were compared with those of Atlantic cod Gadus morhua from the same system, other main demersal fish species in the fjord, and haddock from the adjacent fjord, Sorfjord. Our results indicate that both haddock and cod are opportunistic feeders, feeding on the abundant crustaceans in the fjords. Several of the large demersal fish species in Ullsfjord had very similar stable isotope signatures, which reflected their predation on similar prey types.