δ15N isotope studies of life history and trophic position of Fundulus heteroclitus and Menidia menidia

Abstract

We used δ15N isotopic signatures to follow the trophic position as well as the life history of 2 of the most abundant fish species of the Waquoit Bay estuarine system: common killifish Fundulus heteroclitus and Atlantic silverside Menidia menidia. Juvenile fish bore isotopic signatures specific to the estuary in which they had been growing. The fish changed in δ15N as they grew and had significant, estuary-specific differences in δ15N signatures. F. heteroclitus increased in length by 3 to 4 cm as they increased in δ15N by up to 4‰. The magnitude of the shift in δ15N is consistent with movement of the fish ∼1 trophic step up the food web. F. heteroclitus remained within their native estuaries year-round, and retained the isotopic signatures of their prey in each estuary. M, menidia moving offshore during the winter months lose estuary-specific differences in δ15N, and appear to feed at a higher trophic level than fish residing in the estuaries. Evidence from gut content analysis, coupled with results of the stable isotopic measurement, closely concurs with the life history characteristics of these 2 well-known fish species, and provides additional insight into life history habits and feeding.