Otolith chemistry, stomach contents and stable isotope analysis of a snapper (Pagrus auratus) caught in the Waikato River at Ngaruawahia

Abstract

Analyses of stomach contents, stable isotopes and otolith microchemistry were carried out in order to ascertain the length of freshwater residence of a snapper (Pagrus auratus) caught in the Waikato River at Ngaruawahia in a net set for grey mullet(Mugil cephalus). Results of all analyses suggest that the snapper had spent all of its life in a marine environment with no evidence of freshwater residence in the otolith. Stable isotope analyses (δ15N = 17.0‰, δ13C = –17.1‰) indicated an entirely marine diet, and the stomach contents (two New Zealand screwshells, Maoricolpus roseus, and a hermit crab, Pagurus novizelandiae), suggested that the fish had not fed while in freshwater. However, this does not preclude the possibility that the snapper quickly travelled up the river, without eating, and was caught very soon after.