Nursery habitat use and foraging ecology of the brown stingray Dasyatis lata determined from stomach contents, bulk and amino acid stable isotopes

Abstract

Identification of nursery habitats and knowledge of the trophic ecology and habitat use of juvenile fishes within these habitats are fundamental in developing sound management and con- servation strategies. The brown stingray Dasyatis lata is a large benthic predator that inhabits the coastal waters of Hawai'i. Although abundant in these ecosystems, little is known about its basic eco - logy. Stomach content, bulk and amino acid stable isotope analyses were used to assess diet and habitat use of juvenile brown stingrays and to examine the possibility of competitive interactions with juvenile scalloped hammerhead sharks Sphyrna lewini that are sympatric with brown stingrays in K¯ ane'ohe Bay, Oahu. Based on stomach contents, brown stingrays fed almost exclusively on crus- taceans. An ontogenetic shift in stingray diet and an increase in relative trophic position (TP) were apparent from stomach content and stable isotope analysis. Stingray bulk δ 13 C and δ 15 N values indi- cated long-term foraging fidelity to subregions of the bay. Use of K¯ ane'ohe Bay as a nursery habitat was supported by nitrogen isotopic analysis of individual amino acids from stingray muscle samples. Our results clearly demonstrated that stingrays foraged within the bay for the majority of their juve- nile lives then shifted to offshore habitats with the onset of sexual maturity. Trophic enrichment fac- tors used to estimate TPs from amino acid analysis in previous studies may underestimate TPs in elas- mobranchs owing to urea retention for osmoregulation. Potential prey resources were partitioned between stingrays and juvenile scalloped hammerhead sharks, and TP estimates from each analyti- cal method indicated that juvenile scalloped hammerhead sharks forage on higher TP prey than do juvenile brown stingrays. These results show that the study of foraging ecology and habitat use of marine animals can greatly benefit from integrating traditional stomach content and bulk stable isotopic analyses with nitrogen isotopic analyses of individual amino acids.