Examining spatial and trophic ecology of Bahamian stingrays, Styracura schmardae and Hypanus americanus, using stable isotope analysis

Abstract

In this thesis I use stable isotope analysis to investigate the spatial and dietary ecology of two species of tropical stingray, the southern stingray (Hypanus americanus) and the Caribbean whiptail ray (Styracura schmardae) from Eleuthera island, The Bahamas. In Chapter 1, I directly compare stable isotopes of carbon, nitrogen and sulphur between the two species (S. schmardae, n = 96 ; H. americanus, n = 102) to investigate if and how these sympatric stingrays exhibit resource partitioning. I show that mangrove creek systems may be important habitat for S. schmardae, mitigating competition with H. americanus, and that trophic resource partitioning may also be occurring, with H. americanus feeding at a higher trophic level than S. schmardae. In Chapter 2, I explore the use of stable isotope analysis in detecting ontogenetic shifts in H. americanus (n = 110) and S. schmardae (n = 94). Here, I use breakpoint analysis to pinpoint shifts in mean δ15N and δ13C as body size increases, on three metabolically distinct tissues, which therefore give insights into different time periods: whole blood, white muscle and cartilage (barb). There were four breakpoints in white muscle samples, two in blood and in cartilage only one. We recommend that future research determining ontogenetic shifts via stable isotopes utilise this range of tissues. Breakpoints in δ13C were observed in both species, indicating ontogenetic habitat shifts occurring at juvenile sizes. A second shift was detected at larger body sizes in both δ15N and δ13C for S. schmardae, we suggest this second ontogenetic niche shift indicates a return to mangroves and concurrent increase in higher trophic level prey by adults. The findings presented in this thesis are novel for both species, emphasising the significance of mangroves habitats as well as providing the first ever assessment of resource use by the poorly studied Caribbean whiptail ray. Findings could be used to build conservation frameworks to protect southern stingrays, Caribbean whiptail rays, and the mangroves that appear to be intrinsic to their ecology.