Tracking historical changes in the trophic ecology of the green turtle "Chelonia mydas" in the Hawaiian Islands

Abstract

[eng] Hawaiian green turtles Chelonia mydas were heavily exploited for their fat, meat and eggs by the Polynesians in the pre-contact era and continued to be commercially exploited until 1978 when they were officially protected. These conservation measures allowed its population to rebound, although it is still considered threatened by the Endangered Species Act. Hawaiian green turtles have been the focus of intense research, but surprisingly, little is known about its trophic ecology and how it might have changed due to the anthropogenic impacts in the coastal ecosystems of the archipelago. This thesis aims to better understand the current habitat use and diet of green turtles in the Hawaiian Islands and track possible historical changes in their ecological niche. Underwater censuses in Oahu and the Kona coast revealed that green turtles had a strong preference for shallow, flat platforms covered with dense macroalgal pastures. Green turtle abundance was much lower in coral reefs, where they also had a modest contribution to the total biomass of herbivores, dominated by sea urchins and fishes. Not surprisingly, the stable isotope ratios of C, N and S in the epidermis of modern green turtles from east Oahu and the Kona coast confirmed a macroalgae- dominated diet, but seagrasses and mangroves had also relevant contributions to their diet in east Oahu, as well as fish in the Kona coast. Furthermore, the ontogenetic diet shift associated with the settlement of juvenile green turtles in neritic habitats is faster in eastern Oahu than in the Kona coast, perhaps because of the higher availability of macroalgae in the former. The stable isotope ratios of C, N and S in the squamosal and the ribs of the same green turtle individuals revealed similar patterns of geographic and ontogenetic variability, hence confirming that unprocessed bone samples are informative of diet prior to death. This is the base for retrospective analysis using museum specimens. However, mixing models using the trophic discrimination factor (TDF) derived experimentally for cortical bone yielded unreliable results when used on unprocessed bone samples, suggesting that trabecular bone has a different TDF value. This is

relevant, because the skulls and carapaces preserved at museums are made of skeletal elements with a thick core or trabecular bone. Despite such limitation, the stable isotope ratios of C, N, and S in the skeletal elements of green turtles preserved in museums revealed minor changes in the isotopic niche of green turtles from east Oahu during the past 120 years. Nevertheless, the breadth of the isotopic niche decreased in the most recent years, indicating that ancient green turtles exhibited a broader diversity of individual foraging strategies, with a few individuals relying mostly on seagrasses and others consuming substantial amounts of animal matter. These trophic specialists are currently gone from eastern Oahu, where green turtles have converged on the use of the most abundant resource, red macroalgae, probably because of the homogenization and simplification of coastal habitats Nevertheless, results confirm that macroalgae were the staple diet for the majority of the green turtle population before the introduction of exotic red macroalgae and hence it is a trait characteristic of the Hawaiian population. To understand the relationship between diet and the morphology of the skull and the mandible, geometric and traditional morphometrics were used, comparing skulls and mandibles of populations relying mostly on seagrasses and populations relying mostly on macroalgae. Results showed that macroalgae consumers have longer and narrower skulls than seagrass consumers, the former is more suitable for selective browsing and suction feeding and the latter is better adapted for grazing and stronger bite force. The skull morphology of Hawaiian green turtles fits that general pattern but is different from that of the green turtles inhabiting the Mexican Pacific, although both are genetically related.