Spatial and trophic ecology of the bluntnose sixgill shark: environmental drivers of behavior and comparative trophic position in two distinct habitats
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2012-12
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University of Hawaii at Manoa
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The bluntnose sixgill shark, Hexanchus griseus, is an apex predator and scavenger that has adapted to highly varied thermal structure, light penetration, productivity, and food web structure throughout its large geographic range. The behavioral adaptations to varied environmental conditions are poorly understood, and knowledge is particularly lacking in tropical latitudes where sixgill sharks inhabit deep water. This study aimed to investigate environmental drivers of movements of sixgill sharks in a tropical habitat, and to compare the trophic ecology of H. griseus in two distinct habitat types. Pop-up satellite archival tagging revealed diel vertical migrations between ~275-700m, and light and temperature were identified as important factors in determining depth habitat. Low oxygen concentrations did not appear to limit depth, and the animals spent about 50% of their time in hypoxic water (<60μmol). Home range size was expected to be small, but two mature males left the island slope where they had been tagged and traversed deep water. This long-distance pelagic movement could allow gene flow between distant populations. A preliminary investigation of prickly sharks (Echinorhinus cookei) revealed that they have an overlapping but shallower depth range, are much more sedentary, and are less light-averse than sixgill sharks. Amino acid compound-specific isotopic analysis of nitrogen indicated that sixgill sharks in Hawaii had a greater ontogenetic shift in trophic position than sharks from Puget Sound. This result suggests that scavenging may become important for large size sixgill sharks in deep oligotrophic habitats such as Hawaii, while adults in the more productive ecosystems are able to be selective predators. Alternatively, commercial fishing pressure in Puget Sound may have depleted stocks of sixgill shark prey items such as hake, forcing them to feed at a lower trophic level. Prickly sharks had a higher trophic position than most sixgill sharks, indicating that their feeding strategies may be different despite overlap in depth and geographic ranges. As a whole, this study provided insight on how a single species has adapted to be successful in very different habitats through isothermal submergence in low latitudes and a generalist feeding strategy, and has identified light and temperature as important parameters which determine sixgill shark habitat and behavior.
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bluntnose sixgill shark, Hexanchus griseus
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Theses for the degree of Master of Science (University of Hawaii at Manoa). Oceanography.
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