Movement patterns, foraging ecology and digestive physiology of blacktip reef sharks, Carcharhinus melanopterus, at Palmyra Atoll: A predator dominated ecosystem

Apex predators may have a strong regulatory function in marine ecosystems through both density and behaviorally mediated effects. Understanding the ecological impacts of apex predators is particularly important in predator dominated ecosystems where intra-specific competition may be high. While a number of techniques are available for quantifying predator movement patterns and distribution, little is known of the causative factors that regulate these behaviors. One important aspect of predator behavior is foraging, and an important regulating aspect of foraging is digestion. To advance our understanding of the interrelationship between gastric function and foraging behavior, I tested two types of data loggers for deployment in shark stomachs. One type of tag measured stomach acidity, the other the motility of the stomach wall. Both types of tags were deployed in the stomachs of captive free-swimming blacktip reef sharks to determine the effects of feeding and fasting on gastric digestive function. Gastric pH was maintained low during long periods of fasting, suggesting continuous secretion of acid. Gastric motility was higher for meals of mackerel than for similar sized meals of squid with maximum motility occurring at meal sizes of 1% body weight. Based on diel patterns of gastric motility and pH, I predict that blacktips will feed daily and preferably forage during times of low water temperature.;Palmyra Atoll is a remote, predator dominated ecosystem, and has a large population of blacktip reef sharks. Blacktips at Palmyra are smaller than those at other locations, which may be the result of food-limited growth due to intra-specific competition. Palmyra consists of two lagoons (east and west), and abundance of sharks appears to be similar in both lagoons. Active and passive tracking was used to study the movement patterns of the sharks at Palmyra. Sharks in the west lagoon utilized small home ranges over scales of days to weeks. Adult sharks selected ledge habitats, while smaller individuals selected sand-flats, and small pups were found in very shallow waters. Fractal analysis revealed that sharks used patches that were 3--17% of the spatial scale of their home range, and that sharks move with a directed walk while in patches but move randomly between patches.;Sharks in the west lagoon showed strong site fidelity with some individuals being detected there for over 3 years. Sharks showed little movements between lagoons and sharks in the east lagoon had shorter residence times. Sharks in the west lagoon had higher body condition indices than those in the east lagoon and stable isotope analysis revealed that trophic structure was different between the two lagoons. Conditions differ between the two lagoons which may be driving differences in foraging success. This study reveals the importance habitat can play in the movement patterns, home range and foraging success of sharks and suggests that intra-specific competition could be a strong regulator of apex predator populations in pristine predator dominated ecosystems. Studies of the digestive system revealed that physiology may also regulate some aspects of movement patterns, although field studies will be required to test these hypotheses.