Combining abundance and performance data reveals how temperature regulates coastal occurrences and activity of a roaming apex predator.

The redistribution of species has emerged as one of the most pervasive impacts of anthropogenic climate warming, and presents many societal challenges. Understanding how temperature regulates species distributions is particularly important for mobile marine fauna such as sharks given their seemingly rapid responses to warming, and the socio-political implications of human encounters with some dangerous species. The predictability of species distributions can potentially be improved by accounting for temperature's influence on performance, an elusive relationship for most large animals. We combined multi-decadal catch data and bio-logging to show that coastal abundance and swimming performance of tiger sharks Galeocerdo cuvier are both highest at ~22°C, suggesting thermal constraints on performance may regulate this species' distribution. Tiger sharks are responsible for a large proportion of shark bites on humans, and a focus of controversial control measures in several countries. The combination of distribution and performance data moves towards a mechanistic understanding of tiger shark's thermal niche, and delivers a simple yet powerful indicator for predicting the location and timing of their occurrences throughout coastlines. For example, tiger sharks are mostly caught at Australia's popular New South Wales beaches (i.e. near Sydney) in the warmest months, but our data suggest similar abundances will occur in winter and summer if annual sea surface temperatures increase by a further 1-2°C.

Telemetry and random-walk models reveal complex patterns of partial migration in a large marine predator.

Animals are often faced with complex movement decisions, particularly those that involve long-distance dispersal. Partial migrations, ubiquitous among all groups of vertebrates, are a form of long-distance movement that occurs when only some of the animals in a population migrate. The decision to migrate or to be a resident can be dependent on many factors, but these factors are rarely quantified in fishes, particularly top predators, even though partial migrations may have important implications for ecosystem dynamics and conservation. We utilized passive acoustic telemetry, with a Brownian bridge movement model and generalized additive mixed models, to explore the factors regulating partial migration in a large marine predator, the tiger shark, throughout the Hawaiian Islands. Although sharks tended to utilize a particular "core" island, they also demonstrated inter-island movements, particularly mature females that would swim from the northwestern Hawaiian Islands to the main Hawaiian Islands (MHI). Immigration to another island was a function of season, sea surface temperature (SST), and chlorophyll a concentration. Our results predict that 25% of mature females moved from remote French Frigate Shoals atoll to the MHI during late summer/early fall, potentially to give birth. Females with core home ranges within the MHI showed limited movements to the NWHI, and immigration to an island was better explained by SST and chlorophyll a concentration, suggesting a foraging function. Dispersal patterns in tiger sharks are complex but can be considered a mix of skipped-breeding partial migration by mature females and individual-based inter-island movements potentially linked to foraging. Therefore, sharks appear to use a conditional strategy based on fixed intrinsic and flexible extrinsic states. The application of Brownian bridge movement models to electronic presence/absence data provides a new technique for assessing the influence of habitat and environmental conditions on patterns of movement for fish populations.

Habitat geography around Hawaii's oceanic islands influences tiger shark (Galeocerdo cuvier) spatial behaviour and shark bite risk at ocean recreation sites.

We compared tiger shark (Galeocerdo cuvier) spatial behaviour among 4 Hawaiian Islands to evaluate whether local patterns of movement could explain higher numbers of shark bites seen around Maui than other islands. Our sample consisted of 96 electronically-tagged (satellite and acoustic transmitters) tiger sharks, individually tracked for up to 6 years. Most individuals showed fidelity to a specific 'home' island, but also swam between islands and sometimes ranged far (up to 1,400 km) offshore. Movements were primarily oriented to insular shelf habitat (0-200 m depth) in coastal waters, and individual sharks utilized core-structured home ranges within this habitat. Core utilization areas of large tiger sharks were closer to high-use ocean recreation sites around Maui, than around Oahu. Tiger sharks routinely visited shallow ocean recreation sites around Maui and were detected on more days overall at ocean recreation sites around Maui (62-80%) than Oahu (<6%). Overall, our results suggest the extensive insular shelf surrounding Maui supports a fairly resident population of tiger sharks and also attracts visiting tiger sharks from elsewhere in Hawaii. Collectively these natural, habitat-driven spatial patterns may in-part explain why Maui has historically had more shark bites than other Hawaiian Islands.

Growth and maximum size of tiger sharks (Galeocerdo cuvier) in Hawaii.

Tiger sharks (Galecerdo cuvier) are apex predators characterized by their broad diet, large size and rapid growth. Tiger shark maximum size is typically between 380 & 450 cm Total Length (TL), with a few individuals reaching 550 cm TL, but the maximum size of tiger sharks in Hawaii waters remains uncertain. A previous study suggested tiger sharks grow rather slowly in Hawaii compared to other regions, but this may have been an artifact of the method used to estimate growth (unvalidated vertebral ring counts) compounded by small sample size and narrow size range. Since 1993, the University of Hawaii has conducted a research program aimed at elucidating tiger shark biology, and to date 420 tiger sharks have been tagged and 50 recaptured. All recaptures were from Hawaii except a single shark recaptured off Isla Jacques Cousteau (24°13'17″N 109°52'14″W), in the southern Gulf of California (minimum distance between tag and recapture sites  =  approximately 5,000 km), after 366 days at liberty (DAL). We used these empirical mark-recapture data to estimate growth rates and maximum size for tiger sharks in Hawaii. We found that tiger sharks in Hawaii grow twice as fast as previously thought, on average reaching 340 cm TL by age 5, and attaining a maximum size of 403 cm TL. Our model indicates the fastest growing individuals attain 400 cm TL by age 5, and the largest reach a maximum size of 444 cm TL. The largest shark captured during our study was 464 cm TL but individuals >450 cm TL were extremely rare (0.005% of sharks captured). We conclude that tiger shark growth rates and maximum sizes in Hawaii are generally consistent with those in other regions, and hypothesize that a broad diet may help them to achieve this rapid growth by maximizing prey consumption rates.