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

Abstract

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.

Figure 1

Location estimates for tiger sharks equipped with dorsal fin-mounted satellite transmitters (SPOT tags). Top panel: Overview of filtered Argos locations (yellow points) from 32 tiger sharks captured off Maui and Oahu. Red box indicates area shown in detail in bottom panel. Bottom panel: Tiger shark filtered Argos locations (yellow points) within coastal habitats of the Main Hawaiian Islands. Maps were created using R software v.3.1.2 (R: A Language and Environment for Statistical Computing, R Core Team, R Foundation for Statistical Computing, Vienna, Austria (2014) https://www.R-project.org).

Figure 2

Filtered Argos locations (yellow points) of six individual tiger sharks (M/F = sex, TL = Total Length in cm) showing concentration over insular shelf habitat (0–200 m depth, red shading). Light blue squares indicate tagging locations of each shark (Two were tagged off Oahu, four off Maui). Maps were created using R software v.3.1.2 (R: A Language and Environment for Statistical Computing, R Core Team, R Foundation for Statistical Computing, Vienna, Austria (2014) https://www.R-project.org).

Figure 3

Home range isopleths for tiger sharks captured around Maui and Oahu. Yellow square indicates original tagging location. Isopleth levels indicate the proportion of total satellite locations enclosed by the polygon (e.g. area in red = 10%, dark blue = 50%, light blue = 95%). Maps were created using R software v.3.1.2 (R: A Language and Environment for Statistical Computing, R Core Team, R Foundation for Statistical Computing, Vienna, Austria (2014) https://www.R-project.org).

Figure 4

Home range core use areas (0.1 iso level - translucent red polygons) of 19 satellite-tagged tiger sharks captured around Maui (n = 13) and Oahu (n = 6). Note the cluster of core areas in waters off SW Maui (overlapping core areas of 7 individuals were identified in waters between Maalaea and Makena). Light-blue shaded area indicates insular shelf (depth 0–200 m). Yellow points indicate locations of documented shark bite incidents along high recreational-use (pink) and low recreational-use (dark-blue) coastlines. Maps were created using R software v.3.1.2 (R: A Language and Environment for Statistical Computing, R Core Team, R Foundation for Statistical Computing, Vienna, Austria (2014) https://www.R-project.org).

Figure 5

Depth frequency distribution (mean ± standard error) of bathymetry corresponding with Argos locations of female (top panel) and male (bottom panel) tiger sharks equipped with dorsal fin-mounted satellite transmitters. Note that these depths do not represent the swimming depths of satellite-tagged sharks (see Supplementary Figs S13 & S14), but the depth of water below the shark when the satellite location was obtained while the animal was at the surface.

Figure 6

Depth and temperature time-series data from SPLASH-tag equipped tiger shark 132062. Top panel: Overview of entire time series. Bottom panel: Detail of 72 h sample of larger time series (from dashed box in upper panel). Yellow points are harvested bathymetry values (water depth below shark estimated from surface positions).

Figure 7

Matrix plot illustrating Site Fidelity Index (SFI) values for all Maui and Oahu-tagged sharks on all Maui & Oahu receivers. The blue bars indicate sharks tagged around Oahu, black bars are sharks tagged around Maui. Red points indicate the receiver station closest to the capture/tagging location of each shark. The dashed line bisecting the plot indicates the boundary between islands (Oahu receivers to the left, Maui receivers to the right).

Figure 8

Frame grabs from video recovered from shark-mounted camera deployed on 436 cm TL, sexually mature male tiger shark captured off Oahu in January 2015. (a) Rear of tiger shark approached by camera shark showing no evidence of claspers on the pelvic fins (1), indicating this shark is female. (b) View of dorsal surface showing evidence of mating scars (2) behind the trailing edge of the dorsal fin (3). (c) Profile view of female tiger shark as camera shark approaches, showing nictitating membrane is retracted (4). (d) As camera shark makes closer approach to female tiger shark, nictitating membrane can be seen entirely-covering the eye (5). See also Supplementary video.