Opportunistic visitors: long-term behavioural response of bull sharks to food provisioning in Fiji

Abstract

Shark-based tourism that uses bait to reliably attract certain species to specific sites so that divers can view them is a growing industry globally, but remains a controversial issue. We evaluate multi-year (2004-2011) underwater visual (n = 48 individuals) and acoustic tracking data (n = 82 transmitters; array of up to 16 receivers) of bull sharks Carcharhinus leucas from a long-term shark feeding site at the Shark Reef Marine Reserve and reefs along the Beqa Channel on the southern coast of Viti Levu, Fiji. Individual C. leucas showed varying degrees of site fidelity. Determined from acoustic tagging, the majority of C. leucas had site fidelity indexes >0.5 for the marine reserve (including the feeding site) and neighbouring reefs. However, during the time of the day (09:00-12:00) when feeding takes place, sharks mainly had site fidelity indexes <0.5 for the feeding site, regardless of feeding or non-feeding days. Site fidelity indexes determined by direct diver observation of sharks at the feeding site were lower compared to such values determined by acoustic tagging. The overall pattern for C. leucas is that, if present in the area, they are attracted to the feeding site regardless of whether feeding or non-feeding days, but they remain for longer periods of time (consecutive hours) on feeding days. The overall diel patterns in movement are for C. leucas to use the area around the feeding site in the morning before spreading out over Shark Reef throughout the day and dispersing over the entire array at night. Both focal observation and acoustic monitoring show that C. leucas intermittently leave the area for a few consecutive days throughout the year, and for longer time periods (weeks to months) at the end of the calendar year before returning to the feeding site.

Figure 1. Receiver array on the southern coast of Viti Levu, Fiji.

(A) Off Shark Reef (inset) stations (7–16), and (B) locations of Shark Reef stations (1–6).

Figure 2. Fast Fourier transform of the time series of number of detections per hour at Station 1 for one representative C. leucas individual (ID 75).

A 24 h pattern was evident for 11 of the 12 C. leucas tested (see text for details). The x-axis shows the frequency, a function of periodicity, and the y-axis is the spectral density, indicating the most important cycle periodicities.

Figure 3. 24 h timeline showing the period of time a C. leucas was present at Station 1 on non-feeding and feeding days.

Each line in the timeline (y-axis) represents an individual's occurrence at Station 1 on a given day. The red shaded area denotes the time of the day when feeding occurred at the Shark Reef Marine Reserve (09:00–12:00).

Figure 4. Circular plots showing the percentage of hours C. leucas were detected at Station 1, Shark Reef and off Shark Reef over the 24 h diel cycle.

Figure 5. Circular plots showing the percentage of hours C. leucas were detected at grouped stations over the 24 h diel cycle.

Sample sizes (i.e. number of hours): Shark Reef, n = 12,688; Station 7, n = 438; Stations 8–10, n = 514; Stations 11–16, n = 1,466.

Figure 6. Site fidelity indexes (SFI_v) determined from direct observation of visually identifiable C. leucas at the Shark Reef Marine Reserve between 2004 and 2011.

Mean SFI_v values of 25 ‘unequivocal’, 9 ‘easy’ and 14 ‘challenging’ to identify C. leucas individuals (see text for details). SD  =  variation between years. Red dots denote individuals for which also acoustic monitoring data are available (see Table 3). Refer to Table S1 in for description of natural marks of individuals.

Figure 7. Examples of intraspecific variation of C. leucas monthly encounter rates.

Representative examples of mean (±SD) monthly SFI_v values from (A) Rip, (B) Crook, (C) Hook and (D) Long John. Red dots denote SFI_v values; blue dots denote SFI_a 1_9-12 values. Standard deviations represent variation between years (SFI_v = 2004–2011; SFI_a = monitoring period). Refer to Table S1 in for description of natural marks of individuals.

Figure 8. Timeline of the daily detections of C. leucas monitored for >50 days.

Grey dots denote detections at Shark Reef (Stations 1–6) and black dots detections off Shark Reef (Stations 7–16).

Figure 9. Time-depth series of satellite tagged C. leucas Rip.

In addition to acoustic tag ID 32, the female shark was tagged with a PSAT (standard rate X-Tag; for tag specifications see F14 in [20]) for 9 days in February 2009. Red dots denote acoustic detections at Shark Reef (Stations 1–6) and off Shark Reef (Stations 7–16). Grey shaded areas denote night (18:00–06:00); light red shaded areas denote days and time (09:00–12:00) of feeding.