Multi-method assessment of whale shark (Rhincodon typus) residency, distribution, and dispersal behavior at an aggregation site in the Red Sea

Abstract

Whale sharks (Rhincodon typus) are typically dispersed throughout their circumtropical range, but the species is also known to aggregate in specific coastal areas. Accurate site descriptions associated with these aggregations are essential for the conservation of R. typus, an Endangered species. Although aggregations have become valuable hubs for research, most site descriptions rely heavily on sightings data. In the present study, visual census, passive acoustic monitoring, and long range satellite telemetry were combined to track the movements of R. typus from Shib Habil, a reef-associated aggregation site in the Red Sea. An array of 63 receiver stations was used to record the presence of 84 acoustically tagged sharks (35 females, 37 males, 12 undetermined) from April 2010 to May 2016. Over the same period, identification photos were taken for 76 of these tagged individuals and 38 were fitted with satellite transmitters. In total of 37,461 acoustic detections, 210 visual encounters, and 33 satellite tracks were analyzed to describe the sharks’ movement ecology. The results demonstrate that the aggregation is seasonal, mostly concentrated on the exposed side of Shib Habil, and seems to attract sharks of both sexes in roughly equal numbers. The combined methodologies also tracked 15 interannual homing-migrations, demonstrating that many sharks leave the area before returning in later years. When compared to acoustic studies from other aggregations, these results demonstrate that R. typus exhibits diverse, site-specific ecologies across its range. Sightings-independent data from acoustic telemetry and other sources are an effective means of validating more common visual surveys.

St. Helena: An Important Reproductive Habitat for Whale Sharks (Rhincodon typus) in the Central South Atlantic

“…This confirms the value of validating visual observations with unsupervised passive acoustic monitoring as suggested by Cagua et al (2015); Cochran et al (2019). Residence indices were similar to those from other seasonal sites (Shib Habil: R max = 0.36, R min = 0.05) and in contrast to sites that had cryptic residency of whale sharks year-round (Mafia: R min = 0.24) further supporting the seasonal nature observed from the sighting data (Cagua et al, 2015;Cochran et al, 2019). The maximum residence indexes calculated for tagged sharks shows that they spend roughly 23% of their time, or ∼84 days, in St. Helena waters.…”

“…Seasonality of detections in the array matched those of surface-based assessments, viz animals begin to arrive at the island in November and December, with the aggregation peaking in January and petering out by May. This confirms the value of validating visual observations with unsupervised passive acoustic monitoring as suggested by Cagua et al (2015); Cochran et al (2019). Residence indices were similar to those from other seasonal sites (Shib Habil: R max = 0.36, R min = 0.05) and in contrast to sites that had cryptic residency of whale sharks year-round (Mafia: R min = 0.24) further supporting the seasonal nature observed from the sighting data (Cagua et al, 2015;Cochran et al, 2019).…”

“…Maximum residence index (R max ) was calculated by dividing the number of days an individual was detected in the array by the total number of days between initial tagging and last detection. Both indices are directly affected by study duration, which can bias animals that were tagged later in the course of the study or individuals that were detected over short periods of time (Cochran et al, 2019), therefore, both results are reported in an attempt to provide upper and lower values for true residency.…”

“…Insights from these tags have shown that whale sharks are capable of very large horizontal movements (e.g., Hueter et al, 2013) and able to conduct deep dives into the bathypelagic zone (e.g., Tyminski et al, 2015). Acoustic telemetry can help explore residency patterns (such as site use and fidelity) (Cagua et al, 2015), fine-scale horizontal movements, and validate photographic identification data (Cochran et al, 2019). Used in combination, these techniques can elucidate both the large and fine-scale movements of whale sharks in a particular area.…”

Regional Movements of Reef Manta Rays (Mobula alfredi) in Seychelles Waters

“…Some of the shortcomings of satellite and acoustic tagging, and photo-identification techniques, may be overcome by combining these approaches (Meyer et al, 2010;Kneebone et al, 2014;Vianna et al, 2014;Braun et al, 2015). Detections of acoustic tags within receiver arrays, individually or combined with sighting records of tagged individuals at aggregation sites, can be used to reduce the uncertainty surrounding location estimates derived from geolocation analyses (Werry et al, 2014;Cochran et al, 2019). This combined approach improves the accuracy of individual tracks provided by archival pop-up tags.…”