Simple criteria to determine detachment point of towed satellite tags provide first evidence of return migrations of whale sharks (Rhincodon typus) at the Galapagos Islands, Ecuador

Abstract

Background: Attachment of towed, floating satellite tags to large marine organisms has provided scientists with a wealth of information on the movements of these species. These tags generally are not programmed to detach at a particular time, yet are often prone to detachment by natural means after only a few days or weeks. It is important to be able to distinguish between the tracks provided by a detached, floating tag, and one that is attached to the subject. To this end, we placed three SPOT-5 and one SPLASH tag on large female whale sharks at Darwin Island (Galapagos Islands), and compared their tracks with those of two floating SPOT-5 tags released at the same site. We present a set of criteria to determine whether a towed satellite tag encased in a float is still attached to the study organism. Results: None of the whale sharks remained in the vicinity of the island. Three of the tracks lasted 31 to 48 days, yet one shark was tracked for 167 days. This was the first recorded bona fide homing migration of a whale shark, travelling 1,650 km west then returning to Darwin four months later. Two other sharks also returned to Darwin. We found that at the time of detachment, a clear change in the daily timing and quality of transmissions became evident. This, in conjunction with daily depth and temperature summaries, and knowledge of currents and the biology of the subject, can be used to justify endpoints on tracks that continue to accrue positions as the tag floats with the currents.

Peer Review #1 of "Laser photogrammetry improves size and demographic estimates for whale sharks (v0.1)"

“…7). By contrast, the coastal aggregation in Saudi Arabia had about equal numbers of juvenile males and females, whereas offshore sites in the Gulf of California and the Galapagos Islands mainly had large females (Ketchum, Galván-Magaña & Klimley, 2012;Ramírez-Macías, Vázquez-Haikin & Vázquez-Juárez, 2012b;Hearn et al, 2013;Berumen et al, 2014). The apparent sex bias and the narrow size range of whale sharks across the Indian Ocean aggregation sites raises intriguing questions concerning the location of newborn, female, and larger mature sharks.…”

“…The reason for the prevalence of juvenile male whale sharks at known aggregation sites is unclear, and although different diet preferences for juveniles and adults has been suggested (Ketchum et al, 2012), this does not explain the sex bias. The segregation (Hazin et al, 2008;Hearn et al, 2013;Afonso, McGinty & Machete, 2014). Their larger size and superior swimming efficiency may enable them to move further horizontally and vertically and thus forage more successfully in a patchy offshore prey landscape (Sims et al, 2006).…”

Peer Review #1 of "Satellite tagging highlights the importance of productive Mozambican coastal waters to the ecology and conservation of whale sharks (v0.1)"

“…Transmitted data included tag location and accuracy (location classes 3, 2, 1, 0, A, B, Z), as well as sea surface temperature (SST) at the time of transmission. We used standard methods by Hearn et al (2013; time of transmissions and time-at-temperature data) to determine when a tag detached from the shark, and removed the floating portion of the tracks before analyses were conducted. We only used location classes 3, 2 and 1 for further analyses.…”

Peer Review #1 of "Satellite tracking of juvenile whale sharks in the Sulu and Bohol Seas, Philippines (v0.1)"

“…Locations transmitted before tag deployment, and after the tag detached and floated, were removed. The latter situation was detected through transmission of constant temperature histograms and early morning transmissions (00.00-03.00 hh) over five Manuscript to be reviewed consecutive days (Hearn et al 2013). Locations on land (10.7% of total transmissions) were removed by extracting bathymetry data from the ETOPO dataset (Amante & Eakins, 2009) for each location, using the xtractomatic package in R (Mendelssohn, 2017).…”