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

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 48days, yet one shark was tracked for 167days. This was the first recorded bona fide homing migration of a whale shark, travelling 1,650km 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. Conclusions: The data provided by floating tags is sufficiently distinct to be able to determine a detachment date. After detachment, daily transmissions are received in the first hours of the day after midnight (Coordinated Universal Time), the location quality of the transmissions is consistently high, and temperature or depth summaries are consistent with surface records. Prior knowledge of subject behavior and general ocean circulation patterns in the region reinforces the ability to determine detachment date. In some cases, after a prolonged period of more than three or four weeks, the detection pattern may change, yet caution should be exercised in assuming that the tag is, after all, still attached.