Article written by Vanessa McCole, B.S. Marine Biology, Image provided for Thresher Shark Divers
Scientific Study: Acoustic Tracking of Bigeye Thresher Shark Alopias superciliosus in the Eastern Pacific Ocean by Hideki Nakano, Hiroaki Matsunaga, Hiroaki Okamoto, and Makoto Okazaki.
Migration Study
In July of 1996 off the coast of the Galapagos Islands, researchers used Acoustic Telemetry to study the movement ecology of 2 immature female Bigeye Thresher sharks. The females had a precaudal length of 175 and 124 cm and were tracked for 96 and 70 hours, respectively. This was the first time the movement of this species had been studied and it revealed a very interesting daily vertical migration in which the sharks would remain deep in the water during the day and would swim to the surface at night. It was also determined that the Bigeye Thresher is a primarily nocturnal species with a vertical distribution from the surface of the water to about 500 m which means that it tends to be found deeper in the water than the other Thresher shark species. The separation of depths may help reduce competition between the different species or may indicate differing niches. This study also provided information about the swimming speed, physiology, and niche of the Bigeye Thresher. Overall, this study helps us understand this creature better.
Daily Patterns Of The Bigeye Thresher
The larger Bigeye Thresher studied, Shark 1, stayed around a depth of 200 – 300 m during the day and 50 – 80 m at night, while the smaller shark, Shark 2, stayed around a depth of 400 – 500 m during the day and 80 – 130 m at night. Every day, the sharks would exhibit slow ascents at a low angle occurring about 30 minutes after sunset and quick descents at a high angle occurring about 30 minutes before sunrise. This behavior was remarkably predictable throughout the observation time. The researchers believe that the upward angled pupils of this species evolved to help them identify prey during their daily slow ascension. It may also be easier for them to identify prey which are highlighted against the surface from below.
Fig 1. Alopias superciliosus. Vertical movement of bigeye thresher Shark 1 superimposed on a 1 degree C isotherm plot drawn from expendable bathythermograph casts. Black bold lines along the upper horizontal axes indicate nighttime feeding.
Fig 2. Alopias superciliosus. Vertical movement of bigeye thresher Shark 2 superimposed on a 1 degree C isotherm plot drawn from expendable bathythermograph casts. Black bold lines along the upper horizontal axes indicate nighttime feeding.
A Strategy In Their Behavior
During the night, the sharks would exhibit large vertical oscillations. This behavior is thought to assist in hunting as larger prey require a more active hunting strategy. These oscillations did not occur during the day, instead, they would remain in a steady position, meaning this species most likely does not hunt during the day. They most likely remain at lower depths during the day to avoid predation. Their swim speed was determined to be around 1.32 to 2.02 km h–1 which is slower than most other pelagic sharks. Because of their slow speed, it is likely advantageous to avoid other sharks as much as possible, hence their hiding in deeper water during the daytime.
Fig 3. Alopias superciliosus. Recordings of bigeye thresher shark movements over a 4 h period, day and night. Oscillatory swimming patterns were recognized at night only. Lines represent a moving average of 3 records.
Water Temperature and Depth
It has been established that the sharks would stay around the same depths each night, but why? What caused them to choose the depths they resided in? It turns out that the sharks would return to depths with a desired temperature. During the day, Shark 1 would always return to a depth with a temperature of about 11°C and Shark 2 would always return to a depth of about 7 to 8°C. During the night, they would navigate through temperatures from 19 to 26°C. It seems the preferred temperature of each shark is what dictated their depth. The deepest dive ever recorded for this species was established during this study where Shark 1 travelled all the way to 723 m where the temperature was 5°C. This dive indicates that the Bigeye has a capacity for thermo-tolerance.
What Else?
There is a lot more to learn about this species. The study found that the sharks seemed to orient themselves westwardly along whatever current they were in, but the larger implications of this behavior is unclear. It is also unknown how the migration habits may change with maturity, size, or sex of the shark. What we do know is how their vertical location changes throughout the day which can be used to reduce bycatch of this species. Limiting longline fishing in shallow water during the day is one suggestion made by the researchers to help decrease such incidents. With more information of this and other species, fisheries can become more sustainable.