News
New Scientific Report from the Marine Megafauna Foundation: Sharks and Lasers
Laser photogrammetry determines whale shark size segregation and life-history parameters to improve conservation management of the world’s largest fish
The whale shark (Rhincodon typus) is the world’s largest fish. Due to their large size, their tendency to spend a lot of time at the surface and predictable aggregating behaviour in known coastal areas, they are vulnerable to human threats such as directed fisheries, boat strikes and net entanglement. They are globally threatened, but a lack of biological and demographic information prevents an accurate assessment of their vulnerability to further decline or capacity to recover. Determining life-history parameters is therefore crucial to improving whale shark management.
The Marine Megafauna team used paired-laser photogrammetry to describe the size ranges of sharks, assess the length at which males reach sexual maturity and examine whether this technique can detect growth rates over a one to three year period, at two coastal aggregation sites in the Western Indian Ocean; Tofo Beach in southern Mozambique and Mafia Island in Tanzania.
The findings highlight that the size range and sex ratio of whale sharks from Mozambique and Tanzania are similar to those at most other coastal aggregation sites globally, in that mostly male juvenile sharks are present. The size at maturity was ~200 cm longer than for whale sharks in the Atlantic, suggesting biological differences among sharks in the two oceans. Growth rates could not be determined because the slow growth of the sharks in combination with the accuracy of photogrammetry means that a longer time interval is needed. The population segregation by size and sex observed here and elsewhere reinforces the need to determine the whereabouts of newborn sharks, immature female sharks, and mature sharks of both sexes to improve conservation and management for this globally threatened species.
“The segregation by size and sex in whale sharks is intriguing – where are we going to find the newborns, females and mature sharks in the Indian Ocean?”
Laser photogrammetry improves size and demographic estimates for whale sharks
Christoph A. Rohner1,2*, Anthony J. Richardson2,3, Clare E. M. Prebble1, Andrea D. Marshall1,4, Michael B. Bennett5, Scarla J. Weeks6, Geremy Cliff7,8, Sabine P. Winter7,8 and Simon J. Pierce1,4
1 Marine Megafauna Foundation, Praia do Tofo, Inhambane, Mozambique
2 CSIRO Oceans and Atmosphere Flagship, Brisbane Queensland 4102, Australia
3 Centre for Applications in Natural Resource Mathematics (CARM), School of Mathematics and Physics, The University of Queensland, St Lucia Queensland 4072, Australia
4 Wild Me, Praia do Tofo, Inhambane, Mozambique
5 School of Biomedical Sciences, The University of Queensland, St Lucia Queensland 4072, Australia
6 Biophysical Oceanography Group, School of Geography, Planning and Environmental Management, The University of Queensland, St Lucia Queensland 4072, Australia
7 KwaZulu-Natal Sharks Board, Umhlanga 4320, South Africa
8 Biomedical Resource Unit, University of KwaZulu-Natal, Durban 4000, South Africa
Abstract
Whale sharks Rhincodon typus are globally threatened, but a lack of biological and demographic information hampers an accurate assessment of their vulnerability to further decline or capacity to recover. We used laser photogrammetry at two aggregation sites to obtain more accurate size estimates of free-swimming whale sharks compared to visual estimates, allowing improved estimates of biological parameters. Individual whale sharks ranged from 432–917 cm total length (TL) (mean ± SD = 673 ± 118.8 cm, N = 122) in southern Mozambique and from 420–990 cm TL (mean ± SD = 641 ± 133 cm, N = 46) in Tanzania. By combining measurements of stranded individuals with photogrammetry measurements of free-swimming sharks, we calculated length at 50% maturity for males in Mozambique at 916 cm TL. Repeat measurements of individual whale sharks measured over periods from 347–1068 days yielded implausible growth rates, suggesting that the growth increment over this period was not large enough to be detected using laser photogrammetry, and that the method is best applied to estimating growth rates over longer (decadal) time periods. The sex ratio of both populations was biased towards males (74% in Mozambique, 89% in Tanzania), the majority of which were immature (98% in Mozambique, 94% in Tanzania). The population structure for these two aggregations was similar to most other documented whale shark aggregations around the world. Information on small (<400 cm) whale sharks, mature individuals, and females in this region is lacking, but necessary to inform conservation initiatives for this globally threatened species.
For information on this publication please email chris@marinemegafauna.org.
Blogs
Four opportunities to go pro in 2024 with Dive Friends Bonaire
Dive Friends teaches the Instructor Development Course (IDC) several times a year to students who are eager to share their passion for diving with the world.
Dive Friends is known for the personal approach throughout the course. Their in-house course director will lead the students through every essential step, mentoring them to achieve their fullest potential as a dive instructor.
Applications for the following IDC start dates are now open:
- 12 April
- 5 July,
- 20 September
- 29 November
Partnership with Casita Palma
If the student opts for the IDC-Deluxe or IDC-Supreme package, their accommodation will be arranged for them at Casita Palma. This small and quiet resort is within walking distance from Dive Friends Bonaire’s main dive shop location and has everything you need to relax after an intense day of IDC training. Breakfast is included, so the student will always be fuelled and ready for their day.
Contact Dive Friends Bonaire’s Course Director Eddy for more information: coursedirector@divefriendsbonaire.com.
Blogs
SOMABAY: Scubaverse Trip Review (Watch Video)
In a video created exclusively for Scubaverse.com, Jeff Goodman visits SOMABAY in the Egyptian Red Sea to experience the diving on offer there.
Book your next Red Sea dive adventure with SOMABAY! For more information, visit www.somabay.com.
Stay at the Breakers Diving & Surfing Lodge when you visit! For more information, visit www.thebreakers-somabay.com.
Find out more about ORCA Dive Clubs at SOMABAY at www.orca-diveclubs.com/en/soma-bay-en.
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