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. Rohner^1,2*, Anthony J. Richardson^2,3, Clare E. M. Prebble¹, Andrea D. Marshall^1,4, Michael B. Bennett⁵, Scarla J. Weeks⁶, Geremy Cliff^7,8, Sabine P. Winter^7,8 and Simon J. Pierce^1,4

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.

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