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New Scientific Report from the Marine Megafauna Foundation: Sharks and Lasers

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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.

View or download the full paper.

Marine Life & Conservation

Shark Trust launches Oceanic 31 Shark Art Auction

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After a two-year tour of UK art galleries, community spaces and aquariums, the Shark Trust’s acclaimed Oceanic31 exhibition takes its final bow at the Royal Geographical Society later this month. And the unique collection of artwork, depicting 31 species of oceanic sharks and rays, donated by 31 artists, is now open for bids from art lovers and shark enthusiasts. The online auction, launched today, will close on the 7th December at 8pm (BST). The money raised will support the Shark Trust Oceanics Programme.

People can now bid on 27 of the artworks by visiting this website:

https://superstars-auctions.com/sharktrustauction

It is a chance to own a beautiful piece of original art and to support the Shark Trust. The timing of the auction also means that these would make a very special Christmas gift for any shark-lover.

The diversity of pieces mirrors that of the sharks and rays they represent. You can bid on paintings, digital creations, sculptures, mixed media and more. You can pick your favourite artist or species of shark. Or you can select the perfect artwork to make a statement in your home or office. Whichever you choose, you will be supporting the work to protect these amazing animals.

One of the pieces of art has been selected to be auctioned live by Steve Backshall at the For the Love of Sharks event at the Royal Geographical Society in London on the 29th November. In addition to this, two further pieces will be raffled at this event, giving people a chance to win an incredible piece of shark art. For the Love of Sharks is the Shark Trust’s flagship evening. A night to celebrate sharks. Steve Backshall is the headline speaker at this event that will see other prominent shark advocates join him on stage.

Tickets for the event can be snapped up here:

https://thesharktrust.org.uk/Event/flos24

Those that would like to see the Oceanic 31 exhibition have one final chance. It is being displayed at the Pavilion at the Royal Geographic Society from 26th November until the 7th December. Entry is free.

Find out more here:

https://www.rgs.org/events/upcoming-events/oceanic-31

 Paul Cox, Shark Trust CEO, Said “This exhibition has given us the opportunity to reach out to a new audience. And inspire more people with the wonderful sharks and rays on which our Big Shark Pledge campaign is based. We are immensely grateful to the 31 artists who have worked so hard to create these works.”

Bid for your favourite Oceanic 31 artwork here:
https://superstars-auctions.com/sharktrustauction


Banner Image: Smooth Hammerhead by Alicia Hayden

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Gear News

Introducing the RS Series Drysuit from DYNAMICNORD

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drysuit

Cozy and warm in cold water.

It’s drysuit season all year round. With the new RS series premium drysuits from DYNAMICNORD, divers are perfectly equipped for all cold waters.

The sophisticated cut of the RS drysuits allows complete freedom of movement, providing maximum comfort. The high-quality 350 g/m² ripstop material with double ripstop zones on the elbows, knees, and crotch protect ensures extreme robustness as well as long life.

drysuit drysuit

The suit’s features leave nothing to be desired: A high-quality TiZip zipper, easily adjustable inlet and outlet valves, upgradeable with electrically heated underwear, HP bottleneck cuffs made of high-strength latex on the wrists, a 3mm neoprene neck cuff for maximum comfort, a 7mm hood with air outlet and 3mm boots made of double-sided, cross-layered neoprene with ankle straps and fin strap stoppers and a protective but very flexible sole.

drysuit

Noteworthy is the Molle fastening system, which can be used in a variable and compatible manner. This way, the two detachable leg pockets can be attached easily, safely and comfortably.

Alterations of boots to a different shoe size or adjustments to neck or wrist cuffs are done free of charge at the own DYNAMICNORD service centre in Germany in only a couple of days.

The matching Dynaskins made with high-quality Polartec® technology are available in various designs. As an undersuit under the drysuit, as a single suit in warm waters or as a two-piece suit. Polartec® technology’s main features are neutral buoyancy, breathablilty and quick-drying.

drysuit

Dynaskins are made largely from recycled plastic bottles.

The drysuit RS-351 for men and RS-352 for women are now available at your DYNAMICNORD dealer and in the online store.

Colors men and women: Black-Blue and Black-Burgundy

Sizes men: XS-4XL incl. intermediate sizes MS, MT, MLS, MLT, LS, LT, XLS, XLT, 2XLS, 3XLS, 4XLS

Sizes Women: XS-XL incl. intermediate sizes MS, MT, LS, XLS

DYNAMICNORD – Your Outdoor Companion.

www.dynamicnord.com

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