Laser photogrammetry improves size and demographic estimates for whale sharks

Area of Science:

• Marine Biology
• Conservation Science
• Wildlife Ecology

Background:

• Whale sharks (Rhincodon typus) are globally threatened, with conservation efforts hindered by insufficient biological and demographic data.
• Accurate size and demographic information is essential for assessing the vulnerability and recovery potential of whale shark populations.

Purpose of the Study:

• To improve size estimations of free-swimming whale sharks using laser photogrammetry.
• To estimate biological parameters, including length at maturity and population structure, for whale sharks in Mozambique and Tanzania.
• To evaluate the suitability of laser photogrammetry for growth rate studies.

Main Methods:

• Laser photogrammetry was employed at two whale shark aggregation sites to measure total length (TL) of free-swimming individuals.
• Measurements of stranded whale sharks were combined with photogrammetry data to calculate length at 50% maturity for males.
• Repeat measurements over 1-3 years were analyzed to assess growth rates, with limitations noted for short-term studies.

Main Results:

• Individual whale sharks measured ranged from 420-990 cm TL across both sites.
• Length at 50% maturity for male whale sharks in Mozambique was calculated at 916 cm TL.
• Both populations exhibited a strong male bias (74-89%) and were predominantly immature (94-98%).

Conclusions:

• Laser photogrammetry offers a more accurate method for estimating whale shark size compared to visual estimates.
• The studied populations in Mozambique and Tanzania show demographic structures similar to other global aggregations, characterized by a male bias and immaturity.
• Further research focusing on smaller individuals, mature animals, and females is critical for effective conservation strategies for the threatened whale shark.