A new study by researchers from Marine Megafauna Foundation (MMF) and its collaborators highlights the need for long-term monitoring and precautionary management of whale shark populations.
The study, published in the journal Endangered Species Research found that, although the population of whale sharks monitored over eight years showed a stable trend in numbers, modelling applied to the data found that small changes in the population’s abundance would not have been detected. Only a dramatic change of around 25 per cent in the whale shark population would have been detected over the eight-year period. A decrease of this magnitude would have devastating consequences for a whale shark population, but as smaller changes take much longer to detect, the study recommends that precautionary long-term monitoring is crucial to saving the species from extinction.
The new study investigated the abundance and population trends of whale sharks aggregating off Mafia Island in Tanzania, a population known for its high residency, as the same individuals are routinely re-identified in the region. The researchers used ‘capture-mark-recapture’ (CMR) models used to calculate the probability of new sharks entering the population or of individual animals being re-sighted, and used the results to estimate the the population size in each year.
‘Mafia Island in Tanzania was the perfect place for this investigation,’ said Dr Chris Rohner, lead author of the study and Principal Scientist with MMF. ‘We see many of the same whale sharks year after year and have gotten to know them really well.’
‘These high re-sighting rates also mean that the models estimated abundance with a high level of precision,’ added MMF Senior Scientist, Dr Steph Venables.
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Whale sharks are identified by their spot pattern, unique to each individual shark in the same way fingerprints are to humans. This means that although the model used to calculate population trends is termed ‘capture -mark-recapture’, whale sharks can be identified without interference. ‘To get the data to monitor whale sharks, we swim alongside each shark and take a photo of their unique spot pattern,’ said Dr Clare Prebble, Senior Scientist at MMF. ‘Back on land, we let an algorithm compare the spots of the sharks we saw to the whole Sharkbook Database, where it either re-identifies them as previously seen sharks, or we get to add a new one to the catalogue.’
The number of whale sharks in Tanzania fluctuated over the study period, but the overall trend was stable. There are few direct threats to whale sharks at Mafia Island, although mortalities in fishing nets in the country are sometimes reported. ‘Our study shows that even a small level of mortality from human causes can stop whale sharks from recovering from historical overfishing in the Indian Ocean,’ said Dr Simon Pierce, MMF Founder and global authority on whale shark conservation. ‘Whale sharks take decades to reach adulthood and probably only breed every few years. We need to protect whale sharks, particularly at important aggregation sites like Mafia Island, to help them rebuild a healthy population.’
The study estimated a total of 208 whale sharks spent time at Mafia Island over the 8 years. ‘This is a comparatively small population and we need to look after these gentle giants,’ said Dr Baraka Kuguru from the Tanzania Fisheries Research Institute. ‘Whale sharks are important to Mafia Island’s economy, attracting marine tourists and local fishers often find their catch when they see whale sharks.’
‘Our case study in Tanzania highlights that whale sharks need protection even when abundance trends are not yet conclusive, which is known as precautionary management,’ said Dr Jesse Cochran from the King Abdullah University of Science and Technology, one of MMF’s research partners in the study. ‘Long-term monitoring over several decades is key to accurately assessing their trend.’