0:00 - In the waters of Mafia Island, Tanzania, lives an unusual population of whale sharks, the largest fish in the ocean and a species now classified as endangered.

0:09 - Unlike their migrating brethren, these whale sharks stay near the island all year long …

0:15 - A behavior never before observed in whale sharks.

0:21 - Researchers from the Marine Megafauna Foundation want to know why these whale sharks stick around.

0:35 - SIMON PIERCE, PRINCIPAL SCIENTIST AND RESEARCHER, MARINE MEGAFAUNA FOUNDATION:
The fishermen and the tourism operators here, they were only seeing whales sharks for a few months over the year over the summer. When we started tagging the sharks, though, with small acoustic tags—and we got a network of receivers in the bay — we started finding that actually the whale sharks don’t leave. The just move slightly further up shore and slightly deeper, and they don’t come to the surface quite as often.

1:04 - ALEXANDRA WATTS, RESEARCH ASSISTANT, MARINE MEGAFAUNA FOUNDATION:
One of my jobs while I’m in Mafia is to take tissue biopsies of the whale sharks. Usually those bits are tissues are less than one gram. So it’s a very small amount and the sharks, very very rarely react.

Some of the things these tissue samples can tell us about is that they can actually give us an idea of the location of the sharks and where they spend most of their time. For example, the profiles from the fatty acid analysis have shown that although the sharks do mostly feed on the shrimp here they also have other diet preferences out in deeper water and at night. So, we can actually use it to show that the sharks often stay in an area such as Mafia. We can prove that biochemically.

1:39 - The researchers theorize that the whale sharks stay because there is plenty of food throughout the year.

1:43 - The sharks’ constant presence makes Mafia Island and ideal place to study this mysterious species.

1:48 - SIMON PIERCE, PRINCIPAL SCIENTIST AND RESEARCHER, MARINE MEGAFAUNA FOUNDATION:
Even though whale sharks are quite a popular species with divers and scientists, there’s still a lot we don’t know them. And some of the stuff we don’t know is really fundamental. We don’t know how old they are, we don’t know when they become adults, we don’t know how old they get. Here we can get to know individuals so we can learn about what whale sharks are doing over their lifespan.
2:07 - SIMON PIERCE, PRINCIPAL SCIENTIST AND RESEARCHER, MARINE MEGAFAUNA FOUNDATION:
It’s pretty crazy that the whale sharks here often look like a mobile ecosystem. There’s loads of other species of fish feeding on the same stuff that’ll often associate with the whale shark. So you’ll get this huge cloud of fish biomass just moving around, often almost blocking the shark from view.


2:25 - For local fishermen, whale sharks have become a tool of their trade.

2:30 - Because smaller fish swim with the whale shark, they use the whale shark's size to spot their targeted catch.

2:38 –LIBERATUS MOKOKI, TOUR GUIDE, MAFIA ISLAND, TANZANIA:
I’m very proud to see the largest fish in the world. It’s very important for the whale shark because [it’s] the sign for the fish. We spot the mackerels and sometimes the tuna fish.

2:52 - Because the fishermen understand the value of the whale shark to the ecosystem and their sustained fishery, they make sure the shark are not harmed by their fishing activities.

3:03 - SIMON PIERCE, PRINCIPAL SCIENTIST AND RESEARCHER, MARINE MEGAFAUNA FOUNDATION:
The issues are likely to be if the fishermen accidentally get the whale shark in the net and it can be quite difficult and dangerous to free such a large animal. So one of things we’re working at here is just helping the fishers to be able to safely release them and as much as possible avoid catching them in the first place.


3:22 - SIMON PIERCE, PRINCIPAL SCIENTIST AND RESEARCHER, MARINE MEGAFAUNA FOUNDATION:
As a conservation biologist, one of the things that particularly motivates my research is that whale sharks are now a globally endangered species as of 2016. We think their populations have halved over the last few decades and that’s purely due to human influence. So we got to make sure that places like this remain a sanctuary for the whale sharks because they spend so much time here. We really got to try and minimize the human threats and help them recover as a species.