That makes them difficult to study. And I should know. I first started working on whale sharks, the world’s largest fish, when I moved to Mozambique back in 2005. They’ve stymied and frustrated me ever since.

Unlike a lot of scientific work, the fundamentals of whale shark field research aren’t complicated to explain. Every shark has a distinctive and unique pattern of white spots. That makes it easy to identify individuals. I can take a photo of each side of the shark and work out who it is later. Assigning a sex isn’t challenging either. I dive underneath each shark to look for claspers, the male reproductive organ, on their inner pelvic fins. If there are claspers, it’s a male. No claspers? It’s a female.

How big is it? Well, that is more difficult to answer. First, I tried guessing their length. Unfortunately, it doesn’t work well. When I was first processing whale shark sizes, including only size estimates from trained, experienced research staff, I found that individual sharks had been assigned size estimates of 3-8 metres over just a few years. Not helpful.

Some time after, I was chatting to a friend studying sharks in Australia. She told me about parallel lasers. You aim the two lasers at the side of the shark, take a photo, and use the laser dots as a scale. Awesome.

To test that approach, my research buddy Andrea Marshall and I built a frame with attachments for the lasers to measure manta rays and whale sharks. We are very clever, so we made it out of stainless steel to ensure it wouldn’t rust. It weighed 10 kg. It almost drowned Andrea. Didn’t rust though. Phase two was rebuilt from marine aluminium. It worked. We now had two green laser dots on each shark, calibrated to be exactly 50 cm apart, providing a scale bar to measure total length. That allowed us to determine that male and female whale sharks both become adults at 8–9 metres.

How do we know? I mentioned above that male sharks have paired reproductive organs called claspers. The claspers rapidly increase in size as the shark hits the final stage of maturity. We can measure clasper size using the lasers, too. This may be an invasion of privacy, but at least we don’t have to physically measure them. Lasers are better.

It’s harder to confirm adulthood in female sharks. Only one female, harpooned in Taiwan in 1995, was definitely pregnant. She was 10.6 metres in length and had 304 tiny pups inside. Field studies off Mexico and the Galapagos, where large females are seen around seamounts, have shown that the smallest females presumed to be pregnant are about 9 metres long.

We also know, from fisheries records, that whale sharks attain a maximum length of 18-20 metres. A shark that size is old. Exactly how old, though? We don’t know. Sensing a theme yet?

Most species of sharks have growth rings, like a tree, in the middle of their vertebral column. Whale sharks have rings too, but… it’s complicated. A tree doesn’t move. It grows faster or slower depending on environmental conditions. That means you’ll get a “light” band in summer, when the tree was growing fast, and a dense ‘dark’ band in winter, when it grew slowly.

Whale sharks give a bubbly chuckle at the concept of winter. They can easily swim over 10,000km each year, which lets them choose their own preferred water temperature. That screws up their growth rings, as we can’t tell if they correspond with a calendar year. Fortunately, by re-sighting and re-measuring individual sharks in the wild, we can determine their growth under natural conditions.