Is visiting Australia as a marine biologist a happy or sad trip to make when the Great Barrier Reef is under such threat?

It’s always a happy experience. It’s a country I have admired and loved, from afar and up close in person for years since I first started visiting in the 1970s. But there are concerns of course – and not just in Australia – about the impact 7 billion people are having on a planet that doesn’t get any bigger.

It causes a kind of darkness and hope. Because now we know what we did not know and could not know. Going back even 10 years, we couldn’t see some of the issues that are now sharpening into focus.

Since your 2009 TedTalk on protecting our oceans, do you see things changing for the better or worse?

The good news is that awareness is growing of why the oceans and natural world matter – things we used to take for granted. But there’s dismay as I think of how vulnerable our actions leave us. About half of the reefs globally have disappeared or are in a state of sharp decline. Then there’s the acidification of the ocean. That was really just beginning to be noticed in 2009. Now it’s a very high concern.

To the extent there has been improvement, we must recognise what had led to that improvement and not ever go back – for example, by placing restraints on the large industrial scale of fishing. We know what problems that has caused and continues to cause. There should be no reason to back off on that.

What would you say to the Australian government if you had its ear?

Once you know what you know and see what you see, it’s irresponsible not to do what you can to share that knowledge. This is not a matter of opinion, this is about the evidence. I feel fortunate to have spent thousands of hours under the ocean as a scientist over a period of many decades collecting that evidence. In the process, it has given me a perspective on the before and the after – what has happened to the Great Barrier Reef and the Coral Sea and other areas around Australia.

The good news is that more places are being protected. The Australian government established plans in 2012 for an exclusion zone, a great move that has inspired other countries to deal with reef erosion too. Now that the present government has put a hold on that, around the world there are people holding their breath, hoping it will still be put into action.

You speak of ‘tomorrow’s child’ in your documentary Mission Blue – who is that exactly?

I’ve often observed that some of the world’s great policy meetings say they do their best to represent what they call “the stakeholders”. For four years, I was on the International Whaling Commission making policy about the fate of whales. And it struck me: there are two constituents missing. The whales aren’t here to express their view. Nor are those from the future, 50 years ahead, who will inherit the consequences of the decisions that are made now. That’s tomorrow’s child.

We have to represent those who aren’t at the table. Think for them, speak for them, imagine for them the consequences of our actions, and not just in the short, self- centred term. Think in terms of how it will impact future generations.

You were part of the first all-female team of aquanauts in 1970. What does feminism mean to you?

I applied to be part of the Tektite living under water project in 1969. It didn’t occur to me that women were not expected to apply – the form did not make that clear. Most scientists and divers at the time were men but some of us applied anyway. And it caused consternation. The head of the program, James Miller, made history when he said: “Well, half the fish are female. I guess we could put up with a few women.”

They could have said no. All the astronauts at the time were men. But in the end, the women’s team held their own. We had more hours in the water than any of the men’s teams. We produced great research results. It opened the eyes of a lot of people. And they said that our success in the program helped break down barriers, allowing women to become astronauts. I hope that’s the case. I don’t like to think what would have happened if we’d lived up (or down) to expectations.

For someone who spends so much time under water, you have a way with words.

I’ve always loved language. I just wish I had greater confidence in speaking different ones. I speak fish. And whale. But I’ve long wished to express myself in the many languages that humans have developed. I was inspired by the scientist Thomas Huxley who wrote a book in 1857 that said you have an obligation as a scientist to communicate with the public. I speak from the scientist’s perspective but I also speak, I suppose, as a human being – from the heart.

The one thing you must always do is tell the truth. You can use many different kinds of words to say the same thing. You can speak in scientific jargon or shorthand. Or you can take that same information and explain what it means to an eight-year-old so that a child or anyone who hasn’t developed the language of science can understand. That’s what I’ve learned: you can tell the truth by telling a story.

The media likes to paint sharks as predator number one – are they?

They’re certainly not enemies. They are elements of the natural system. Occasionally a shark will take a bite. But we are not on their menu – they are on ours. We kill them for sport, kill them for their fins, their liver, their meat. But they ignore us for the most part. We shouldn’t really have trouble in their presence.

A successful dive is usually a dive where you are fortunate to see sharks of any sort. Their numbers have dropped precipatively since when I first began diving in the 1950s – 90% of them are gone, most of them in the last 30 years. We’ve become extremely good at killing them. If they take a bite from us, it should be no surprise.

They are not out to get us and actually the number of attacks has not increased. There are just many more of us. Some attacks in recent times may be related to sharks having fewer opportunities to find food. We’re consuming not just sharks, but what sharks eat. And if their normal food items are in short supply ...

Of the thousands of hours you have clocked under water, do you have a favourite dive?

Oh, there are so many. I was at a place called Marion Reef, diving in 70ft (21 metres) of water, and these grey reef sharks circled us. I could not count them, there were so many – at least 100. They were forming a great wheel around us but were quietly curious, not aggressive. It was a little hair-raising – had they chosen to gang up on us, they could have easily consumed us. But they were just looking. I remember it so well in my mind’s eye.

There are others dives I remember in Sydney Harbour. There are these giant cephalopods near the mouth of the harbour, cuttle fish as much as a metre long with great eyes. They are so beautiful, they look almost other-worldy. I’ve had some wonderful encounters with those lovely, curious, intelligent creatures.

How optimistic do you feel about our ocean’s future?

Cautiously optimistic. We are seeing an increase in awareness and an inroad into why the ocean matters and the consequences of not protecting it. We are relating the extinction of ocean wildlife to the carbon cycle which in turn is related to climate change. Excess carbon dioxide is affecting the ocean with acidification – that’s another big awakening. And with knowing comes caring. As information gets out there, people become tuned in. They can make personal decisions as well as joining with other to change policy and just do what it takes to ease the pressure.