快猫短视频

IT WAS an unusually windy day on Heron Island in Australia鈥檚 Great Barrier Reef. From the beach near the island鈥檚 small research station, I could see a few hundred metres of calm, shallow water. But beyond that, the seabed dropped away quickly and the ocean was being whipped into fierce waves. There was no way we would be going out to count fish in our little boat.

Life as a marine biologist does have moments like this. You travel to some far-flung spot to observe amazing marine life, only for the elements to turn against you or your cameras to malfunction. I wandered back to the station, wondering how to make the best of this bad luck, and my eyes fell on a huge photograph that gave me an idea.

It was a satellite image of the island and the surrounding lagoon, clearly showing hundreds of small patches of coral circled by bright 鈥渉alos鈥�. It almost looked like someone had taken an eraser to the image and run it around the edges of each reef. In truth, I knew these were areas of bare sand and that many coral reefs had them. I also knew that no one was sure what caused them.

I decided to wade out into the lagoon and take a look at the halos. We could learn something about them without needing to motor out into the rough, deeper water. That would at least be a contribution to science. I didn鈥檛 know at that moment that the riddle of the reef halos would draw me back to this place for years to come.

Reef halos were first discovered in the 1960s, but only a handful of biologists ever bothered to study them. The explanation that emerged for their formation was that halos form because of fear. Fish and invertebrates hide in a patch of coral and venture out to eat algae and seagrass that cover the seabed. They only dare stray a certain distance from cover in any direction for fear of predators, which explains why the cleared area in the midst of the lagoon鈥檚 algal meadows is circular. But no one had produced definitive evidence that this is the case.

It was 2010 and I had travelled to the island with my husband and collaborator , our 1-year-old daughter and my mother-in-law. For the next week, as the bad weather persisted, Josh and I waded around a kilometre into the lagoon in chest-deep water each day, towing equipment in a bin wrapped with a makeshift pool noodle flotation system, and trying not to step on giant stingrays.

We did a simple experiment, setting out weighted pegs on the seabed attached to strands of seaweed at intervals across the halos and into the areas carpeted with algae beyond. The next day, we saw definite signs of nibbling all the way out to about 9 metres from the central coral, after which the seaweed was mostly untouched. It appeared to neatly back up the fish fear hypothesis.

A year later, we returned to Heron Island to carry out the research we had planned to do on our previous trip. This was part of an effort to count fish and invertebrates at sites along the east coast of Australia and so better understand how fishing was affecting them. We had five of Josh鈥檚 graduate students in tow to help. But I couldn鈥檛 get out on the high seas this time either. I was pregnant with our second child and had terrible morning sickness 鈥� boating was out of the question.

鈥淲e waded out into the lagoon towing our equipment in a bin and trying not to step on stingrays鈥�

Stuck in the research station, my thoughts turned once more to the halos. I had begun to wonder if they might be a tool that could help with my wider research into overall reef health. If formation of the halos was driven by the fear small fish have of being eaten, the number of predators in the area should be linked to whether these bare patches appear and how big they are. With fewer predators, you would expect the grazing fish to be less fearful and so venture further from the reef, resulting in wider halos. If we could prove that, the halos might offer an indirect way to measure the health of reef ecosystems, one that could be spied from space, with none of the inconvenience of seasickness.

I enlisted the help of Josh鈥檚 students and started surveying satellite images of the Great Barrier Reef to see if halos were present and, if so, whether they were larger around reefs where predators had been extensively fished. There is a well-defined network of areas where fishing is and isn鈥檛 allowed that provided an opportunity to compare the two.

The work took a long time to complete, but even as the results from a few reefs rolled in, we could see the pattern we expected wasn鈥檛 borne out. Halos seemed no different in size on reefs where predators could be fished or on those that were protected. Our experiments with the pegs and seaweed had supported the fear-driven halos hypothesis, but the satellite imagery seemed to tell a different story.

Rather puzzled, I returned to Heron Island in 2013, this time with two colleagues from the University of Queensland. The plan was to set up a small army of GoPro cameras around the reefs to identify which creatures were creating the halos. But on the first day, as we ventured into the lagoon by boat, we found the halos had vanished. It was beginning to feel like these strange sea circles were playing tricks on me.

I looked back at some historical Google Earth images of the island and saw that, sure enough, the lagoon was full of halos in some years, while in others they were nowhere to be seen. This just added to the halos鈥� mystery.

We decided to set up the cameras anyway. Then we had to watch all the footage and catalogue every fish and invertebrate that appeared, minute by painstaking minute. From the underwater photographs of the seabed that we also took, we realised that the halos were actually still there, it was just that they were too faint to be seen from the water鈥檚 surface, presumably because the algae wasn鈥檛 as lush as usual. But the truly surprising thing was that none of the plant-eating fish we filmed ever strayed all the way to the edge of the halos. What, then, was devouring the seaweed way out there, in the riskiest territory?

The only possibility we could think of was that other creatures were clearing the algae under cover of darkness, when we couldn鈥檛 see them. We wanted to test this, but had no way to film at night. Using lights to illuminate the seabed might change the wildlife鈥檚 behaviour. Another option was the 鈥渋nvisible鈥� infrared cameras used to film land animals at night. However, infrared radiation doesn鈥檛 travel far through water, only illuminating up to about 30 centimetres from the camera.

In 2016, I led yet another trip to Heron Island. By now, we had solved the night filming problem by going to the far red end of the visible spectrum. It turns out that many fish can鈥檛 see this light, but it is enough to illuminate the seabed for human eyes. We experimented with all sorts of ways of creating this light, but eventually found that a handheld, commercial dive light covered with a red film worked well enough to be picked up by a GoPro camera. This set-up revealed that fish called emperors and sweetlips were foraging for invertebrates buried in the sand, disrupting the algae out to the halos鈥� edges. We cheered when we saw this on video. It felt like the mystery was solved.

When reefs breathe

That was a little hasty. Soon after that trip, we had the full data back from our satellite survey of the whole Great Barrier Reef, the one I had begun with Josh鈥檚 students. This showed that, although halos are more common on reefs that are protected from fishing and so should have more predators, they weren鈥檛 any smaller on average in protected areas. It suggested that the fish we had seen grazing and foraging in the halos couldn鈥檛 be the whole story 鈥� there must be other factors too.

What those factors are is a question we are now trying to answer. We have a number of clues. For one, we are starting to find that the overall number of fish of all types 鈥� not just the predators 鈥� in the vicinity of a reef seems to affect the halo size, but in surprising ways that we are struggling to grasp. If we can understand what these patterns are, and if they hold true at reefs in different locations, it might explain more of the riddle.

鈥淭he halos appeared to blink on and off the reefscape, like lights on a Christmas tree鈥�

There might also be environmental drivers. We have recently investigated historical satellite imagery of other places around the world where we have found halos. We see similar patterns in lots of places. From time to time, halos blink on and off the reef, like lights on a Christmas tree, with no apparent relation to things like seasons, temperature, wind or water motion. Even stranger, we have seen that many halos in an area can change size at the same time, almost as though the reefscape is breathing, but again with no obvious relationship to environmental influences.

We don鈥檛 yet have all the answers. But it is clear that, as with so many natural phenomena, reef halos are far more interesting than we first assumed. This week, my colleagues and I have published much of the research we have conducted so far, in the hope that we and others can eventually get to the bottom of it.

I still believe that halos will ultimately prove to be a handy tool for quickly and inexpensively revealing aspects of change in coral reef ecosystems without scientists or conservationists having to get their feet wet. It is fitting, then, that it was trouble with getting out on the water that started me down this reef halo rabbit hole in the first place.