Fish & Trips
This article was printed in THE BULLETIN on July 20, 1999. It is reproduced with permission of the Author and Publisher. Text copyright Bob Beale - email@example.com.
Far from being at the mercy of the elements, the larvae of reef fish survive by prodigious feats of swimming - often into the open sea. It means, Bob Beale writes, that no coral reef can be considered self-contained in the battle to save ocean diversity.
A flamboyant angelfish weaves its way through the intricate maze of coral and stops abruptly to hover beside a nondescript rock. Then it gapes its mouth and flares its gills, looking for all the world as if it is about to throw up.
But no, it freezes in this odd position and glances sideways towards the rock. At this signal, a tiny torpedo-shaped figure with a fluoro-blue tail darts out from beneath a crevice and confidently approaches.
Incredibly, the smaller fish swims gently head first into the open gill covers of the stationary angelfish and starts pecking away at the delicate blood-red fronds of its gills.
We are on the Low Isles, a couple of island specks of marine reserve in the Great Barrier Reef, a short boat ride from Port Douglas in far north Queensland.
We are witnessing one of the marvels of reef ecology: the little guy is a common cleanerfish (a species of wrasse), who makes his living by setting up a drive-by cleaning station - in this case the rock - and waiting for trusting clientele to cruise by to have their parasites picked off with surgical precision.
Watch for a short while and you'll see a technicolour parade of all kinds of other fish who drop by and signal that they're ready to be preened as well.
The way the wrasse survives is a marvel in itself. But the gathering of all these fish in their coral oasis is the result of an even greater marvel. Most of them arrived from the open water as larvae, spiny tiddlers no bigger than your baby fingernail. They are the new recruits that ensure diverse fish communities and ecological balances are maintained.
As a general rule, reef fish make lousy parents. They spawn and expel their pin-head-sized eggs to the wide watery blue yonder in spring and summer and have nothing to do with them again.
Within a day of their conception a tiny tail emerges from the egg sac and the larvae gradually develop as free-swimmers that are carried off the reef and into open water for between two and 20 weeks, although many disappear down the throats of larger creatures.
It had long been assumed that where the survivors ended up was pretty much a matter of chance, that it was mainly good luck if they were carried by currents to another reef where they could settle and mature.
But thanks to the painstaking efforts of Australian and French researchers, a very different picture is now emerging. And apart from uncovering some intriguing natural history, the research has important implications for the way tropical marine reserves and fisheries are managed.
Far from being helpless drifters, it turns out that the many reef fish larvae are much better equipped to find a home than anyone ever realised. Not only are they relatively speedy swimmers, but they have extraordinary stamina and a mysterious ability to sense exactly where a reef is.
The first studies of their swimming abilities yielded stunning results. For her doctoral research, Ilona Stobutzki, now of CSIRO Marine Research in Queensland, built special laboratory raceways through which seawater flowed at a steady 13 centimetres a second. That's a pretty typical current speed around Lizard Island, the site of the research station where her experiments were carried out.
Then she caught wild reef fish larvae of many species and placed them in the raceways and monitored their performance. The largest of these larvae were no more than 2 cm long, and their physical abilities varied from species to species. But the best - surgeonfish larvae - swam without food or rest for eight days straight, travelling the equivalent of almost 100 kilometres.
"In nature, where they can rest and get food, they could certainly go a lot further," Stobutzki points out.
Those findings were initially met with disbelief by many researchers. After all, the top-performing larvae were doing the marathon equivalent of Susie Moroney swimming the Channel from Dover to Calais and back 10 times non-stop.
But the idea that these little creatures were mainly passive drifters was based on observations of the larvae of northern hemisphere cold water ocean fish, such as cod. As Stobutzki puts it, "We now know that they are pretty pathetic swimmers."
Related research by Jeff Leis, who leads a group at the Australian Museum, has shown that the swimming feats of the reef fish larvae are remarkable - not just for their endurance but for their pace as well.
Some can swim up to 50 cm - or 20 of their own body lengths-a second, much faster than most of the currents they encounter. If you could scale them up to human size, their performance would be the equivalent of Michael Klim swimming the 100-metre freestyle event in 2.5 seconds (the current human record for that event is about 48 seconds), Leis points out. With collaboration from colleagues at the University of Perpignan, France, and the Australian Institute of Marine Science, Leis' team has gone beyond the laboratory into the open ocean and reef lagoons of tropical waters in Australia and French Polynesia to watch larvae in the wild.
This is much harder than it sounds (not to mention nervous encounters with circling sharks, inquisitive marlin and annoying sucker fish) and is highly labour intensive. Larvae are captured with light traps at night, then released one at a time during the day, with two scuba-diving scientists at the ready. Because they are so small - and usually almost transparent - the larvae are extremely hard to see, so one diver does nothing but follow and watch his quarry like a hawk.
"You can't even take your eye off them for a moment to check your air pressure gauge or they disappear," Leis says. The other diver watches out for the first and records the direction, depth and speed at which the larvae swim for about 10 minutes, during which time they cover about 120 metres. Repeated releases are painstakingly tracked and recorded to build up a more comprehensive picture of how and where they move.
Stobutzki's research also revealed another big surprise. She placed captured larvae in an ingenious device in the sea at several locations near a coral reef. The small cage was placed in the water at night, with the larvae held in a closed central chamber.
The chamber door was held closed with a Lifesaver sweet, giving Stobutzki time to motor her boat away so as not to distract the larvae with its sound. When the sweet dissolved, the chamber sprang open and the larvae could remain there or swim to its outer edge by choosing a path either towards or away from the reef.
Most chose the path that took them closest to the reef, regardless of where the device was placed in relation to prevailing ocean currents. Since they had no apparent directional cues from light or waterborne chemicals or odours, the question of how they knew where the reef was remains open.
Leis points out that there's evidence that this reef-sensing ability may operate over distances of up to a kilometre and he shares Stobutzki's hunch that the larvae may home in on reef sounds.
If you've ever dived on a reef, you'll know that the unaided human ear can pick up many weak sounds coming from it, a whole range of clicks, pops and clacks. Sensitive microphones pick up a veritable symphony, such as the scraping of parrot fish as they feed, and the overall impression is like listening to a frying pan sizzle.
Leis plans to do experiments later this year in which recordings of those sounds will be played to larvae released in open water, well away from any reef. If they swim towards the speakers, the answer will be clear.
There's also emerging evidence, as yet unpublished, from other researchers that a significant proportion of the larvae use these amazing abilities to ensure that they don't travel far at all from their birthplaces. It seems that many may stay in the vicinity of their reef of origin, mature, and then return to the safely of their parents' lagoon.
So, put all that evidence together and you've got some good and bad news for managers of reef reserves, the declaration of which is flavour of the moment in marine conservation circles.
The good news is that we now understand much better how reef fish reproduce and repopulate their favoured areas.
The bad news is that this markedly changes the way we should be managing those reserves, as Leis explained to colleagues at the Pacific Science Congress in Sydney on July 6.
"This all totally demolishes the idea of passive larvae at the mercy of the currents," Leis says. "In turn, that shatters our ideas of how reef systems operate and the models we've made to explain them."
It can no longer be assumed, for example, that all you have to do is protect one reef habitat to provide new larvae fish stocks for reef areas downstream from it in the prevailing currents. Each reef area may actually be repopulating itself, or larvae from one reef may deliberately avoid or select another.
If one reef area is presently allowed to be fished by commercial and recreational anglers in the mistaken belief that its fish stocks will be repopulated from a reserve elsewhere, trouble may be looming. In the Caribbean, for example, overfishing of herbivorous fish has allowed the algae they once ate to multiply out of control and smother and kill many coral systems.
Among the Australian reef fish affected by these new insights are such commercially important species as coral trout and emperors.
It may turn out that on large reserve designed to serve a whole region is less useful than a series of small reserves, with the same total area, in each reef system, Leis says. Or detailed studies of each fishery may need to be undertaken and quotas may need to be set to maintain the right balance of adult fish.
"An important point is that our results are only for coral reef fishes," he says. "The traditional view may be valid for corals or crown-of-thorns starfish. Therefore, the possibility remains that different reserve designs may be required for different species."
An extreme view of reserves is sometimes expressed that if we have them, then any regulation outside of reserves is unnecessary because they will always repopulate the unprotected areas, he says. These new insights into reef biology also shoot down that idea.
That means going to a lot more trouble than we are presently doing to manage wild reef fish stocks and their conservation in most of the tropics. And if such fundamental discoveries about their biology are still being made, the present enthusiasm to declare new marine conservation zones must be tempered with the strong suspicion that - because the scientific theory behind them has been found wanting - we may be getting their design and distribution quite wring. We have to change tack as we unravel more and more of these wondrous webs of life.
Mark McGrouther , Collection Manager, Ichthyology