The smallest fishes on Australia’s Great Barrier Reef are sending out a warning that profound change is taking place in the Reef’s natural systems, probably as a result of human activity.
At 25-45mm in length, the gobies are so small and cryptic they are often invisible to the casual visitor – but they make up almost half of all the fish life on the reef, says ichthyologist Professor David Bellwood of the ARC Centre of Excellence for Coral Reef Studies and James Cook University.
A small goby species, Eviota sp. The loss of such small fishes warns of a change in the world’s coral reefs. Photo: J.P. Krajewski
“These fish may be tiny, but they are very important. They are telling us that the world has changed, and in ways we do not understand. That we may not be able to manage things as well as we hoped,” he says.
“In 1998 there was a major coral bleaching event that affected corals across a huge area of the reef. After some years, quite a lot of the coral has recovered – and looks more or less as it once did.”
“But the gobies have not come back. Something is not right if the fastest breeders of the reef are still missing. Overall, the coral fish fauna are still in a degraded state – after 30 generations.”
Professor Bellwood has devoted almost 20 years to the study of what many might imagine to be the least significant of fish on the reef. He feels they may be far more important than might appear, as indicators of the health status of the Reef.
“Gobies are among the Reef’s most plentiful species. They live fast and die young, in vast numbers. Many big reef fish live ten years or more: a typical goby lasts just 100 days. Everything eats them – they are the ‘Tim Tams’ of the Reef. For every ten that wake up in the morning only nine go to sleep at night.
“Because their generations turn over so quickly, gobies provide a highly sensitive indicator of changes that may be taking place, far more so than the longer-lived species, like large fishes or turtles.
“Normally gobies breed up quickly and replace their numbers. But in many areas which have suffered from the effects of coral bleaching and other impacts caused by human activity, many are largely missing today – a signal that something is seriously wrong.
“Gobies divide up the ecological niches on the Reef very finely – they tell us if the Reef system, as whole, is thriving or failing. Now, they are telling us that the foundations of the Great Barrier Reef and other coral reefs around the world are shifting in profound ways.
“This may be a sign that coral reefs will never be the same again, and that we should be planning for an unstable, uncertain future.”
Prof. Bellwood says it is not yet possible to diagnose the precise reasons for the loss of the gobies, but the combination of climate change – and the vast southward movement of reef species which is now starting to show up in scientific surveys – impacts such as bleaching (caused by rising global temperatures), ocean acidification (caused by CO2 emissions), pollution and runoff from human activities on the land, outbreaks of weed, Crown of Thorns starfish and coral diseases and overfishing of other species may be combining to take their toll, producing subtle ecosystem-wide impacts that are not easily seen, except by small fish.
“I’m one of a handful of scientists in the world working on goby ecology, and we still know little more about them than we did when Charles Darwin visited Australia. But because of their vast numbers, rapid growth rates and fast turnover, they are a real powerhouse for the reef, providing nutrition for a great many other species directly or indirectly. If they disappear from the system, it signals something is profoundly amiss.”
Studying something as small as gobies is not easy. Usually it consists of encasing a coral bommie in a large mosquito net, and then meticulously collecting anaesthetized fishes from within the net. Comparisons of goby populations over many years before and after coral bleaching, even in reefs where the coral is apparently healthy, shown signs of significant change.
Professor David Bellwood, CoECRS and JCU, ph +61 7 47 814447
Dr Martial Depczynski, AIMS, UWA ph +61 8 6369 4025
Jenny Lappin, CoECRS, +61 7 4781 4222
Jim O’Brien, James Cook University Media Office, +617 4781 4822 or +61 0418 892449
Sources: Depczynski, M., C.J. Fulton, M.J. Marnane and D.R. Bellwood (2007) Life history patterns shape energy allocation among fishes on coral reefs. Oecologia 153: 111-120.