A team of Australian, American and French coral reef scientists has achieved a world breakthrough in tracking fish that could revolutionize the sustainable management of coral reefs and help restore threatened fisheries. Working on coral reefs in a marine protected area in Papua New Guinea, the team has pioneered a new way to study fish populations by "tagging" adult fish with a minute trace of a harmless isotope, which they pass on to their offspring.

In the process, they have established that Nemo – the lovable orange, black and white clownfish of movie fame – really does come home, with around 60 per cent completing the astonishing journey back to their tiny home reef after being swept into the open ocean as babies.

Working on pristine coral reefs in a marine protected area in Papua New Guinea, an international team - led by Dr Geoff Jones and Dr Glenn Almany of the ARC Centre of Excellence for Coral Reef Studies at James Cook University - has pioneered a new way to study fish populations by 'tagging' adult fish with a minute trace of a harmless isotope, which they then pass on to their offspring.

The team's findings were announced in the international journal Science today, and contain three scientific world-firsts:

  • the first field-based use of a new method of tagging fish larvae
  • the first larval tagging study of a free-water (pelagic) spawning fish species
  • the first larval tagging study of a bottom (benthic) spawner and a pelagic spawner in the same location and comparison of their dispersal patterns.

The tag is enabling the researchers to understand the extent to which young fish return to their 'home' area or go off to interbreed with more distant populations.

This helps to build a picture of the extent to which fish populations are connected or isolated from one another – currently a vital missing link in the sustainable management of fish stocks.

"If we can understand how fish larvae disperse, it will enable better design of marine protected areas and this will help in the rebuilding of threatened fish populations," Dr Almany explains.

In trials at Kimbe Bay, PNG, researchers tagged over 300 female clownfish and vagabond butterflyfish with a barium isotope. Females pass the isotope to their offspring and it lodges in their offspring's ear-bones (otoliths).

"The isotope is stable, non-radioactive and quite harmless to the fish in these minute amounts – or to humans if it were to be used to tag a table fish," Dr Almany explains. "It's simply a way of telling one group of fish of the same species from another."

The team later returned to confirm the tags had worked and study how many of the offspring had returned to the home reef or had dispersed to other reefs. They found around 60% of the juvenile fish returned to the home reef – a tiny dot in the ocean only 300m across – after being carried out to sea as babies.

"Just as importantly, 40% of the juveniles came from other reefs that are at least 10 kilometers away, which indicates significant exchange between populations separated by open sea," he adds.

"This shows how marine protected areas can contribute to maintaining fish populations outside no-fishing zones."

Their latest research, at an aquaculture facility in Bali, is exploring the possibility of applying the tag to coral trout – a candidate for better management in waters where it is overfished for human consumption.

If the tagging experiment proves successful the team hopes to conduct trials with coral trout off Great Keppel Island on the Great Barrier. Similar studies are planned for coral trout in PNG and in the Caribbean with a threatened species, the Nassau groper. In both cases the team hopes the new insights from larval tagging will improve management and the fish populations' chances of recovery.

"In a situation where you are trying to protect fish caught for the table, tagging would help you to select the right reefs to protect, in order to maintain the overall population – and the fish catch into the future," Glenn explains.

The team consists of Glenn Almany and Geoff Jones of CoECRS, Michael Berumen of the University of Arkansas, Simon Thorrold of Woods Hole Oceanographic Institute and Serge Planes of the University of Perpignan.

Source: James Cook University.