Coral Reef Fish Make Their Way Home


May 3, 2007

Coral reef fish hatchlings dispersed by ocean currents are able to make
their way back to their home reefs again to spawn, says a
groundbreaking study published today in the journal Science.
The study, whose findings are considered a major advance for fish
conservation biology, was conducted by an international team of
scientists from Australia, France, and the U.S. using a novel tagging
method to track two populations of fish, including the endearing
orange, black, and white reef-dwelling clownfish made famous in the
movie “Finding Nemo.”

Led by Dr. Geoff Jones and Dr. Glenn Almany of the Australian
Research Council Centre of Excellence for Coral Reef Studies at James
Cook University, the study took place on coral reefs in a marine
protected area in Papua
New Guinea. Scientists tested a new method to trace fish from birth to
spawning and detect the percentage of fish hatched on one reef that
return there to spawn. The techniques used in this study can reveal the
extent to which fish populations on separate reefs are isolated
breeding populations, or connected by fish movements (known as
‘connectivity’).  Such information is critical to effective
management of reef fish populations.

Following two fish species, the clownfish (Amphiprion percula) and the vagabond butterflyfish (Chaetodon vagabundus),
the scientists found that young of both species made it back to their
home reef about 60 percent of the time⎯a surprising result for fish
larvae that had dispersed from a small reef habitat into a large
area.  The researchers tagged fish at the reef surrounding a small
island, Kimbe Island, within a recently-designated Marine Protected
Area in Kimbe Bay, Papua New Guinea.

“If we 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,” said Almany, lead author on the Science
article. Other members of the team were Michael Berumen of the
University of Arkansas, Woods Hole Oceanographic Institution (WHOI)
biologist Simon Thorrold, and Serge Planes of the Universite de
Perpignan.

The study’s results highlight three notable achievements. This is the
first time scientists have successfully used a new internal tagging
method in the field, as well as in the lab. It is the first larval
tagging study of a pelagic (open water-swimming) spawning fish. It is
also the first comparison between two fish species with different
reproductive strategies and dispersal patterns.

The tagging method the team employed was developed by Simon Thorrold at
WHOI. The process involves injecting minute quantities of harmless
stable barium isotopes into breeding female fish of both species. “The
isotopes are passed to the offspring and incorporated into the ear
bones – or otoliths – of the developing embryos,” said Thorrold,
“thereby labeling the hatchlings at birth with the isotopes as
permanent traceable tags.”

Two months after injecting females, the scientists returned and
captured newly settled fish at the same reef to determine how many had
returned to their home reef and how many had migrated from other nearby
reefs. The percentage of fish whose otoliths were labeled with the rare
barium isotope was identified at WHOI through a technique known as
laser ablation inductively coupled plasma mass spectrometry (ICP-MS).

The two species have different reproductive styles. Butterflyfish
release eggs and sperm into the water, and the larvae drift and swim
freely for more than a month before finding a home reef. In contrast,
clownfish spawn eggs that are attached to the reef for a week before
the larvae hatch and disperse in oceanic waters for 10-14 days. 
The larval clownfish must then find a reef, and a suitable anemone,
that will be home for the remainder of its life.  Currents
inevitably carry both species away from the parental reef, because
larval fish cannot swim well, but this study confirms that the majority
of both species appear to find their way home after completing the
oceanic larval phase.

Reef fish conservation programs utilizing marine protected areas are
based on assumptions about how many fish migrate in from other areas
and how many return to home areas to spawn. At a time of increasing
pressures on coral reef ecosystems, this study provides an important
piece for planning the optimum size of coral reef protected areas and
breeding populations.

“Just as importantly,” said Almany of their results, “40 percent of the
juveniles came from other reefs that are at least ten kilometers (five
miles) away, which indicates significant exchange between populations
separated by open sea. This shows how marine protected areas can
contribute to maintaining fish populations outside no-fishing zones.”

The successful test of this method in the field offers scientists and
managers a powerful new way to evaluate the effectiveness of management
models and practices based on direct information. Thorrold is
continuing this work, using the maternal labeling technique to evaluate
the degree of connectivity in other fish populations, including
endangered Nassau grouper in the Caribbean.

This work was funded in part by the National Science Foundation.

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The Woods Hole Oceanographic Institution is a private, independent
organization in Falmouth, Mass., dedicated to marine research,
engineering, and higher education. Established in 1930 on a
recommendation from the National Academy of Sciences, its primary
mission is to understand the oceans and their interaction with the
Earth as a whole, and to communicate a basic understanding of the
ocean’s role in the changing global environment.