Magazine article Oceanus

Tracking Fish to Save Them

Magazine article Oceanus

Tracking Fish to Save Them

Article excerpt

The Reef Fish Connectivity and Conservation Initiative

For decades, the Nassau grouper (Epinephelus striatus) was one of the most sought-after fish species in the Caribbean and Gulf of Mexico, from the Bahamas to Central America. These large, delicious fish live among coral reefs and have a breeding behavior that makes them especially vulnerable. They come together in aggregations of thousands to spawn at specific times and places, making them easy to catch-and to overfish.

Nassau grouper populations have been severely depleted by humans throughout most of their range. The environmental and economic ramifications are alarming, and regional governments are responding by restricting or prohibiting fishing for them.

But several large spawning aggregations of this species still exist in the western part of its range near Belize, in the Meso-America Barrier Reef System. These aggregations may provide our last opportunity to learn if and how fish populations are connected among isolated reef sites. This information will be critical if we are to save the Nassau grouper populations from local extinction, as has already occurred on some Caribbean islands.

No fishing allowed

Marine ecosystems in all the world's oceans are under considerable and increasing stress from human activities, precipitating urgent calls for new ways to counter the impacts of people. Resource managers are increasingly using Marine Protected Areas (MPAs)-areas completely closed to fishing-as a means to maintain fisheries and biodiversity.

But scientists, fishermen, environmentalists, and governments continue to debate the effectiveness of MPAs. (See "Do Marine Protected Areas Really Work?" page 42, and "Can We Catch More Fish and Still Preserve the Stock?" page 45.) Monitoring mobile animals under water over long distances and times is difficult, if not impossible, so scientists use mathematical models of population growth to predict the effectiveness of fishing closures. Yet we don't know enough about one important component of such a model-how fish move in and out of MPAs. To predict how well MPAs work, we will need models that accurately describe the movement of individuals between geographically separated sites-what is termed population connectivity.

A critical part of estimating connectivity among geographically separated groups offish is tracking the dispersal of larval fish. Until recently, there has been no way to tell if adult fish living in one reef habitat were spawned in a different location. We did not have the means to determine where fish were spawned, because most fish larvae are too small to be tagged by conventional means.

Revolutionary tagging technology

But now, scientists at Woods Hole Oceanographic Institution have achieved a breakthrough that is poised to revolutionize the study of larval dispersal in marine fish. We have demonstrated that it is possible to introduce a unique chemical tag into the ear bones (otoliths) of fish embryos by injecting the female before she spawns with a nontoxic isotope.

The isotope is a variant of the elements barium or strontium, which would normally be incorporated in small amounts (along with calcium) into the fish's ear bone as it grows. But the isotope has a slightly different mass than the common form of the element.

The otolith grows as the fish grows, with layers that are laid down like tree rings during the fish's life. All the material in the otolith remains where it was originally deposited; it is not continually turned over, as happens in other bones. …

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