Simon Thorrold, Associate Scientist, Biology Department, and
Anne Cohen, Research Associate, Geology & Geophysics Department
Woods Hole Oceanographic Institution The ocean’s once-abundant fisheries—a resource that helps feed the
world and drives multi-billion-dollar economies—are rapidly being
depleted. Seventy percent of the ocean’s fish are being fished at or
above catch limits that would sustain the fish stocks, according to a
recent report by the National Research Council.
This dismal situation has led to calls for Marine Protected Areas
(MPAs)—areas completely closed to fishing—as a means to protect both
fish stocks and the environments they inhabit. Instead of trying to
manage single species in isolation, the idea is to manage and preserve
whole ecosystems.
But which areas should we protect, to protect fish stocks most
effectively? To make these decisions, we need to know details about
fish life cycles, movements, and migrations. Unfortunately, large gaps
remain in our knowledge about the secret lives of fish.
Following fish in a vast ocean
On land, the
task is much easier. To learn about movements of terrestrial animals,
researchers usually do tag-recapture studies. They place tags on a
number of animals, release them, and then keep track of where the
tagged animals were released and where they were found at later times.
Such studies are difficult in marine environments.Larval fish,
generally measuring 5 millimeters or less, are too small to tag. In
addition, fish typically lay millions of
eggs, of which 99.9% do not survive. Even if we could tag hatchlings,
we would lose nearly all of our study subjects before they reached
adulthood.
Consequently, fisheries scientists have no way to know where an
adult haddock caught on Georges Bank was spawned, or the location of
the nursery area where it spent its adolescence, or the likelihood that
it would return as an adult to spawn in the same place. Yet, this is
exactly the information about fish species that we need to select and
design MPAs that will effectively conserve and replenish fish
populations.
Their ears can tell tales
Our recent research
points to a promising new way to reveal where and how fish live their
lives. Within all fish are ear bones, called otoliths. They grow
throughout each fish’s life, adding annual rings, similar to the growth
rings in trees. For more than a century, biologists have used otoliths
to estimate fish’s ages.
But otoliths may be able to tell us far more. Otoliths consist of
alternating layers of calcium carbonate and protein, which are
deposited in daily increments. Through a complicated process, the
chemical composition of the calcium carbonate is influenced by the
chemical composition and temperature of the water the fish inhabit. If
a fish swims into waters with different chemical or physical
properties, those differences will be recorded chemically in its
otoliths.
In other words, the otoliths can tell us where the fish has been.
And because otolith layers remain unchanged once they are deposited,
they can tell us when the fish was there. In addition, in some fish
species, the width of each daily growth increment in the otoliths can
be correlated with the growth rate of the fish.
Keys to unlock the ‘black box’
In many ways,
otoliths can be thought of as the fish-equivalent of an airplane’s
flight data recorder. They are continually logging information about
the growth and health of the fish and about the water it swims in.
Since otoliths begin to grow just before or after hatching, the entire
life history of individual fish is available to be read, albeit in
code.
Unfortunately, accessing information from flight data recorders is
simpler than it is from the otolith “black box.” Scientists can
determine the chemical composition of samples taken from many calcium
structures, such as coral skeletons or clamshells, by using a mass
spectrometer. This instrument sorts individual elements within a sample
according to their mass and measures the amounts of each.
But such analyses generally require fairly large amounts of
material. Each day, fish deposit only an extremely thin layer of
otolith—about 10 micrometers (0.0004 inches) in width. Most mass
spectrometers cannot be used on such small sampling scales.
To determine the chemical composition of daily growth increments,
scientists need to analyze thin (5- to 10-micrometer) sections of
otoliths. To analyze these thin sections, they require special types of
mass spectrometers that use microbeams of ions or laser probes.
Scientists are fortunate to have access to such state-of-the-art
mass spectrometers, including the Northeast National Ion Microprobe
Facility (NNIMF) and the Plasma Induced Multi-Collector Mass
Spectrometer (PIMMS) facility, located at Woods Hole Oceanographic
Institution. These provide precise measurements of minute quantities of
trace elements and isotopes in thin sections of the otoliths. These
measurements give us the ability to discern small differences in
chemical composition that occur within time periods as short as days.
Cracking the chemical code
Once collected, the
data are still difficult to interpret, however. When otoliths form,
they are surrounded by the fish’s internal fluids. These fluids are
separated from the ambient water on the other side of the fish’s
scales. So the possibility has existed that otolith chemistry has no
relationship to the chemistry of the ambient seawater outside the fish.
Our research shows evidence, however, that chemistry of the water
the fish swims in does indeed influence the chemical composition of its
otoliths. We demonstrated in the laboratory that for at least two
elements, barium and strontium, there is a direct, linear relationship
between concentrations of these elements in the ambient water and in
the otoliths. This may hold true for other elements, too.
If the properties of ambient water do influence the chemical
composition of the otoliths on a daily basis, can we use the variations
in composition as natural records of a fish’s hatching location and
subsequent travels?
A treasure trove of fish data
We have recently shown that we can do so with a natural, wild population of weakfish (Cynoscion regalis).
Currently, these fish are managed as if they are a single population
along the whole U.S. East Coast. That is because weakfish living from
Florida to Maine show no genetic differences. Weakfish are an important
commercial and recreational species that hatch in estuaries, spend
their adulthood near the bottom in coastal waters, and return to
estuaries to spawn.
Juvenile weakfish, however, hatch in each of five different East
Coast estuaries. They are Doboy Sound, Georgia; Pamlico Sound, North
Carolina, Chesapeake Bay, Virginia; Delaware Bay, Delaware; and Peconic
Bay, near the west end of Long Island, New York.
We have found that otoliths of fish born in each of the five natal
estuaries had different, unique isotope and element compositions, or
“signatures.” All their lives, these fish had carried a natural tag,
encoding the location where they were hatched.
We then analyzed otolith cores (the first portions deposited by
hatchlings) from adult fish in those estuaries, and we found that most
of the adult fish were returning to their birthplaces to reproduce—not
randomly to any of the five possible natal estuaries. Knowing this
means that protecting just one or two natal estuaries might not be
sufficient to maintain the fish stocks.
We now believe that fish otoliths are a rich source of demographic
information for fisheries scientists all over the world. At least one
million otoliths are sectioned in laboratories every year, primarily to
determine the fish’s ages. Now we know that annual and daily growth
increments in otoliths contain significantly more information about the
secret lives of fish than simply their age. Chemical signatures in the
otoliths offer the potential to reveal where and when a fish traveled
throughout its life.
The development of techniques for decoding this otolith archive
gives us a powerful new tool to help manage fisheries resources. If we
know where fish hatch and travel, and where the spawning adults
originate, fisheries managers will be better able to choose the most
effective locations to site MPAs and to restrict fishing—to protect the
world’s diminishing fish resources.
Please see related articles:
Tracking Fish to Save Them
In Tiny Ear Bones, the Life Story of a Giant Bluefin Tuna
Posted: August 27, 2004 [top] |
