Fishin the 90's
Fishin the 90's
 Allen Andrews and Erica Burton, graduate students of ichthyology, check out the dating scene as Phil (Phil's Fish Market, Moss Landing, CA) looks on. | What do you do when you feel like picking up a fish? You walk into
the fish market, check out the scene and wonder: How about that fish? Sure,
he's ugly, but will my friends like him? Is he fresh or does he smell bad?
Is he right for me? Or perhaps: How old is he? Five, maybe ten years, fifteen
at most. Do you think you should find one closer to your own age?
Although no one can tell you what you like, scientists at Moss Landing Marine Laboratories have developed a unique way to determine the age of many hard-to-guess fishes. The methods are state of the art and the results are surprising. |
| Determining fish age is a crucial part of understanding the life history
of a fish. From the fish's size, age and sexual maturity, scientists can
tell a lot about a population. From this life history information, managers
can tell whether a fishery is likely to increase or decline. Based on age at first maturity,
population dynamics,
recruitment, and other
life history data, sustainable
yield estimates, and ultimately, catch limits are derived from this information.
We live in a changing world where fishermen and women are no longer going after the same old guys. In fact, many of the old dives have been fished out. Many of the species, once prevalent, have now been depleted beyond their ability to repopulate their habitats and are now largely missing from these, once teeming, nearshore waters. This has forced the industry to search farther and deeper for other species in other habitats. More and more, the deepwater rockfish species find themselves the target of an industry, struggling to keep afloat. These fish inhabit the waters from 500 to 1000 meters, and sometimes deeper (over half a mile down) where food is limited and growth rates can be slow. The problem for scientists and regulators is that fishermen are now harvesting large quantities of some of these species, about which we know very little. Yet, their survival depends upon our ability to make responsible decisions about their lives and their future. "Sometimes ya just gotta date 'em to find out for sure" says ichthyology graduate student Erica Burton, working with Dr. Gregor Cailliet and Dr. Kenneth Coale. The catastrophic crash of the orange roughy fishery in New Zealand waters was the consequence of management using poor age estimates to create policy. There, a fishery was managed using sustainable yield estimates based on a species that live to be 10 to 20 years old. As it turned out, these species actually lived to be over 100 years old. Fish were being caught much faster than they could reproduce and they just couldn't keep up with the fishing pressure. We saw prices climb from less than a dollar a pound to over $10 per pound. |
| But how does one date fish in the '90's? They don't carry driver's licenses. Historically, fish have been aged by several methods. Some fish we can grow in tanks and watch them age and mature directly. Others that live nearshore we can catch, tag with an identification marker, let them go, then recapture them at a later date to see how much they have grown. These methods don't work for species which live in deep waters and cannot survive being brought to the surface. These fish have a very small probability of being recaptured, even if they did survive being caught in the first place. |
Erica Burton gets fresh with a fish at Phil's Fish |
| Fortunately, nature has provided us with a "fish diary" in
the form of an
otolith,
or ear stone, which is accreted within the inner ear. These otoliths are
made of calcium carbonate which grows in increments in much the same way
as tree rings. If we can read this diary and interpret the bands in the
otolith, we can determine the fish's age and other personal information.
But not all bands are laid down annually, so sometimes we don't know if
the fish is six months or six years old. We, therefore, need an independent
way of ageing fish to make sure we are reading the otolith diary correctly.
Again, nature pulls through with a built in chronometer in the form of
a naturally occurring radio isotope pair: radium-226 and its great-great grandaughter,
lead-210.
Radium, discovered by madam Curie, is a naturally occurring radio nuclide that has been used to illuminate old watch dials. It behaves chemically just like calcium and the fish can't tell the difference. As the fish builds its otolith, it incorporates radium along with calcium. As the otolith ages, radium decays to lead. The longer it ages, the more lead-210 builds up (the older the great-great grandmother, the more the great-great grandchildren). If we can measure both the radium-226 and the lead-210, we can determine the age of the otolith. Simple? NOT! |
| Lucky for the fish, scientists at MLML, working with colleagues at UC Santa Cruz, have been able to develop a method to separate tiny quantities of radium from these otoliths. Rather than waiting for the radium to decay and measuring its activity, the radium atoms in these samples are counted directly using Thermal Ionization Mass Spectrometry (TIMS) on an instrument at UC Santa Cruz. Lead-210 can be measured by alpha particle spectrometry at MLML. This project, therefore, represents a novel collaboration between oceanographers, radiochemists, ichthyologists and earth scientists using the latest detection equipment in the field. The radium-226 data together with the lead-210 data generated at MLML are combined to decipher the bands in the otolith diary. |
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For several rockfish species we find that they can live to be over 80 years old. Several species may live to be over 120 years old! These findings will have important applications for those who seek both to regulate these fisheries and to understand the members of our marine community. These are, perhaps, the senior citizens of the sanctuary. The next time you are in the fish market, you may find that the fish you are bringing home is older than your grandfather.
