POSTED 18 APR 2002
Headline at top of page: Left sockeye salmon from photo by Kate Guthrie, NOAA.
Why do fish come and go? Why are some years good, and others dreadful? These questions are much on the mind of Northern-Hemisphere anglers as they head out for another season of "drowning worms," as they ruefully call their obsessive dunking of bait into the drink.
A salmon that died after spawning will feed a hungry bear and release nitrogen and phosphorous. These elements will fertilize algae that are grazed by zooplankton, a favorite food of young salmon.
Courtesy Bruce
Finney, UAF Institute of Marine Science.
Fish scientists are almost equally at-sea when it comes to the long-term variations in fish quantity. Does a particular species increase in response to the amount of available food? Is overfishing to blame, or changes in climate? Perhaps an increase in predators? Has the evolutionary fitness of the species changed for unknown reasons? Has the water or landscape changed?
Most likely, large variations in fish stocks reflect several interacting factors. But that's little solace to the folks who study, not to mention regulate, fishing.
Amidst a growing furor over depletion of ocean fishes around the world, a new study points a finger not at people but at climate, in causing fluctuations in population of sockeye salmon in Alaska over the past 2,200 years.
Handy isotope
The study, directed by Bruce Finney of the University of Alaska at Fairbanks, looked at N-15, a naturally occurring but rare heavy nitrogen isotope. N-15 is somewhat more common in the ocean than the atmosphere, and it builds up in the bodies of salmon and other marine predators. Because N-15 is deposited in the lakes when salmon die after spawning, a higher the ratio of N-15 to N-14 indicates that more salmon croaked in a lake.
To document old salmon populations, Finney and colleagues extracted cores from the sediment of three lakes on Kodiak Island, Alaska. Two lakes have had salmon for thousands of years; the third lake lacked them until quite recently.
The proportion of N-15 to N-14 reflects the number
of salmon that died in the lakes. Frazer Lake had no salmon until the
1950s, when the N-15 proportion started rising. Courtesy
Bruce Finney.
As you can see from the graph, around 100 BC, the percentage of N-15 declined sharply, especially at Karlak Lake, but also in Akalura Lake. In contrast, Frazer Lake remained pretty stable until salmon were introduced in the 1950s.
In Karlak and Akalura Lakes, salmon numbers - as reflected in the isotope record -- rose around 800 AD, and remained high until about 1900, when commercial fishing increased.
Although fishing and habitat destruction are blamed for modern-day declines in salmon, they played a lesser role in the past, says study co-author Irene Gregory-Eaves, a doctoral student at Queen's University, Canada.
During the late Kachemak period (see graph), salmon were scarce, and the archeological record shows that local inhabitants probably ate more marine mammals than fish.
Then the human population -- and salmon numbers, as reflected by N-15 -- both rose. Huh? Don't people often gobble fish and make them scarce? Yes, but in this case, Gregory-Eaves says, the local folks may have been attracted to the fish, but did not eat enough to plunder the population.
Sockeye salmon. Courtesy
Washington Department of
Fish and Wildlife.
Blame the climate
If the salmon's population swings did not reflect
fishing pressure, what then? The study's authors concluded that they probably
grew from broad climatic changes in the North Pacific Ocean.
Why blame climate among the many factors can affect fish? Because the two big changes in salmon populations -- 100 BC and 800 AD -- coincided with major climatic shifts -- and with significant changes in numbers of Pacific sardine and northern anchovies. "Seeing these broad geographic patterns," Gregory-Eaves says, "it's hard to explain it other than by climate."
The exact mechanism linking climate to salmon numbers is unclear, due to the paucity of good records going back 2,200 years. But, Gregory-Eaves says, "There's a pretty strong theory that climate affects salmon through food availability."
In warmer years, she explains, warmer water in the Gulf of Alaska tends to rest above colder water, reducing vertical mixing and making the water more transparent. Floating plants near the surface get more light and grow faster. Because these plants are the base of the ocean food chain, that eventually benefits salmon and other predators.
But too warm may be no better than too cold, Gregory-Eaves adds. A recent study showed that if global warming continues, during July, salmon will only be able to survive in the remote Bering Sea.
In other words, a small rise in temperature may be enough to cook them out of the Gulf of Alaska. But this time around, the effects of climate will be multiplied by intense commercial fishing.
Whether you like salmon for economic or gustatory reasons, it's something to think about as you skewer the bait with the hook.
-- David Tenenbaum
BIBLIOGRAPHY
Fisheries Productivity in the Northeastern Pacific Ocean... Bruce Finney et al, Nature, 18 April, 2002.
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