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erring babies
and juveniles. Note dime for comparison. |
These factors affect all life stages. If more adult herring spawn, more eggs will be laid. More eggs will survive is there are few storms, small waves, and few birds eating the eggs. Baby herring that are physically or genetically damaged will not survive. Water currents that keep the baby herring in Prince William Sound and move them to nursery areas in bays are good for survival. To survive, herring juveniles must avoid being eaten by other fish and birds as they grow from August through October. In addition, during that time, they must have a lot to eat, because they need lots of stored food to survive over winter. A warm winter is bad for herring survival because metabolism is high and they use up their stored food too quickly. Not only do all these factors affect the survival of herring until they are one year old, but the effects are not the same in all parts of Prince William Sound.
There is a range of survivals expected at each of these life stages. I looked at the high and low extremes of possible survival for each stage to calculate possible survival of herring to age one year. My estimates show that for every one million herring eggs that are laid, the range of survivors is between one and 6,500. It takes approximately 40 to 100 adult herring to produce those one million eggs. The same amount of eggs are not laid every year, so how many juveniles survive depends on the number which were laid. As a high example, in 1988 approximately four trillion eggs were deposited. That number of eggs should yield between 4.5 million and 25 billion juveniles. As a low example, in 1997 two trillion eggs were deposited, yielding approximately 2.4 million to 13 billion juveniles. The extremes of outstanding or failed survival are quite rare.
A minimum amount of spawning
fish is needed to guarantee the continued existence of
herring in Prince William Sound. If the estimated amount of
herring falls below the minimum herring needed, the
commercial herring fishery is closed. That is what happened
in 1993 - 1996 and again 1999 - 2001. Many herring juveniles
survived in 1988, but very few have survived since then. The
Exxon Valdez oil spill killed baby herring in 1989.
It also badly affected their food and spawning.
Additionally, the oil spill lowered the resistance of
herring to disease. Many fish became diseased and died in
1993. Without more fish surviving to take their place, the
amount of herring stayed at low levels and the fisheries
stayed closed. Again in 1999, disease was killing the
herring, and again, there are few fish alive the following
years. Though the cause of the disease in 1999 was not
related to oil, the impact was the same, the commercial
fisheries were closed for several years. The herring
industry used to be valued at between six and 11 million
dollars per year in Prince William Sound. For the last three
years, the value has been zero. This has had a profound
impact on the economy of the region.
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Balls of
herring juveniles in bays in Prince William
Sound. |
Humans cannot change the environmental factors. Closing the fishery to protect the number of spawning adults is the only part of the ecosystem that humans can control. Though it is not a guarantee of how many juveniles will survive, it does increase the odds of survival. More herring need to survive to support the commercial fishery as well as to support all the animals that depend on them for food. Closing the fishery now will be good for both the fishery and the ecosystem in the future.
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For information on the Harriman Retraced Expedition e-mail: harriman2001@science.smith.edu |