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Risks to Wild Populations from Hatchery Fish
Risks to Wild Populations from Hatchery Fish
Genetic Risks
Genes determine the characteristics of living things. Human intervention in
the rearing of wild animals has the potential to cause genetic change. These
genetic changes impact salmon diversity and
the health of salmon populations. Hatchery programs vary and therefore the risks
identified below vary by hatchery. Genetic risks of artificial propagation on
wild populations include:
1) Inbreeding
Inbreeding can occur when the population for a hatchery comes from a
small percentage of the total wild and/or hatchery fish stock (for example,
100 adults are used out of a population of 1 million). If only a small number
of individuals are used to create the new hatchery stock, genetic diversity
within a population can be reduced. Inbreeding can affect the survival,
growth and reproduction of salmon.
2) Intentional or artificial selection for a desired trait (such as growth
rate or adult body size)
Although not common practice today, some hatchery programs intentionally select
for larger fish (or other specific traits). This selection changes the genetic
makeup of the hatchery stock, moving it further away from naturally reproducing
salmon stocks.
3) Selection resulting from nonrandom sampling of broodstock
The makeup of a hatchery population comes from a selection of wild salmon and/or
returning hatchery salmon that are taken into captivity (i.e., broodstock).
If, for example, only early-returning adults are used as broodstock,
instead of adults that are representative of the population as a whole (i.e.,
early, normal, and late-returning adults), there will be genetic selection for
salmon that return early.
4) Unintentional or natural selection that occurs in the hatchery environment
Conditions in hatchery facilities differ greatly from those in natural environments.
Hatcheries typically rear fish in vessels (i.e., circular tanks and production
raceways) that are open and have lower and more constant water flow than that
which occurs in natural streams and rivers. They also tend to hold fish at higher
densities than those that occur in nature. This type of environment has the
potential to alter selection pressures in favor of fish that best survive in
hatchery, not natural, environments.
5) Temporary relaxation during the culture phase of selection that otherwise
would occur in the wild
Artificial mating disrupts natural patterns of sexual selection. In hatcheries,
humans select the adult males and females to mate, not the salmon. Humans have
no way of knowing which fish would make the best natural breeders. In addition,
selection is relaxed up until the time when juveniles are released from the
hatchery (because they don't face the same predation and foraging challenges
as wild juvenile fish). Fish raised in hatchery environments face very different
pressures than those raised in the wild.
Ecological Risks
Hatchery-produced fish often differ from wild fish in their behavior, appearance,
and/or physiology. Ecological risks of artificial propagation on wild populations
include:
1) Competition for food and territory
Competition between wild and hatchery fish can occur. It is most likely to occur
if the fish are of the same species (wild chinook salmon and hatchery reared
chinook salmon) and they share the same habitat (quiet, shallow water or deep
fast water) and diet.
2) Predation by larger hatchery fish
If hatchery-released salmon are larger than wild salmon, evidence suggests that,
for certain species, hatchery-released salmon can eat wild salmon.
3) Negative Social Interactions
Juvenile salmon establish and defend foraging territories through aggressive
contests. When large numbers of hatchery fish are released in streams where
there are small numbers of wild fish, hatchery fish are more likely to be more
aggressive, disrupting natural social interactions.
4) Carrying Capacity Issues
Carrying capacity is a measure of the amount of a population (like the amount
of salmon) that can be supported by a particular ecosystem. Carrying capacity
changes over time with the abundance of predators and resources such as food
and habitat. When hatchery fish are released into streams where there are wild
fish, there can be competition for food and space. Many streams and watersheds
are degraded due to contamination, development, etc. Questions are now being
asked about the numbers of fish that different streams and ecosystems can support.
Behavioral
Hatchery environments are different than stream environments. Hatcheries typically
rear fish in vessels (i.e., circular tanks and production raceways) that produce
sterile environments where there is no underwater structure (i.e., sticks and
wood), little or no overhead cover (such as cover from nearby trees and shrubs),
and a predictable food supply. Consequently, hatchery fish tend to have different
foraging, social, and predator-avoidance behavior.
Overfishing
Large-scale releases of hatchery fish have supported commercial, Tribal, and
sport fishing practices for many years. However, large-scale releases of hatchery
fish in a mixed-population fishery create a risk of overfishing for wild populations.
For example, if fishers are allowed to catch half of the more abundant, hatchery
stocks, half of the wild stocks will also be harvested if they occur at the
same time and place as the hatchery fish. Because hatchery populations have
high survival in the hatcheries, they can generally support higher harvest rates.
Wild stocks, on the other hand, are typically much smaller, and their population
could be harmed by such high harvest rates. The National Marine Fisheries Service
and fisheries managers are currently looking for opportunities to support selective
harvest of hatchery fish (i.e., harvest that doesn't impact wild stocks). Selective
harvest opportunities could be supported through catch and release programs
and/or in places where hatchery stocks are isolated from wild stocks (i.e.,
if hatchery stocks use a different stream or enter the stream at a different
time than wild stocks).
Fish Health
The effects of disease on hatchery fish and their interaction with wild fish
are not well understood. Hatcheries can have disease outbreaks. Once released,
these fish can transmit disease to wild fish.
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last modified
03/09/2006
Web site owner: Northwest Fisheries Science Center
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