> The other day on discovery channel, I saw a show on the Hoover Dam... What can
you tell me
> about this dam and it's impact even today, on natives.
Dams effect the survival and reproduction of Colorado pikeminnow (and other
native Colorado River fishes such as humpback chub and razorback sucker) in a
number of cumulative ways. First of all, the fish are not adapted to the deep,
still-water reservoirs behind the dams, nor the cold, clear water that flows
from them. Colder temperatures downstream of dams reduce the survival rate of
pikeminnow embryos, and slow the growth rate of pikeminnow fry, making them more
vulnerable to predation from other fishes. Sometimes the temperature drop is
sudden and lethal. One study showed that when juvenile pikeminnow and humpback
chub move from the 20C (68F) tributaries to the 10C (50F) tailwaters, they
enter a "cold coma" for 5-90 minutes. If they're not eaten by predators during
this period, physical damage and death may occur from abrasion against rocks,
burial in sediment, and simply being swept away in high velocity currents. In
addition, dams block migrating adults from reaching their upstream spawning
grounds in the rapids. And they effect the ecology of rivers in ways not easily
seen. For example, dams confine the flow of nutrients that are essential to the
survival of aquatic organisms. Channelization below dams reduces the number and
size of backwaters that pikeminnow use for nursery areas. And when dams release
water to provide power or irrigate summer crops, they disrupt the river's
natural cycle of flood and drought, causing peak flows in the summer instead of
the winter and spring.
Another dam-related problem is the introduction of nonindigenous fishes into
reservoirs and tailwaters for the benefit of recreational anglers. These
fishes, such as largemouth bass and channel catfish, are well-adapted to
slow-water habitats. They quickly spread throughout the Colorado River system,
out-competing native fishes for food and breeding sites, and preying upon their
eggs and fry. Even the pikeminnow's own predatory habits hastened its decline.
Colorado pikeminnow preyed on channel catfish, but since the two species had not
evolved together, the pikeminnow had no defense against the catfish's fin
spines. As a result, many pikeminnows suffocated when catfish got lodged in
their throats. Even seemingly harmless baitfish proved a danger to the once
formidable predator. In the small backwaters where young pikeminnow mature,
introduced red shiner (Cyprinella lutrensis) out-compete the pikeminnow for
space and food. And, finally, with nonindigenous fishes came nonindigenous
diseases, from which the native fishes have no natural resistance.
Because it is unlikely that any of the major western dams will be removed, the
U.S. Fish and Wildlife Service's recovery plan for the Colorado River fishes is
limited to restoring self-sustaining populations of the fish above major dams in
three specific "Recovery Areas" in the Colorado's Upper Basin. The plan calls
for maintaining natural river flow patterns by releasing more water from dams in
the spring, stabilizing flows in late summer to protect juveniles, making sure
stocked gamefish do not conflict with recovery efforts, and building passageways
and ladders around selected barriers.
A major part of the recovery program is to breed Colorado River fishes in
captivity and release the juveniles into the wild. But captive propagation will
only succeed if there is healthy habitat for the fishes to grow and spawn.
What's more, there is some doubt among biologists whether Colorado River fishes
-- which have complex life histories -- can be reestablished with hatchery
reared stock. For exmaple, would a captive-bred pikeminnow know whether its
spawning grounds are upstream or downstream? Would it even know when to spawn
considering it grew up in a holding tank that does not exhibit seasonal flow
variations? And do different spawning runs represent genetically isolated
populations that should be managed as separate spawning stocks? Researchers are
still studying these and other questions.
Let us hope that the Colorado pikeminnow and other endemic fishes of the
Colorado River, which evolved in a highly-changing environment, can adapt to
some of the environmental changes brought about by man.
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