>From: Christopher Scharpf <ichthos_at_charm.net>
>Reply-To: nanfa_at_aquaria.net
>To: NANFA Mailing List <nanfa_at_aquaria.net>
>Subject: NANFA-- Hoover Dam (was "website")
>Date: Wed, 12 Jul 2000 21:37:27 -0400
>
>Rob "our man in Ohio" Carillio asked:
>
> > 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.
>
>
>Christopher Scharpf
>Baltimore
>
>
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/ nanfa_at_aquaria.net. To subscribe, unsubscribe, or get help, send the word
/ subscribe, unsubscribe, or help in the body (not subject) of an email to
/ nanfa-request_at_aquaria.net. For a digest version, send the command to
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/ For more information about NANFA, visit our web page, http://www.nanfa.org