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<br />16 . Executive Summary
<br />
<br />Dam, only in the LCR; an unlikely scenario
<br />considering all other populations spawn in
<br />similar mainstem conditions.
<br />
<br />2. Cold releases forced mainstem spawners to
<br />switch to an earlier spawning mode and ascend
<br />the LCR to coincide with temperatures of that
<br />tributaIy; a possible scenario considering many
<br />fish species are capable of switching spawning
<br />times under changed environmental conditions,
<br />e.g., temperature, photoperiod.
<br />
<br />3. Two population components existed in Grand
<br />Canyon-one spawned in the mainstem, and one
<br />in the LCR The mainstem component
<br />experienced unsuccessful reproductive efforts
<br />following Glen Canyon Dam and few if any
<br />individuals remain; this is the most likely
<br />scenario and an important consideration for a
<br />second population in the mainstem if unique
<br />genetic stocks exist.
<br />
<br />Some spawning may be occurring in the mainstem
<br />downstream of the LCR in lower reaches of warm
<br />tributaries, or in warm mainstem springs, as
<br />indicated by young humpback chub captured by
<br />other investigators. These fish may have been
<br />hatched locally, or they may have been hatched in
<br />the LCR and survived the thennal
<br />transition to the mainstem and
<br />downstream transport. Young
<br />captured in downstream reaches
<br />during 1990-93 could have originated
<br />from the LCR and dispersed to any
<br />area downstream within days.
<br />Assuming average transport rate of
<br />about 1.1 to 3,6 km/hr (0.3-1.0 m/sec,
<br />Graf 1995), a small suspended object,
<br />like a larval fish, could be transported
<br />from the LCR (RM 61.3) to Diamond
<br />Creek (RM 226) in about 241 to 74
<br />hr.
<br />
<br />IMPORTANT
<br />INTERACTIONS
<br />
<br />BIOTIC
<br />
<br />Fourteen species of fish were
<br />sympatric with humpback chub during
<br />this investigation, including 3 native
<br />species and II non-natives. These
<br />interacted with humpback chub as
<br />known or potential predators,
<br />
<br />
<br />1400
<br />
<br />>a
<br />~1200
<br />C
<br />i1000
<br />E
<br />:::l
<br />II)
<br />5800
<br />o
<br />.Q
<br />.,2600
<br />o
<br />.lI:
<br />~400
<br />.Q
<br />c.
<br />E
<br />:::l 200
<br />J:
<br />
<br />o
<br />o
<br />
<br />Final Report
<br />
<br />competitors, and vectors for parasites and diseases.
<br />Known predators included brown trout, rainbow
<br />trout, and channel catfish. Potential predators
<br />included striped bass, green sunfish, brook trout,
<br />black bullhead, and walleye, but these occurred in
<br />small numbers and probably had an insignificant
<br />impact, Carp could also be significant predators of
<br />incubating eggs in the LCR and warm springs,
<br />Small cyprinids, such as fathead minnows and red
<br />shiners are known predators of early life stages of
<br />native species (Gregory and Deacon 1994, Ruppert
<br />et al. 1993).
<br />
<br />It was estimated that brown trout, rainbow trout,
<br />and channel catfish in the mainstem potentially
<br />consume 250,000 young humpback chub annually
<br />(Fig, 15), significantly affecting survival of 1, 2, and
<br />3-year old fish, and reducing recruitment to the adult
<br />portion of the population. The most significant
<br />predator was the brown trout, with a potential
<br />annual consumption of 230,000 chubs. This is the
<br />estimated predation occurring between the LCR
<br />(RM 61.3) and Bright Angel Creek (RM 87.7), in
<br />the area where the species are sympatric and young
<br />humpback chub densities are highest. Although the
<br />young chubs originate from the LCR, we believe
<br />that fluctuating flows and cold dam releases
<br />destabilize shoreline habitats and transport the fish
<br />
<br />/~;.
<br />
<br />600 ~
<br />c
<br />3
<br />"C
<br />500i
<br />n
<br />ox
<br />o
<br />4OQg'
<br />C'
<br />o
<br />o
<br />300;
<br />c
<br />3
<br />CD
<br />200 c.
<br />l>
<br />:::l
<br />:::l
<br />C
<br />100 !!!.
<br />-<
<br />
<br />,.
<br />
<br />_\~ /'
<br />~/
<br />li;)' .'
<br />~,/
<br />.o~/"'
<br />""..-/
<br />
<br />qJo~:,/"
<br />
<br />,"
<br />
<br />"
<br />
<br />,."
<br />
<br />,
<br />--
<br />
<br />"
<br />
<br />....150/0) --
<br />to"" --
<br />_hC)'lf"t _-----
<br />p\'':---
<br />--
<br />--
<br />
<br />.'~.
<br />
<br />.'
<br />
<br />-'
<br />
<br />"
<br />
<br />.'
<br />
<br />---
<br />
<br />---
<br />
<br />Channel Catfish (1:_~~L..
<br />
<br />><
<br />Rainbow Trout (1%) ....
<br />o 0
<br />2 3 4 5 6 7 8 9 10 .9
<br />Number of Adult Predators (X1000)
<br />
<br />Fig. 15. Potential daily and annual consumption of humpback chub
<br />by adults of three predator fish species in the Colorado River in Grand
<br />Canyon. Relationships assume 2.0 chubs consumed daily by 10.4% .
<br />of adult brown trout; 1.0 chub consumed daily by 1,5, or 10% of adult
<br />rainbow trout; 1.0 chub consumed daily by 1.5% of adult channel
<br />catfish.
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