7766

• UCREFRP Catalog Number: 7766
• Author: Scholz, A. T., et al.
• Title: Notes on the Development and Measurement of Egg and Larval Thyroxine Concentration in Flannelmouth Suckers ‎(
• USFW Year: 1992.
• Copyright Material: NO

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4.0 DISCUSSION

Flannelmouth sucker eggs were observed to hatch from 5.5 to 7.0
days post-fertilization at 21.0 :t:: 1.0oC. Swim-up occurred on day 9.0 to
, 10.0 post-fertilization at 21.0 :t:: 1.OoC. Alevins had completely absorbed
their yolk sac and started to gain weight by this time. Flannelmouth
sucker development proceeded at about the same rate as development of
the related razorback species held in uniform conditions (Scholz et al.
1992b) This is not unexpected since flannelmouths are reported to
hybridize with razorback suckers in the wild (Hubbs and Miller 1953;
Sigler and Miller 1963). Individual weight of flannelmouth sucker eggs
recorded in the present study ranged from 25.0 to 30.0 mg, compared to
10.8 to 11.0 mg reported for razorback suckers (Scholz et al. 1992b).
Individual weight of flannelmouth alevins ranged from 10.0 to 15.0 mg,
compared to 4.1 to 4.9 mg reported for razorback suckers (Scholz et al.
1992b).

One suspected cause of the decline of razorback suckers in the Green
River is altered flow and temperature regimes, caused by construction and
operation of Flaming Gorge Dam, which could impact egg and larval
development and survival (Miller et al. 1982; Wick e1 al. 1982). After the
dam became operational, water temperatures in the river below the Dam
were colder later in the year compared to pre-dam conditions. Apparently,
. flannel mouth suckers have not been as severely impacted as razorbackS,
even though both species occur sympatrically and spawn at about the same
time in the Green River. (It is not known that they actually spawn in the
same location.) Therefore, we speculate that perhaps the difference
observed in egg or larval size could effect relative survival rates of each
species in cooler water. Cooler water would delay hatching or swimup.
The fish is living off stored energy during this period. Since species with
larger egg biomass could potentially store more energy than those with
lower biomass, perhaps species with larger eggs could survive the stress
of delayed hatching better than species with smaller eggs. An interesting
experiment would be to measure the relative energy stores present in the
eggs of each species via bomb calorimetry, the energy utilization rate
(metabolic rate) of eggs of each species held at different temperatures,
and the time required for eggs to hatch and larvae to swim-up at different
temperatures; then use this information to calcualte the energy stores
remaining for each species at the time of hatch and swim-up.

Our results also indicated that developing flannelmouth suckers
experienced thyroid hormone surges at the time of hatch and the time of
swim-up. Concentration of the surge at the time of hatch (8.5 to 18.6

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