7821

• UCREFRP Catalog Number: 7821
• Author: Hamilton, S. J., et al.
• Title: Evaluation Of Toxicity To Larval Razorback Sucker Of Selenium-Laden Food Organisms From Ouray NWR On The Green River, Utah.
• USFW Year: 1996.
• USFW - Doc Type: \
• Copyright Material: NO

them more energy and subject them to greater predation pressure.

The zooplankton fed in the present study were predominantly (::::: 80-90 %) cladocerans
and copepods in about equal numbers with small numbers of rotifers and algae. Marsh and
Langhorst (1988) found that stomachs of larval razorback sucker in Arizona Bay backwater
contained rotifers 53 % of the time, Bosmina (Cladoceran) 55 %, and copepoda 45 %. They
also reponed larvae showed a positive selection for cladocerans (especially Bosmina) and
strong negative selection for rotifers. Langhorst and Marsh (1986) in a more detailed report
of their Lake Mohave research with larval razorback sucker (Marsh and Langhorst 1988)
reported larvae used all available habitats (limnetic, benthic, and macrophytic) as evidenced
by the presence in larvae stomachs of Macrothrix, a Cladoceran associated with vegetation.
Papoulias and Minckley (1992) reported that razorback larvae initially ate sessile diatoms,
phytoplankton, and detritus, but by 7 days posthatch larvae were eating rotifers, nauplii.
cladocerans, insect eggs, and chironomids, which suggests larvae were opportunistic feeders.

In the present study, zooplankton less than 0.425 mm were fed during the exposure.
Langhorst and Marsh (1986) reported that larval razorback sucker 11-14 mm total length
selected smaller plankton to feed on compared to sizes available in the plankton population at
large. They found size selection was especially notable for Daphnia, which averaged 0.66 to
0.86 mm in length, in larvae stomachs, about half the average size (1.42 to 1.60 mm) in
plankton samples. Papoulias and Minckley (1992) found larval razorback sucker showed
prey-size selection (as body width), but larvae did consume prey from 0.1 to 0.4 mm.
Consequently, the size of zooplankton fed to larvae in the present study was in the
appropriate size range.

Larval razorback sucker in studies 3 and 4 grew to 12-14 mm total length by 44-48
days posthatch. This growth is comparable to that reported by Papoulias and Minckley
(1990) for laboratory-reared larvae, which were 14-16 mm total length at 50 days posthatch.
Growth of larvae in studies 3 and 4 was also similar to that in the supplement feeding study
with 24-day old larvae, which were 14-15 mm total length at 44 days posthatch. Even
though the feeding rates in the supplement feeding study were four-fold different (20 versus
80 food organisms/fish in the low and high treatments), the difference between feeding
treatments in total length was only 13 %. In contrast to studies 3 and 4, Papoulias and
Minckley (1992) reported larvae reared in fertilized ponds for 42 days posthatch had total
lengths of 16-19 mm. Likewise, Marsh and Langhorst (1988) reported larval razorback
sucker grew up to 16 mm in a Lake Mohave backwater in a few (:::::6) weeks, but larvae in
Lake Mohave reservoir showed no growth beyond their initial size of 10.6 mm.

Weight of larvae was not affected in study 3, but was reduced in study 4 in fish fed
zooplankton from S5. Weights of larvae in the present study were close to those of larvae in
the supplemental feeding study that were fed either 20 or 40 organisms/fish. However,
weights of larvae in studies 3 and 4, and even in larvae fed 80 organisms/fish in the
supplement feeding study, were lower than those of Papoulias and Minckley (1992) who
reported 6-week-old larvae weighed 25 mg in ponds with invertebrate densities of 12

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