7031

• UCREFRP Catalog Number: 7031
• Author: Deacon, J. E.
• Title: Endangered and Threatened Fishes of the West
• USFW Year: 1979
• USFW - Doc Type: Great Basin Naturalist Memoirs
• Copyright Material: YES

1979

THE ENDANGERED SPECIES: A SYMPOSIUM

gust and November was extensive enough to
provide a good representation of population
structure in woundfin. The major fact re-
vealed is that in 1977 young-of-the-year com-
prised a very small (nearly inconsequential)
proportion of the woundfin population above
the narrows. By contrast, a comparison of
population structure in woundfin above the
narrows in 1977 and 1978 (Fig. 3) indicates
that young-of-the-year dominated the popu-
lation in 1978.
When sampling is extensive enough, and
stunting can be discounted as a significant
factor, much of the information gleaned from
an examination of length frequency can be
summarized by calculation of a mean length
for the population. In this case, for both
woundfin and roundtail chub, small mean
length indicates relatively high reproductive
success and vice versa. Figure 4 and Table 2
present data available on mean length of
woundfin above the narrows in 1973, 1977,

30

25

z
0 20
~
<t
....J
::)
a.. 15
0
a..
-
0
~ 10
0

5

20

30

40

50 60
SIZE

90

and 1978, together with a hydro graph of
mean monthly flows. They show that in 1973
and 1978, with high winter and spring flows,
reproductive success was high, but in 1977,
with low flows, reproductive success was
low.
A similar situation appears to have existed
for the round tail chub, Gila robusta semi-
nuda (Fig. 5, Table 2), except that the species
was so rare in 1977 that very few were cap-
tured in spite of extensive sampling efforts.
This, of course, indicates that not only were
environmental conditions in Virgin River
during 1977 inimical to successful spawning
in this species, they also apparently reduced
the survival of adults. Figure 5 does show
that the species spawned successfully in at
least one location on the upper mainstream
of the Virgin River in 1978. Relatively high
population density or evidence of a successful
hatch was not found at any other location
sampled in the upper or lower Virgin River

0-0 FALL 1977 (N=370)
- FALL 1978 (N= /53)

70

80

Fig. 3. Comparison of length frequency of woundfin in Virgin River above the narrows in fall 1977 and fall 1978.

--

45