1108

• UCREFRP Catalog Number: 1108
• Author: Valdez, R. A.
• Title: Fisheries Biology And Rafting Annual Summary Report - 1987
• USFW Year: 1988.
• USFW - Doc Type: ‎(Contract no. 6-CS-40-03980)‎.
• Copyright Material: NO

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densi ty nursery areas for the species (Biological Sub-Conuni ttee 1984). The
reader is reminded that the CPE's in this report are the result of sampling
for larvae and YOY throughout the summer, in contrast to the FWS ISMP which
assesses CPE for YOY squawfish only from sampling in the fall, September or
October. As a result, the CPE's in this report are probably higher than com-
parable CPE's from the standardized monitoring program for the respective
regions.

ii. Hatching Dates. In order to gain a better understanding of the
envi ronmental factors that influence hatching success and survival of YOY
Colorado squawfish, hatching dates for 490 YOY and larvae were back-calculated
using the length/age regressions developed by Haynes et al. (1985). The
relationships are as follows:

REGRESSION A:

For fish with a total length of less than 22 rom

Age = -76.7105 + 17.4949L - 1.055L2 + 0.0221L3 (r2 = 0.99)

REGRESSION B:

For fish with total lengths of 22.0 to 47.0 rom

Age = -26.6421 + 2.7798L (r2 = 0.99)

When the back-calculated dates of hatching are plotted on a frequency histogram
(Figure 3), it becomes evident that in 1987, hatching of Colorado squawfish,
captured in the Cataract Canyon region, occurred primarily from mid-June
through late July. It appears from this analysis that some fish also hatched
during May. This histogram represents hatching times very different from those
seen in 1986, when hatching peaked much later; in early August. When compared
with a similar histogram developed for YOY captured from the same area in 1985
(Valdez 1985), differences in hatching times between the three years become
evident. In 1985, hatching began the first of June, peaked during the first
week of July, and continued through the first week of August. Clearly,
hatching of Colorado squawfish captured in Cataract Canyon occurred at dif-
ferent times of the summer for the 3 years examined. Spawning dates can be
determined by subtracting 3-5 days (Hamman, 1981) from the hatching dates.

Some of the variation seen in back-calculated hatching dates might be
attributed to differential growth rates, slight errors in length measurements
afield, or spawning events separated by time. But, the most likely explanation
for this variation is that since Cataract Canyon is located below the con-
fluence of the Green and Colorado rivers, it receives recently-hatched fish
being transported from more than one spawning site upstream on ei ther river.
It is also possible that some of these fish originated from spawning sites in
or near Cataract Canyon.

In an effort to assess the effect of the two variables, water temperature and
river flow, on the hatching times of Colorado squawfish, these were plotted
wi th the frequency histograms in Figure 3. Temperature data for 1985 and 1986
are from the bottom readings of the BOR gauge at Clearwater Canyon in Lake
Powell (about 8 miles below Imperial Canyon). Temperatures dates in 1987 is

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