9511

• UCREFRP Catalog Number: 9511
• Author: Portz, D. E. and H. M. Tyus
• Title: Fish humps in two Colorado River fishes
• USFW Year: 2004
• USFW - Doc Type: a morphological response to cyprinid predation?
• Copyright Material: YES

239
0.9
0.8
0.7
CO 0.6
O
m 0.5
Z 0.4
0.3
0.2
0.1
0.3-
0 0.3 m sec'
0.2 110 0.9msec'
C
1 0.1
m
Z
? 0.3 m secs
®0.9msec'
X. texanus X. texanus G. cypha G. cypha
(hump removed) (hump removed)
Figure 3. Drag (Newtons) exhibited by body casts of X. tex-
anus and G. cypha in the experimental flume with water veloc-
ities of 0.3 and 0.9 to s-1. Casts without hump are the same as
those with hump except the hump has been removed by sand-
ing.
exactly proportional to a velocity-squared rela-
tionship when fish casts with humps and humps
removed were tested at velocities of 0.3 and
0.9 m s-1 (Figure 3).
Lift assessment
As expected, net lift varied greatly among the
objects tested, and there was no lift on the sphere.
A sphere is symmetrical; therefore the low pressure
on one side is offset by the low pressure on the
other side, thus, there should be no net force per-
pendicular to the flow (Denny 1988). Lift exhibited
by the fish casts was dependent on species, but
negative lift was unaffected by the presence of a
hump. The nuchal process did not have an influ-
ence on the downward forces related to body de-
sign (negative lift), as indicated by the lift force
values of X. texanus and G. cypha casts before and
after removal of the hump, both tested at 0.3 and
0.9 m s-1 (Figure 4; p > 0.05). A slightly negative
lift was produced by X. texanus; however, G. cypha
produced a substantial positive lift, especially with
higher water velocities (Figure 4). The different
morphologies of X. texanus and G. cypha no doubt
resulted in very different trends in lift performance.
In part, this disparity can be explained by the
noticeable asymmetry of G. cypha. A greater per-
centage of body volume is found above the dor-
soventral mid-point between the leading edge of
the rostrum and the tailing edge of the caudal
-0.1
X. texanus X. texanus G. cypha G. cypha
(hump removed) (hump removed)
Figure 4. Lift (Newtons) exhibited by body casts of X. texanus
and G. cypha in the experimental flume with water velocities of
0.3 and 0.9 m s-1. Casts without hump are the same as those
with hump except the hump has been removed by sanding.
Positive values indicate an upward force and negative values, a
downward force.
peduncle, thus producing positive lift due to lower
pressure on the dorsal surface.
Prey morphology
Prey body depth at the nuchal hump was highly
correlated with body total length for both X. tex-
anus and G. cypha (r2 = 0.984, n = 119,
range = 67-559 mm TL; r2 = 0.986, n = 224,
range = 36.5-410 mm TL, respectively). A similar
relationship also existed for fishes without nuchal
humps, C. latipinnis and G. robusta (r2 = 0.985,
n = 148, range = 34-574 mm TL; r2 = 0.987,
n = 228, range = 39-447 mm TL, respectively;
Figures 5 and 6). However, the slopes of the fishes
without nuchal humps was less than that of the
fishes with enlarged nuchal humps, demonstrating
that fishes with nuchal humps increase in body
depth much earlier in development (Figures 5 and
6).
Predator mouth gape
The dorsoventral measurement of mouth gape in
P. lucius determined the largest size prey that
could be ingested. Predator mouth gape was cor-
related with total length (r2 = 0.911, n = 103,
range = 175-805 mm TL).
When predator mouth gapes and prey nuchal
region depths were considered together it is evident