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

235
rate of removal of momentum from a flowing
fluid; Webb 1975, Vogel 1994).
Reynolds numbers indicate the kinds of forces
that have to be overcome by an object moving in a
fluid. These numbers are a ratio of the inertial and
viscous forces, which for fish are dependant on
three factors: the swimming velocity of the fish,
characteristic length of the fish (where shape is
factored in), and viscosity of the water (Webb
1975, Denny 1988). Streams exhibit Reynolds
number values of approximately 100 000, where
neither viscous nor inertial forces dominate (Webb
1975, Vogel 1994, Lampert & Sommer 1997), and
shapes of fishes that result in a hydrodynamic
advantage may sometimes be difficult to predict.
In most cases, stream fishes reduce structures that
induce drag because increased drag also would
increase energetic costs. Adult stream fishes are
generally streamlined, with an exception in fishes
that do not carry out sustained swimming (Webb
1984, Diana 1995).
In addition to drag, another hydrodynamic
consideration for life in moving fluids is lift. Lift
can be defined as any force that is normal to the
direction of flow, in this case up (i.e., `positive lift')
and down (i.e., `negative lift'; Denny 1988). The
amount of lift generated by morphology depends
on how much the flow is turned by the shape of the
object. If flow above and below the shape are the
same, no net lift results. However, if water pres-
sure on the dorsal surface is less than that below,
the result is a net upward force: in this case, the
increased area of the leading edge deflects the
water so that the streamlines are crowded together
above the object. This crowding thus causes an
increase in water velocity on the dorsal surface
resulting in dynamic lift (Schlichting 1979, Denny
1988, Vogel 1994).
The `nuchal hump hydrodynamic advantage
hypothesis' attributed to Miller (1946) and La
Rivers (1962) can be evaluated by determining the
components of drag and lift produced by the
humps. In this study we determine drag and lift
components associated with large humps to eval-
uate whether the humps confer a benefit for life in
fast flow. In addition, we explore an alternate
hypothesis regarding the advantage of large humps
by examining other factors exerting evolutionary
pressures for hump development, such as a need
for a morphological defense against predators.
Materials and methods
Drag and lift
Three of the fishes we studied, G. cypha, G. elegans
and X. texanus, are endangered species protected
by federal and state statutes, thus live or freshly
sacrificed fishes could not be used in hydrody-
namic tests. Instead, polyester resin casts of mu-
seum specimens were used to obtain information
on how different body forms would be affected by
water flow.
We made whole-body casts of each species from
silicone rubber molds. We suspended fish speci-
mens in a plexiglas molding box filled with GI-
1000 RTV Silicone Rubber (Silicones, Inc.). We
mixed the silicone rubber with a fixed amount of
activator, placed it into a vacuum chamber, and
rapidly degassed it before pouring it into the
molding box. Once rubberized, we removed the
mold from the Plexiglas® box, divided it into two
halves, and retrieved the fish from the mold.
The cast-making procedure was as follows: the
silicone rubber mold was placed back into the
Plexiglaso box to maintain shape and we placed a
brass mounting apparatus (used to connect the
two-dimensional force beam to the fish cast) inside
the mold. We then filled the mold with polyester
resin mixed with 60 pm diameter glass micro-
spheres (3M S60; Minnesota Mining and Manu-
facturing Company) to 40% of the total volume to
add strength to the casts and to serve as filler.
After a day of hardening time, we removed the
casts with embedded mounting apparatuses from
the molds and sanded them to remove any
imperfections.
We made casts from molds of different fish
morphotypes to evaluate the hydrodynamics of
each body plan. We made two different casts of
each species morphotype using a cast most repre-
sentative of a fully developed, mature adult. We
used casts from the fish exhibiting enlarged nuchal
humps (i.e., a 381 mm TL G. cypha and a 458 mm
TL X. texanus) as the treatment group to measure
drag and the direction of force resulting from the
prominent nape. We later removed the humps on
these fish models and retested the casts to assess
performance without the humps. We compared lift
and drag values for those casts with nuchal humps
to the same casts with humps removed. We used