7125

• UCREFRP Catalog Number: 7125
• Author: McAda, C. W.
• Title: Aspects of the Life History of Three Catostomids Native to the Upper Colorado River Basin.
• USFW Year: 1977.
• USFW - Doc Type: Utah State University,
• Copyright Material: YES

where Ud and U1 are experimental measurements corresponding to the tension of the
fish body.
The values calculated according to formula (1) and the experimental values
HE, given in Fig. 1, correspond well. Knowledge of KE in essence reduces the
question of determining Ud to the simpler problem of calculation U1.
The threshold values (Ul)n depend on the electric conductance of the water j
B
and current density S.. The nature and degree of this influence are shown in
Figs. 2 and 3. In Fig. 2 diagrams of the dependence of the threshold values of
Ul and ba on the relative electric conductance 7v are shown for the electroshock
Y¦
reaction in the bream and pike-perch. It is clearly visible from the graphs that
the greatest changes in tle values (U,)„ and (S.). are observed in the area of
the small ratios 7i' and of small values of electric conductance of the water I..
Tp
Graphs of the dependence of (U,)are given in Fig. 3. The sharp
drop in the threshold values (UI)n in the section where y. differs little from Yr
is particularly striking. The spread of the experimental points in this section
is also characteristic. The nature of the curves for the pike-perch and bream
differed considerably, as did the position of the points on the graphs in which
Tp= Y,. All this confirms the position regarding the considerable influence of
boundary conditions (I.,di,) on the threshold Ul in the region where Ya and 1. are
close. As yet it is difficult to explain the nature of the median part of the
graphs Together with this, determination of the overall form of this .?
dependence is of great significance for the forecasting of means of electric fish-
ing and fish-diversion devices. From the diagrams on Fig. 3, in particular, it is'
easily seen that in oligotrophic fresh water it is not possible to obtain stable
reactions from the fish, because the values (U,),, increase sharply, while for marine
water the minimum threshold values U, can be assessed by means of these graphs and
the technical parameters of the equipment calculated. Knowledge of the section in
which a sharp change occurs (U,), is of no less factual significance.
When analyzing the effects of an electric current on a fish the question of
the influence of body length on the threshold values of U1 is one of the main
questions. To establish the actual dependence (U,).=j(1p) it is above all
necessary to assess the influence of conditions under which the threshold values
in aquaria are measured on the result, allowing also for the nature of the de-
pendence obtained
w
s
c 1,0
V
d
---1
Pike-perch '
ream
1 2 J 4 J 6
Relative electric conductance, YOTN
Fig. 1. Calculated (1) experimental (2) values of the
distortion factor of the fish body.
146
'O
U
;S
GQ
QS

0
M
F
w
B.
z
Relative electric conductance, t oTN
Fig. 2. Diagram of dependence on threshold values of
U, and bo on the ratiouft.-
> 6

4
0
u
u 1
a
-2, p
Fig. 3. Diagram of dependence on threshold values of
U, on current density da in the species (A and B be-
ing the points at which the electric conductances of
the water and the fish body coincide).
In the first place the ratio between the size of the aquarium and the body
of the fish influences the values of the threshold reactions as a result of the
different degree of distortion of the electric field by the fish body and the
heterogeneity,,of a field in aquaria of different sizes. The influence of the
distortion of the field by the fish leads to a greater increase in values (U,),
the smaller the ratio of the length of the aquarium to the length of the fish.
The heterogeneity of the field during calculated determination and its in-
fluence are the more marked, the smaller the length of the fish 1,,.
In the second place, when determining the threshold of the reactions of fish
of varying length in aquaria with a constant base L, a change (Ul)n occurs be-
tween the electrodes,' provoked by the change in current density in the water.
For instance with 'a twofold decrease in the length of the fish, in order to ob-
tain threshold value Ul the voltage between the electrodes needs to be approxi-
mately doubled. This provokes an increase by as many times of current density
in`the water (Q.,. In accordance with the diagram of dependence (U,).=f(rp)
(Fig. 3) the increase in 6. leads to a decrease in (U,),,, The closer the values
Yo and Y•, the more marked this influence.
147
Current density, Alm'