One of the factors which contribute to diminishing the efficiency of electrical fishing is the tetanic effect of the current in the first fishing and the fatigue which may follow in subsequent fishings. In the first instance, fish stunned in their lairs have less chance of being caught and those which are already close to the anode are washed away before there is time to catch them. In the second case, tired fish may not be equally catchable, and the efficiency of a second fishing may be affected by this. It is therefore important in the plan of research to determine which currents have the least possible tetanizing and fatiguing effects.
In addition, one of the qualities essential in the electric fishing compared with other methods is avoiding damage to fish. Now it is observed that even in practical electric fishing, fish can be killed.
Fatigue and mortality tests thus constitute a logical complement to the tests of fishing efficiency. Such tests are susceptible to statistical interpretation, but they must be designed for this purpose. In this first experiment, it was felt preferable to organize the tests in an exploratory manner, with the object of obtaining the maximum amount of data as opposed to simplifying the statistical analysis.
The principle employed was the exposure of fish for a definite time to the current supplied by the machines which had been tested for their fishing efficiency, in conditions as close as possible to those of the fishing tests. The fish were stunned for a set time and the stunning or loss of equilibrium persisted after the current was switched off. The time taken for the fish to recover after the current was switched off was taken as the Index of Fatigue. Subsequent mortalities were also recorded.
With the Index of Fatigue to be determined, the experimental variables were:
- the machine under test
- field intensity
- number of exposures to the current
- size of fishes
Each machine was tested against two batches of trout (A and B). Each batch consisted of 6 fish, i.e., two of size-length 5–10 cm, one each of size-length 10–15 cm, 15–20 cm, 20–25 cm and 25–30 cm.
A container consisting of a wooden framework covered with plastic mesh forming an enclosure 40 × 40 cm, and having a plastic mesh partition dividing it across the centre to form front and back compartments 40 cm wide and 20 cm thick, was used to contain both batches of fish (Figure 6).
The anode of the machine under test was located in front of the box, 20 cm from the central partition on the first occasion and 40 cm on the second. This provides four combinations of distances, corresponding to the relative field intensity to which the fish were exposed:
| - | Test at 20 cm: | Fish in compartment | A' | ||
| " | " | " | B' | ||
| - | Test at 40 cm: | " | " | " | A' |
| " | " | " | B' | ||
The cathode was put in the same position relative to the anode which it occupied during the efficiency test.
Table X shows the distribution of the fish in the two compartments, by number and size.
Fish of batch A were placed in the compartments A' and B' according to the plan shown in Table X, and the anode of the machine under test was placed, as previously stated, at 20 cm from the central partition for the first test.
The current was then switched on for 15 seconds. The fish were timed from the start of the exposure period to the instant at which they regained their equilibrium; in order to eliminate variations in the assessment of this point, the timing was done by the same operator throughout. The fish were stimulated with a probe to accelerate their recovery. This method is obviously open to criticism, but in the field conditions of the tests, it was difficult to use a better one.
From the commencement of this first exposure to the current, a time of five minutes was allowed to elapse before the same batch of fish was subjected to a second exposure to the current in the same conditions, and then to a third.
A second test followed with the fish of batch B distributed between compartments A' and B', and the anode located 40 cm from the central partition. The conduct of the second test was identical with that of the first.
We have taken as a measure of the Index of Fatigue the time which elapsed between the commencement of exposure and the instant at which the fish recovered its equilibrium. It would perhaps have been more logical to reckon the time elapsed between the end of exposure and the instant of recovery. The one can be converted into the other by simply subtracting 15 seconds from each Index of Fatigue. This has been done in some cases, and for the mean figures shown at the foot of Table X. This does not affect the conclusions; it merely reduces the variations between the different values to a slight extent.
The results are shown in Table X. Three fish (column 5) were lacking. Only a single fish was dead, which precludes interpretation of the mortality test. In calculating the averages for the Indices of Fatigue, the dead fish was times for 300 seconds. This is in fact a time after which a trout which is not yet breathing has little chance of survival without an artificial breathing.
A matter of some interest is whether there was a correlation, either positive or negative, between the Indices of Fatigue and the efficiencies of the various machines.
Table II shows the Mean Indices of Fatigue for each machine with the corresponding efficiency. There is no obvious correlation.
The following comments may, however, be made. Machine D, with a relatively low efficiency, appeared to cause more shocking; was its low efficiency due to this? Machine C, which had a particularly low efficiency, had by far the lowest Index of Fatigue. Can this be the direct result of insufficient effect on fish?
A great variation of the values for the Index of Fatigue is obvious in Table X, to the extent that it is questionable to what point those in Table XI may be considered significant. This dispersion may be due to the distance from the anode at the instant of the test. They were in fact distributed at random in the two compartments. Those in the compartment nearer to the anode could be at any distance between 0 and 20 cm from it in one test, and between 20 and 40 in the other. In the first case, the potential gradient falls very rapidly in the 20 cm; it follows that the fish adjacent to the anode (0 cm) are subject to much more current than those at the back of the compartment (20 cm).
By taking as a reference value the distance from the centre of each compartment, four groups of distances are obtained: one of 10 cm, two of 30 cm and one of 50 centimetres. By making a differential division of the length classes between the various groups, it is possible to compare them two by two:
- a distance of 10 cm against one distance of 30 cm,
- the other distance of 30 cm against a distance of 50 cm.
The comparison is given in Table XII. It may be considered that even if the Index of Fatigue appears to decrease significantly between 10 and 30 cm, a difference is not recognizable between 30 and 50 centimetres.
This result was readily predictable. The conclusion is, that for future tests, there is no point in using distances over 20 centimetres.
Another point to be taken into account for future tests is to use for all the tests the same electrode. In the present case the anodes were of different shape; it resulted in different values of potential gradient at the fish, independently of the machine voltage.
The two final lines of Table X seem to suggest that fatigue increases with fish length. It is necessary for the interpretation of the results to separate the values of the compartment A' from those of compartment B', since the two are at different distances from the anode. For the trout of compartment A' the variations between the mean Indices of Fatigue are greater than those found in B'; the greater distance probably diminishes the variations.
Table XIII shows the distinct decrease in the Index of Fatigue with successive exposures to the current for the 20-cm distance. This appears to represent a diminished sensitivity to the electric current on the part of the fish, and might be correlated with the diminishing efficiency of successive fishings.
As only a single fish was killed, it is not possible to draw any conclusions. Nevertheless, a mortality test ought to kill fish; on this point, the test was insufficiently rigorous.
The following phenomena have been observed:
- Variation of Indices of Fatigue between the different machines,
- Increased value of the Indices with increasing field intensity,
- Increased value of the Indices with increasing trout length,
- Decreasing value of the Indices with successive exposures to field, indicating a diminishing sensitivity in the fish,
- Virtual absence of mortality which discounts any subsequent damage caused by the machines.
Throughout, the differences are masked where the potential gradient is insufficient.
These results were easy to foresee. The advisability of carrying out such tests in the field, where conditions are variable, is debatable.
It appears to us that this type of test ought to be made in a laboratory. It has a certain importance for improving the efficiency of electrical fishing, but a fatigue and a mortality test should no longer be considered in connexion with the comparison of the machines themselves, but comparison of types of current. The experimental conditions should therefore be standardized in terms of the following parameters:
- water conductivity,
- a standard fish length,
- the dimensions and characteristics of the experimental trough,
- the period of exposure,
- the determination of the Index of Fatigue or Mortality.
These tests will then be susceptible to statistical analysis.
