Dr. Alan Temple posted a blog on December 9, 2015 entitled “Setting Doses for Lab Experiments.” He suggested that I submit a blog on other aspects of lab studies in tanks. This blog covers the setup of tank studies for electrofishing research, and I plan to submit a companion blog on procedures for tank studies. Important aspects to consider for lab studies are the test tank, the electrodes, the power supply and the electrical field.
Test tanks can be of almost any size. For several studies and demonstrations with fingerling size fish, we have used standard aquariums of 15 to 20 gallons (approximately 55 to 75 L). A 15-gallon aquarium has dimensions of 1 ft. W x 1 ft. D x 2 ft. L (or approximately 30 x 30 x 60 cm). The water depth should be sufficient to allow the fish freedom of movement; 8 inches or 20 cm water depth works well for fingerling fish. Longer tanks are required for studies of fish attraction to the anode, called taxis, and larger tanks are needed for larger fish. Photos below are of a variety of tanks used in actual studies.
In addition to the test tank, there should be tanks, tubs or buckets for holding fish prior to testing and separate tanks, tubs or buckets for recovery after testing. Suitable size dip nets are needed for capturing and transferring fish among the various tanks. These holding and recovery tanks, tubs or buckets should be supplied with adequate dissolved oxygen and not be loaded so much as to cause stress on the fish.
Plate electrodes should cover the flooded surface area of the test tank ends, and the two electrodes must be placed parallel to each other and are normally oriented at right angles to the long axis of the tank. There have been studies in which the electrodes are placed parallel to the long axis of the test tank, but that is rare and will not be discussed further in this blog. Usually, the plate electrodes are placed at or near the ends of the tank, but they may be placed closer together and therefore away from the tank ends. The critical point is that they are parallel to each other, that they are perpendicular to the long axis of the tank, and that they completely cover the flooded cross-sectional area of the tank ends. For example, if the tank ends are 30 cm wide and are flooded to a depth of 20 cm, then each plate electrode must cover this area of 600 square cm. By positioning the plate electrodes as described, the field intensity or voltage gradient is the same anywhere inside the tank between the electrodes. That is termed a homogeneous field or a uniform field. Field intensity is usually measured in volts per cm though sometimes in volts per meter or volts per inch. As Alan mentioned in his blog, the power density in µW per cm^3 is also uniform or constant between the electrodes. A uniform electrical field is needed for efficient tank studies of fish response. The uniform field must be confirmed by measuring field intensity with a voltage gradient probe attached to an oscilloscope or suitable digital voltmeter. The voltage gradient probe is moved around inside the energized tank to test the uniformity of the field; the caveat is that the voltage gradient pins must be oriented in the long axis of the tank, i.e. the probe must be turned so that the maximum voltage reading is obtained. A voltage gradient probe can be easily made with some PVC pipe and cap plus some wires. Its construction is the subject of another blog.
The test tank plate electrodes may be constructed of some metal. We have used stainless steel, aluminum, tin, copper and some other metals. The electrodes usually are made of solid plates, but we have used perforated metal and various types of metal mesh. If they are not solid plates, the holes or mesh size must be small so as to provide a uniform field. Large mesh or large holes could produce a non-uniform field in which the fish may be exposed to varying field intensity; that must be avoided for quantitative lab studies. Mesh sizes of 3-4 mm or perforations of 4-5 mm or less may be satisfactory. Again, confirm the uniform field with a voltage gradient probe. One more thing about plate electrodes. Unless they are thick metal, it is a good idea to make them wider than the tank and then put an almost right angle (90 degree) bend on the excess portions on each side. That provides rigidity to thin metal and allows them to fit snugly to the tank sides.
Selection of a suitable power supply for the test tank depends upon what waveforms are to be tested, the distance between the electrodes, the water conductivity, the desired fish response, fish size and, to some extent, the test procedure. From 15-gallon aquaria to the longer tanks shown in these photos, with immobilization as the desired response, the applied voltage should be within the range of about 15 to 40 volts of direct current or pulsed direct current, with rare exceptions. The voltage should be continuously adjustable, or adjustable in one-volt increments, over this range and perhaps beyond. If there are 50 cm between electrodes, that applied voltage range would equate to field intensities of 0.3 to 0.8 volts per cm. Fingerling fish are generally immobilized by effective waveforms with field intensities in that range. Less effective waveforms may require higher field intensity to produce immobilization. As a general rule, larger fish are immobilized with lower field intensities than are small fish. The desired power supply must be able to safely and consistently provide 15 to 40 volts in increments of about one volt or less, preferably with the ability to adjust voltage quickly under load, i.e. while the fish is being exposed to the electrical current.
If pulsed direct current is to be evaluated, then it is important to have a power supply which has independent controls for voltage, duty cycle (or pulse width, also called pulse duration) and frequency, all over the desired ranges to be tested. Duty cycles of 10 to 50% are often used in the field; I have tested taxis in gar using a 95% duty cycle. Continuous DC can be considered a duty cycle of 100%; there is no pulse off time because there is no pulse with continuous DC. Common frequencies used in the field include 30 to 120 pulses per second, although lower frequencies (12 to 20 pps) are used for flathead and blue catfish sampling in the US. There is some interest in using high frequency (300 to 500 pps) pulsed direct current to collect fish. Also, there is interest in evaluating fish response to more complex waveforms such as packets of pulsed direct current and fish response to various shapes of pulses, including alternating current.
The power supplies which I have used for tank studies are two backpack electrofishers made by ETS Electrofishing – the ABP-3 and the ABP-2, both powered by 12-volt batteries. They have continuously adjustable voltage from 1 to 600 volts, continuously adjustable duty cycle from 1 to about 98% (without the continuous DC option), and continuously adjustable frequency from 1 to 1000 pulses per second. We also conducted tank studies using a boat electrofisher powered by building line voltage of 240 volts through an isolation transformer and a custom connection box. In both cases, the power supply was the Midwest Lake Electrofishing Systems Infinity boat pulsator. There may be other electrofishers which are suitable for lab experiments, but most have voltage ranges above the low level required for tanks, and most lack sensitive, quick voltage control in one-volt increments.
These are some aspects of setup for tank studies to evaluate fish response to electrical fields and to various waveforms. Another blog will cover procedures for tank studies.