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Size Matters

Yes, size matters…and that includes fish size when electrofishing. Large fish are immobilized with less field intensity or power density than are small fish.  Large fish sustain a higher total dose of electrical energy than do small fish; this is sometimes referred to as whole body voltage. An excellent paper on this topic is Dolan, C.R. and L.E. Miranda. 2003. Immobilization thresholds of electrofishing relative to fish size. Transactions of the American Fisheries Society 132:969-976. This short blog provides results of a simple study with various sizes of alligator gar.

Alligator gar exhibit a tremendous growth rate.  We have reared them for years and have witnessed average growth rates of about 12% of body weight per day in tanks. That provided the opportunity to determine threshold field intensities (voltage gradients in volts per cm) and head-tail voltages for the same lot of fish but of different sizes and within a short period of time. The head to tail voltage was the measure of whole body voltage used in this study. Head-tail voltage was calculated as the product of threshold field intensity (V/cm) and the fish total length in cm.

Another thing of interest in gar culture is that they exhibit cannibalism at a small size. They must be culled often when only 2-3 cm total length and beyond; sometimes we would cull out the cannibals two times per day. The photos of gar in the test tank are fish of the same lot and age. The larger ones became large by eating their siblings.

The study involved alligator gar subjected to square-wave pulsed direct current of 50 pulses per second and with a 25 percent duty cycle. Thus, the pulse width or pulse duration was 5 ms. The power supply was an ETS ABP-3 backpack electrofisher, and the plate electrodes produced a uniform electrical field inside the test tank, i.e. the field intensity was the same throughout the tank. This study was conducted on three different days, in two different tanks and with slightly different water conductivity. The voltage and field intensity data were adjusted to 100 µS/cm using the maximum power transfer theory model.

These results demonstrate that larger gar are immobilized with a lower field intensity (V/cm) but with a higher overall head to tail voltage, as expected. That is true for fish in static water tanks with a uniform electrical field and with no chance to escape. However, the situation in the wild may be different. Larger fish may be able to burst through a non-uniform electrical field if the effective field size is relatively small. This could be true for the field produced by a backpack or boat electrofisher and even by a small electrical barrier.

Larger fish required a lower field intensity (V/cm) for immobilization.

Larger fish required a lower field intensity (V/cm) for immobilization.

 

Larger fish required more total whole body voltage for immobilization.

Larger fish required more total whole body voltage for immobilization.

 

Test tank with two sizes of alligator gar exposed to the same field intensity (V/cm). The larger fish was immobilized; the smaller ones were not.

Test tank with two sizes of alligator gar exposed to the same field intensity (V/cm). The larger fish was immobilized; the smaller ones were not.

https://youtu.be/ICY3Jc4ESt4

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