In Part A of this blog, it was calculated from a lab study that the effective conductivity of Grass Carp was 62 uS/cm. In Part B, let me show you a simple way of using that information in the field to improve the capture success of Grass Carp based on those results. You may want to download the Electrofishing Tool or App from the Tool section of this site.
The power transfer model uses two parameters to derive a power, voltage or current goal for fish capture. In the lab study of Part A, the two parameters used to describe a curve were effective fish conductivity, Cf, and power density at match, Dm. That works for a tank set up to produce a uniform electrical field, and the results can be quite informative. However, for a typical electrofishing effort in a lake or stream, the electrical field in not uniform; it varies with distance from the electrode in an exponential decay function. Thus, power density decreases dramatically with distance from the electrode.
For field electrofishing, fishing goal tables require inputs of Cf and threshold values (minimum needed for totally successful fishing) for either voltage, current or power from at least one fishing trip. We have developed an Excel file for easily and quickly constructing a fishing goal table for a range of future water conductivity values.
One approach to developing a field-useful power, voltage or current goal for fish capture is to go fishing and adjust voltage (we only adjust current and power indirectly by adjusting voltage, or sometimes by electrode changes, but usually by simply adjusting voltage) until we determine the threshold voltage, power or current for a given ambient water conductivity (see the prior blog on ambient conductivity at this site). Then you can use that threshold value to calculate the threshold values for any other ambient conductivity. It is best to determine threshold values a few times over a range of water conductivity and sampling situations to make the most useful goal table, but an initial goal table can be constructed from one successful fishing event.
In addition to the Excel file, an Electrofishing Tool or App has been developed which allows you to estimate a goal setting for a given water conductivity. The Tool can be used with a smart phone in the field. Even I was able to load it onto my phone; it is a resident file, so an Internet connection is not required in the field, I am told. In fact, I’ve used it on an iPad and on a laptop computer, but the original intent was to use with a smart phone because someone in the field is likely to have a smart phone with them these days. Let me show you how the Tool works using the Grass Carp information.
Here is the first page of the Tool. All of the buttons link to useful information, but click on Standardization Calculators for this exercise.
This is the next page of the Tool, after selecting Standardization Calculators. You may wish to read the information once, but after that you likely will just select Go to Calculators.
When you get to the field, first take water conductivity. If your conductivity meter only measures specific conductivity, then use the Ambient Conductivity Calculator to calculate ambient conductivity from water temperature and specific conductivity. If your conductivity meter displays ambient conductivity, then skip the Ambient Conductivity Calculator and choose either the Current Goal Calculator or the Voltage Goal Calculator. In this example, we will use the Current Goal Calculator.
Okay, here is the example. Let’s say that you have determined that 10 peak amps produced successful fishing for Grass Carp at an ambient water conductivity of 100 uS/cm with your typical two-boom electrofishing boat. Now you want to fish in another stream. You measure water conductivity and determine that the ambient conductivity is 700 uS/cm. Successful fishing is going to require more than 10 peak amps, but how much will it require? Without the benefit of the lab experiment and its Cf value for Grass Carp, let’s say you just use the Cf value of 115 uS/cm reported by Miranda and Dolan (2003) in their channel catfish study. In the Tool Current Goal Calculator input 10 amps at 100 uS/cm and a fish conductivity of 115 uS/cm. Next, simply input the new site ambient conductivity as 700 uS/cm and hit Calculate. The result is a target current value of 38 amps. The idea is to adjust your voltage at the site so that the peak current is 38 amps. Conduct a little trial for a few minutes and adjust voltage as needed so that you capture fish and that they recover in the live well within a minute. Once you determine that threshold, then use that voltage as you conduct your fishing in that body of water.
Now, let’s use the information from the lab study to modify our inputs to the calculator. The only input that will change is the fish conductivity. Instead of using 115 uS/cm, input the 62 uS/cm from the lab study with Grass Carp.
Now the target current is 47 peak amps, an increase from the earlier value of 38 amps. To explain what is happening, let me show you a graph of applied power versus system resistance for a boat electrofisher which has a resistance of 40 ohms at 100 uS/cm ambient water conductivity. When I first saw such a graph, I almost passed out. It is way too much information in one place, or so I thought. Actually, it is quite informative, and dare I say…powerful. Okay, here goes.
At first, forget about the diagonal lines and the curved lines. It is just a graph of peak applied power versus boat electrode resistance. Both measures are on log scales, and note that the x-axis is increasing from right to left. Low resistance is associated with high conductivity, and vice-versa. From right to left, the ascending slanted lines are lines of equal voltage. From left to right, the descending lines are lines of equal current. The blue curved line has its lowest point at an ambient conductivity of 115 uS/cm. The orange curved line has its lowest point at an ambient conductivity of 62 uS/cm. As you can guess, the blue curved line is a power goal line for fish with an effective conductivity of 115 uS/cm, and I don’t need to explain the orange curved line by now. You know it is a power goal line for Grass Carp with a Cf of 62 uS/cm.
So, how is this graph useful to us? What is it showing? The white open circle is the initial success for 10 peak amps at 100 uS/cm. Perhaps you can visually estimate 10 amps from the descending diagonal lines. The white open circle is between 8 and 16 amps and is closer to 8 amps. The 100 uS/cm value equates to 40 ohms, as given previously in the example for the two-boom electrofishing boat being used. In the spirit of transparency, let me say that changing the Cf values to create the curved lines should only shift them right or left, not vertically. Because of the way I constructed the example, there was a slight vertical shift from one curve to the next. Don’t worry about that. Focus on the x-axis at 5.7 ohms; that corresponds to the water conductivity at the new site, i.e. 700 uS/cm. The blue circle is the target of 38 peak amps, for a Cf of 115 uS/cm; and the orange circle is the target of 47 peak amps for a Cf of 62 uS/cm. Because the orange curved line is shifted to the left, its values are higher than the blue curved line at 5.7 ohms, or 700 uS/cm. And your desire is to capture Grass Carp in a body of water with an ambient conductivity of 700 uS/cm, so your electrofishing boat will be operating with a total system resistance of 5.7 ohms. The other parts of that graph don’t matter; you will be operating on the x-axis at 5.7 ohms in that body of water with that electrofishing boat with those same electrodes. If Grass Carp have a Cf of 62 uS/cm, and if the power transfer theory holds, and if you were successful using 10 peak amps at 100 uS/cm, then with all else being equal, success should require 47 peak amps at 700 uS/cm. In other words, 47 peak amps at 700 uS/cm should produce the same size “electrical net” or effective fishing zone as does 10 peak amps at 100 uS/cm.
There is a lot of information presented in this two-part blog. It may require multiple reads to catch it all. I hope it is useful to you as you learn more about this mysterious business we call electrofishing.