Of all the environmental variables which affect electrofishing success, the one for which the most is known, and indeed it has been said is the most important, is water conductivity. This article explores the how and why of conductivity in electrofishing.
By Jan Dean
Colleagues Alan Temple and Jim Reynolds and I have taught electrofishing courses and workshops for several years and in multiple places. We have found that there is much confusion in the measurement of water conductivity as it relates to electrofishing.
Biologists often are expert at operating boats, at catching fish with dip nets, and at processing the catch and recording information. Many are knowledgeable about operating electrofishing control units, and hopefully classes are informing biologists about the science behind electrofishing. However, there has been little formal instruction about water conductivity and how to measure it as it relates to electrofishing.
One may turn on a conductivity meter and record a number and then go sampling. Often, even this is not done, regrettably. I say regrettably because this is the one environmental variable which is easy to measure and which has large and predictable impact on electrofishing success. This is the one environmental variable about which we have both a theoretical, mathematical basis and substantial empirical evidence for how it affects fishing success. That makes it a very important parameter to measure. Fortunately, there are easy-to-use meters for measuring water conductivity. A caution here: it is easy to confuse ambient and specific conductivity and to use the wrong measure to set applied voltage, current or power for electrofishing.
Conductivity is used to measure the concentration of salts or ions in a polar solution such as water, so let’s define it in a few ways. Basically, it is a measure of the ability of water to conduct an electric current. For electrical circuits, resistance is the opposition to current flow; it is defined according to Ohm’s Law as voltage divided by current, so resistance in ohms is the ratio of volts to amps.
Water conductivity is the reciprocal of resistance with a spatial component added because the current is flowing through the water which involves a three-dimensional space. Therefore, water conductivity can be reported or defined as amps/volts/cm. Amps per volt is termed a Siemen, so water conductivity is measured in Siemens per cm. The values are so small that they are often reported as micro-Siemens per cm or µS/cm. In the electrofishing literature, one may find measures such as Siemens per meter and others. An old term for amps per volt was mho, which is ohm spelled backwards.
Let me go a little deeper for those who are interested. This part of the blog can be skipped, if you so choose. Voltage gradient is measured in V/cm. Current density is measured in A/cm2. A definition of water conductivity is current density divided by voltage gradient. Thus, water conductivity equals A/V/cm, or Siemens/cm.
For electrofishing, we are interested in knowing the water conductivity at ambient conditions for the time we are fishing; this is termed ambient conductivity. Limnologists (scientists who study lakes) and water quality personnel often want to compare the ionic concentration of waters from different sources, so they want the values converted to some standard for this comparison.
Water conductivity is a function of water temperature, so water conductivity meters also measure water temperature and convert the ambient water conductivity – what they actually measure – to the value it would be if the water temperature were some standard, called specific, temperature.
In the old days, limnologists used 18°C as the specific temperature because that was close to the average temperature of lakes throughout the world. Conductivity meters today typically use a specific temperature of 25°C because that is near room temperature of water quality labs. Water conductivity meters pass an electrical current through electrodes immersed in the test sample and use the voltage, current and distance between electrodes to determine ambient conductivity.
There are multiple ways to accomplish this, with a variety of electrode designs. That ambient conductivity value is then converted through some equation to specific conductivity, or the conductivity at the specific temperature, usually 25°C. The most basic water conductivity meters only display specific conductivity, whereas more sophisticated ones may display both specific and ambient conductivity. Some meters or their user manuals or even limnology texts will mention the term specific conductance. We teach in electrofishing classes that we use the term conductivity, not conductance, if the current is going through water instead of through wires and components. Thus, what they may be terming specific conductance, we call specific conductivity.
Now, let’s get to the source of confusion that we have seen in electrofishing classes and workshops. Is your meter displaying specific conductivity or ambient conductivity? And which measure do you use for setting the applied voltage to your electrofisher so that you can capture fish effectively today? The second question is easy to answer; always use ambient conductivity for electrofishing. The first question is the main source of confusion and misunderstanding. Here is an easy way to tell if your meter is displaying specific or ambient conductivity, and it will likely require you to do this before you are out on a boat or standing in a stream.
Specific conductivity is made specific to its value at 25°C, so the reading should be the same, or basically so, for any temperature of the sample. In contrast, ambient conductivity varies positively with temperature, usually on the order of about 2% per degree C.
So, here is the simple test. Measure the conductivity of some water sample, then change its temperature several degrees, then re-measure the conductivity of the same sample. If the values remain about the same, then your meter is displaying specific conductivity. If the values changed after the temperature was adjusted, then your meter is displaying ambient conductivity. If your meter is displaying ambient conductivity, then no adjustment is needed. Simply measure and record that value at the lake or stream, use that to adjust your electrofisher, and begin sampling. How to use the ambient conductivity value to adjust your electrofisher is not covered in this blog. Sorry, that is for another blog or class.
If your meter only provides specific conductivity, which is typical for less expensive conductivity meters, then the displayed value must be converted back to ambient conductivity by working the meter’s equation backwards or by using another conversion equation.
It is time to describe some terms or symbols to use in equations. In the literature, Greek symbols such as sigma, gamma or even kappa have been used for conductivity. Let’s keep it simple and use Ca for ambient conductivity and Cs for specific conductivity. Also, let’s set the specific temperature to 25 and use T for the ambient water temperature. Only one additional term is needed; let Greek beta, ß, represent the temperature conversion factor, or the percent of change in Ca per °C.
There are two main forms of equations which can be used to convert Cs to Ca or Ca to Cs. One is linear and the other uses the compound-interest (power) formula. Okay, here goes…
The one we teach in class is the curvilinear (power) form: Ca = Cs*(1+ ß)^(T-25)), where the ^ means to raise to a power. Simply take 1+ß and raise it to the (T-25) power then multiply that answer by the specific conductivity reading to obtain the ambient conductivity. Set ß to 0.02 and the equation becomes easy to use. Here is an example: Cs = 120 µS/cm and T = 15°C. Calculate the conversion factor first as 1.02^(15-25) = 0.82. Therefore, Ca = 120 µS/cm*0.82 = 98 µS/cm.
Now let’s use the linear formula, which is Ca = Cs*(1+ß*(T-25)). In this case, multiply ß by (T-25) then add the 1 to obtain the factor. Using the same values as above. The conversion factor is 0.02*(15-25) = -0.2 + 1 = 0.80. Therefore, Ca = 120 µS/cm*0.80 = 96 µS/cm. As is evident from this example, the two values, 98 and 96 µS/cm, are similar. Differences between the two equations are amplified as the temperature varies more from the specific temperature of 25°C.
Here is a specific example for determining whether a meter is displaying specific or ambient conductivity and for some more analyses of the results. For years, I have used a Pinpoint conductivity meter. It only displayed specific conductivity, so I had to also bring a thermometer to measure water temperature for the conversion.
During a recent sampling trip, the conductivity meter, which was not waterproof, got wet in a sudden rainstorm, and it stopped working. We needed another conductivity meter, and a colleague mentioned the Hanna brand. I choose to purchase and evaluate the Hanna DiST 5 EC/TDS/Temperature tester, which is waterproof. The reported conductivity range is 0 to 3999 µS/cm, and that should cover most of our needs in this area. We also bought the 1413 µS/cm conductivity standard solution for calibration of the pen type meter. The temperature compensation, ß, can be set from 0.0 to 2.4% per °C, so I wondered how well it could be used to measure ambient conductivity when set on a temperature compensation of zero.
After calibration to 1413 µS/cm, I measured the specific conductivity of tap water at room temperature; the result was 206 µS/cm at 25.1 °C, and the temperature compensation was set to 2.0% per °C. Then I changed ß to 0.0 and measured the water conductivity again at that temperature and then again after both heating and then cooling the water. I fit the data to the linear equation shown above using the Solver tool available in Microsoft Excel and graphed the results. Also, I graphed the data plus the compound-interest type, or power, formula shown above and fit with the Solver tool. Both graphs are shown below.
When the temperature compensation was set to zero, the conductivity readings changed with the water temperature. Therefore, the meter was reading ambient conductivity, not specific conductivity. Also, it is clear that the relationship was linear. Water temperatures ranged from 3.4 to 36.6 °C. This basically covered the range of water temperatures in which we would expect to electrofish. The blue line is the least squares linear regression line produced by Excel; the coefficient of determination was 0.9987.
This graph depicts the original data fitted with the linear equation, shown as the black line, and with the compound-interest type equation shown as the red line. The test temperature was set to 25°C, and the data were fit to the equation using the Solver tool in Excel by changing ß. The calculated values of ß for the linear and curved lines above were 1.97 and 2.28% per °C, respectively. Note that both values were near the 2% expected. The linear relationship between the data and the equation line was almost perfect. The line for the compound-interest type curved line agreed closely with the data in the midrange and curved away slightly at the extremes, as expected, i.e. as the temperature diverged from the specific temperature of 25°C. The residual sum of squares for the linear equation was only 33, whereas it was 331 for the curved line equation.
We have been teaching the use of the compound-interest type curved line formula for computing ambient conductivity from specific conductivity and ambient water temperature. Although these data indicate that the introduced error is relatively small when using the compound-interest formula, especially in the temperature range of about 10 to 27°C, I now think that the better approach is to teach the linear form of the equation to convert between specific and ambient conductivity. An even better approach is to use a conductivity meter which can measure and display ambient conductivity directly so that no conversion is needed. That formerly meant having to purchase an expensive meter, but now one can purchase relatively inexpensive conductivity meters which allow setting the beta value to zero.
These results and others indicate that the Hanna DiST 5 EC/TDS/Temperature meter accurately measured ambient conductivity when the temperature compensation factor, ß, was set to 0.0. Further, the meter appears to be using a linear equation to convert ambient conductivity to specific conductivity. The Hanna DiST 5 EC meter is simple and easy to use for measuring ambient conductivity. No conversion from specific conductivity is required if the temperature conversion factor is set to zero.
I hope this blog is helpful in explaining the difference between specific and ambient conductivity and how you can easily determine which is being displayed by your conductivity meter. The goal is more effective electrofishing through accurate measurement of ambient water conductivity and through adjustment of the electrofisher based upon that conductivity value.
Here is an addition to the above blog after some data analysis from another pen-type EC meter for samples of three ionic concentrations, each at three temperatures. The data were supplied by Alan Temple. The calculated beta was 1.99% per °C for both the linear and compound-interest (power) formulas. The error was somewhat higher for the power formula, but the results indicate that either method would provide useful results for electrofishing. The meter was the Extech EC 100.