This blog is being presented it two parts. Part A involves the lab trials to determine Grass Carp effective conductivity, Cf, and power density at match, Dm. Refer to prior blogs at this site on the power transfer theory and on lab experiments for more information about terms, setup and procedures.
In October of 2015, I was invited to the Upper Midwest Environmental Sciences Center (UMESC) of the US Geological Survey in La Crosse, Wisconsin to advise on a zebra mussel shocking project. During a tour of the facility, we saw a tank containing fingerling Grass Carp (Ctenopharyngodon idella). It is rare to have access to these fish in such numbers, of the same size and cultured in tanks so that they are in good physical condition. I asked Research Fisheries Biologist Jim Luoma about the use of a few fish for some shocking trials; the next day, we secured permission to conduct a preliminary study with a few Grass Carp.
After taking care of the main trip purpose, i.e. setting up the study for shocking zebra mussels, we obtained an aquarium, found an ETS ABP-2 backpack electrofisher as a power supply, and constructed suitable plate electrodes for the study. The objective was to shock a couple of fingerlings at each of several different water conductivity levels so as to determine the effective conductivity of Grass Carp fingerlings. I was unaware of such a study with Grass Carp, so this was a perfect opportunity to conduct a small study in our “spare” time while zebra mussel trials were underway. The purpose was to define parameters which may increase capture of Grass Carp in the wild. The UMESC had the ability to produce or mix water of any conductivity desired, so all of the requirements were in one location. After setting up the test aquarium, we added some distilled water and left if for the next day.
The next day, after the zebra mussel work, we conducted the small study using two fish at each of eight water conductivity values and 120 Hz pulsed direct current with a duty cycle of 25%. The fingerlings averaged 12.0 cm in total length with a standard deviation of 0.89 cm. The data produced a pretty good fit to the power transfer model equation of Kolz (1989) using three different approaches. We had only few fish with which to experiment in this preliminary study, but we did obtain some results which may be useful to those trying to collect Grass Carp from the wild. The first day, we used ambient water conductivity values of 22 to 245 µS/cm. The following day, we were able to test a couple of Grass Carp at 684 µS/cm using the same frequency and duty cycle so as to add a data point to the graph at the higher conductivity.
Setup for tank trials with 12-cm fingerling Grass Carp (shown in inset).
From the applied voltages and the ambient water conductivity values, we calculated threshold voltage gradient E, power density D, and current density J values. We fit appropriate power transfer models to each of those parameters, and some of the graphs are presented below. In particular, the fit of each parameter model was used to create graphs of voltage gradients by conductivity to allow visual comparison of each model fit.
Voltage gradient E model fit to voltage gradient data. Cf = 57.7 uS/cm.
Power density D model fit to power density data then shown on voltage gradient graph for comparison with above graph. Cf = 61.7 uS/cm.
Current density J model fit to current density data then shown on voltage gradient graph for comparison with both graphs above. Cf = 65.7 uS/cm.
Power density D model fit to power density data.
Current density J model fit to current density data.
The calculated threshold values of Cf for the E, D and J models were about 58, 62 and 66 µS/cm, respectively. Of these, I will use the power density model value of 62 µS/cm in the next part of this blog. According to the power transfer theory, the parameter most closely associated with fish response is the portion of power density which is transferred to the fish. That is one reason for using the power density model. Another is that fitting the other two models requires taking the square root of their respective formulas, and that may lead to more error versus the power density model which does not require taking the square root. In any event, the three Cf estimates are substantially lower than the often used value of 115 µS/cm as proposed by Miranda and Dolan (2003) in their study using channel catfish (Ictalurus punctatus).
Many thanks to Jim Luoma for making possible and assisting with this preliminary Grass Carp study. We found useful results in a short time with a small number of fish.