Blocking efficiency of electrified barrier for guiding juvenile grass carp (Ctenopharyngodon idella) by pulsed direct current with different electric parameters at different flow velocities.
The necessity for fish to migrate safely is a key requirement for the existence of many fish populations. Anthropogenic constructions, such as dams and weirs, that block rivers, make it difficult, if not impossible, for fish to reach their upriver spawning grounds. Also, these constructions are also very dangerous for fish moving down the river because dams are built associated with hydroelectric power station intakes where fish are injured or killed on the gratings, pump screens, and in turbine chambers. Mild fields of pulsed, direct current (DC) electricity have been used extensively in deterring invasive fish species as well as in reservoir fishery, but directional fish guidance using electric deterrence arrays to guide fish toward desirable passage locations or away from dangerous areas has received little attention. The effectiveness of the electrified barriers is quite variable and testing is often lacking for a variety of water velocity conditions. To explore the effects of different electric parameters of electrified barriers on the avoidance behavior of the juvenile grass carp C. idella with body length of (10.22±2.01) cm and body mass of (34.25±3.62) g, a double-row type of electrified barrier based on pulsed direct current electricity was established and orthogonal designed experiment of three factors, including pulse voltage, frequency and width, four levels was adopted to optimize the blocking performance of the electrified barrier under the static water condition. To explore the effects of water flow conditions on the blocking efficiency of electrified barriers using the optimized electric parameters recommended for static water conditions, three flow conditions at a generalized model of fishway entrance were utilized to test the attractive efficiency of fishway entrance. The results showed that, under static water conditions, the working condition 7 had the highest blocking rate (88.67±1.10)% and lower average stupefaction rate and that the factors affecting blocking efficiency are pulse voltage, pulse frequency and pulse width respectively. Determined by analysis of variance and multiple comparison, the optimal electric parameters under static water conditions for blocking efficiency were the pulse voltage of 160 V/m, the pulse frequency of 6 Hz and the pulse width of 16 ms. Under the optimal electric parameters, the experimental fish had minimal injuries when subjected to electrified barriers. By employing the optimum electric parameters recommended for static water conditions, the active electrified barrier can dramatically boost the aggregation rate and average retention time ratio at the fishway entrance when the channel velocity was 0.15 m/s. When the channel velocity was 0.25 m/s and 0.35 m/s, the aggregation rate and average retention time ratio with the active electrified barrier were not enhanced significantly compared with those in the inactive electrified barrier. Therefore, the recommended electric conditions for optimal blocking effect is to set the pulse voltage of 160 V/m, the pulse frequency of 6 Hz and the pulse width of 16 ms under static conditons. The attractive effects of fishway entrance could be enhanced by the active electrified barriers with the best flowing water velocity being 0.15 m/s. These results will have important reference significance for the practical engineering of the electrified barrier arrangement. The pulsed direct current electric systems have the potential to improve fish passage at anthropogenic barriers. Additional research should investigate the effectiveness at variable electric field voltage and various water flow velocities for fish of various sizes and shapes depending on the species and its ethology.