Effects of repetitive electroshocking on fish populations in experimental raceways and a small headwater stream in southern West Virginia
Fisheries scientists have utilized electrofishing extensively to sample and assess aquatic populations. The physiological responses of fish exposed to electroshock ranges from sensory recognition, through electrotaxis and electronarcosis, to death. This study examined the effects of repetitive electroshocking overtime on population survival, and the effects of mortality on the estimation of population size using single census, multiple censuses and depletion techniques.
Highly significant differences (P < 0.01) were seen between the number of shocks a population of fathead minnows (Pimephales promelas) received per day and the survival of those fish. Populations receiving the two control treatments and a single 4 s electroshock per day exhibited very low mortality (0.40 to 0.67 mean percent mortality per day), whereas mean percent mortality for fish shocked two and three times per day was 2.67 and 7.47, respectively. Handling was not found to significantly increase mortality in fish already subjected to electroshocking.
Known population levels of fathead minnows and dace (Rhinichthys atratulus) were electroshocked daily for 10 days in raceways with sand, gravel, cobble and boulder/bedrock substrates. Actual combined population levels declined 17.2 percent over the study period. Average daily depletion estimates closely reflected this change; however, single census techniques indicated population increases to 48.3. Small and large proportion marked sample single census techniques as one group were significantly different (P < 0.05) from depletion estimates and actual population levels as two groups.
In general, the depletion estimates for both species most closely followed actual population levels in all raceways as compared to single census estimators. Substrates had a significant influence on the estimated population levels. Virtually all fish were available for capture over sand and gravel substrates, whereas, sampling over cobble and boulder/bedrock substrates yielded an average of 43 and 46 percent, respectively of the actual population in those raceways. Rocky substrates provided refuge for fish which resulted in lower percentages entering the catch as compared to sand and gravel substrates.
An "expected mortality" was derived from the average percentage catch per sampling run in raceways and the mortality associated with a comparable number of electroshocks in laboratory tests to predict electroshock mortality over sampling periods and substrate types. A "predicted mortality" derived through the linear regression of depletion estimates over time yielded results very similar to the observed and expected mortality for fathead minnows and was more representative of the change in population size for dace over all substrates than the expected mortality.
The composite analysis of field data showed a 146.7 percent increase in one sample single census estimates, a 44.0 percent increase in two sample single census estimates, and a 55.0 percent decrease in depletion estimates in the final samples as compared to initial samples. Expected mortality estimates were consistently less than predicted mortality when adjusted for the period before intermittent streamflow.
This work has demonstrated that repetitive exposure to electroshocking induces mortality in a predictable manner on fish populations. Depletion estimates are usually accomplished with a much-reduced expenditure of time and effort over mark-recapture methods, however, mortality can be expected due to repetitive electroshocking alone. Researchers are cautioned to examine the objectives of a particular study before deciding on a particular estimation technique.