A boat outfitted with electroshock unit being used by INHS researchers.
There are a number of ways to collect fish for management and research. Everyone knows about hook-and-line fishing and some are familiar with a seine (a net used to herd fish). Researchers also use various trap nets (i.e., slat traps) to capture different types of fish. Few methods, however, are as useful and effective as electrofishing. Many fisheries management research projects require the collection of a relatively large number of fish. With electrofishing, large numbers can be acquired with minimal effort.
Electrofishing boats are used on large lakes and rivers. They usually consist of a 16-foot flat-bottom boat with two booms extended forward from the bow that allow up to six electrodes to hang in the water (see photograph). The electrodes used to stun the fish are connected to a generator in the boat. The crew consists of two or three people. One person drives the boat close to the shoreline and the other(s) stands on the bow and uses large dip nets to collect the fish. When the fish are shocked they usually float to the surface, allowing for easy identification and retrieval. Once collected, the fish are placed into a live well in the boat and remain there until they are worked-up (measured, weighed, etc.) and returned to the water.
A boat outfitted with electroshock unit being used by INHS researchers.
Electrofishing streams usually requires the use of a backpack shocker to sample the fish in the same manner. With this method, one person wears a backpack that has been equipped with a battery power pack and walks through the water while wearing chest waders. In one hand is the cathode and in the other is the anode, both of which direct the current into the water. One or two others walk near the person with the backpack and collect the fish.
When we sample a fish population we collect only a small percentage of the total population of any given species. From this relatively small sample we are able to infer characteristics about a population or community. Electrofishing can have negative effects on fish in certain situations. Studies indicate that of the fish collected, a small percentage may suffer some form of injury or even death. The rate at which this occurs varies from species to species with the salmonids (trout in particular) being the most susceptible. Fish common to Illinois waters (e.g., largemouth bass and bluegill) have been shown to have low incidents of injury or mortality as a result of electrofishing. These injuries are for fish collected from a percentage of the population, not the entire population; thus, any mortalities or injuries do not have a large impact on the population as a whole.
Our research on the effects of electrofishing on bluegill was an attempt to measure any indirect effects on bluegill feeding rates or susceptibility to predation. We are asking, "What happens to the fish that are not collected either because they got away or because they were not the target species?" If there is any extensive long-term depression of feeding in shocked groups of bluegill, there may be consequences in terms of size, reproduction, or condition of the bluegill. Also, if shocked bluegill can be caught by a feeding largemouth bass, we might see a reduction in bluegill population size. Either one of these negative effects would lead to the conclusion that electrofishing may have harmful effects.
The results of our research indicate that bluegill (small and large sizes) have reduced feeding for up to five hours after shocking. This may sound alarming to anglers, but biologically this has relatively little impact on the individual fish and absolutely no impact on the bluegill population. Small bluegill are more likely to be eaten by largemouth bass predators immediately after the shocking experience than unshocked bluegill. In our laboratory experiments, which were performed in an eight-foot diameter pool, the foraging largemouth bass would immediately eat any shocked bluegill that moved (either by gilling or twitching). This increase in the chance of being eaten by a largemouth bass when the bluegill was shocked decreased with time. After 10 minutes, the shocked bluegill recovered enough from the experience to behave like their unshocked counterparts. These results represent the maximum potential effects of electrofishing. The pool used had ideal lighting conditions, no habitat complexity (no aquatic plants, woody debris, or submerged trees), and good foraging contrast (the interior of the pool was painted white); therefore, it was easy for the largemouth bass to see a shocked fish. In a real lake or pond this simplistic environment will not be present. Lakes will have varying degrees of turbidity, habitat complexity, and will not be painted white! This means that our results are most likely the "worst-case scenario" for the effects of electrofishing in lakes and ponds.
The impetus for our research was the growing concern, among researchers and the public, about the potential negative effects of electrofishing on fish. Are we hurting the fish that we are trying to understand and help? Our research indicates that the effects of electrofishing on bluegill feeding rates and susceptibility to predation are negligible. Fish that are sampled with electroshock will not have any long-term negative effects on their feeding or vulnerability to predation due to the shocking experience. Electrofishing is not completely harmless to the fish--there are occasional injuries and even death--and there may be very short-term effects on behavior, feeding, or susceptibility to predation. However, the knowledge acquired with the help of electrofishing surely outweighs these negligible short-term effects.
Sean Callahan, Center for Aquatic Ecology
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