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stimuli of low intensity and short duration can cause impulses to be transmitted. In these instances, there is a cumulative effect (summation) and the impulse is transmitted by the "all or none" phenomena. In general, it's the fish's attraction to the positive electrode, the anode, that makes electrofishing possible. This is caused by a brain reflex in the fish that causes a forced swimming toward the anode (Yibert 1963). External Factors Affectin Electrofishin Success It is well known that electrofishing techniques are selective to larger fish. A bigger fish has more total surface area than a smaller fish, thus receives or is exposed to more current or total energy, and is, therefore, easier to collect. Larger fish usually receive a greater shock because total body voltage increases with length. I've broken the external factors that affect electrofishing into four main groups: size, species, physiological condition, and environmental conditions. Size. The total surface area of a fish can influence the success of capturing it with electric current. Sometimes this principle may be a little deceiving because we tend to think of big fish more in terms of length and weight rather than total surface area. For example, it would appear to be easier to collect a 6-inch northern pike (Esox Zucius) than it would be to collect a 6-inch bluegill (Lepomis macrochimcs) However, the total surface area of the bluegill is greater than the fusiform shaped pike (Figure 1). `. TROUT SUCKER 0 PIKE 0 SUNFISH SCULPIN O Figure 1. Morphology and cross section of various fish species. (Not drawn to scale.) --- Two examples that demonstrate size selectivity of electrofishing gear are: 1) electro- fishing gear was about 25~ effective in collecting 2 1/2 inch long brook trout (SaZveZinus fontinaZis) and about 70~ effective in collecting 8 1/2 inch long brook trout (Figure 2) (McFadden 1961); and 2) small fish have less total surface area, therefore, the fish ab- sorbs less energy and recovers quicker. Common shiners (Notropis cornutus) ranging from 85-95 mm long, took about two minutes to fully recover after receiving a do current, while smaller fish, 65-70 mm long, took about 45 seconds to recover (Figure 3) (Adams et al. 1972). S ecies. Each species is unique and reacts differently to an electrical current. Vulner- a i qty to electrofishing varies among species because of differences in anatomy and be- havior (Reynolds 1983). Some species have large thick scales to rotect them from the penetrating electrical energy (e.g., common carp, G'yprinus carpio~. Others, like the cat- fish. (Ictaluridae), have no scales for protection. Some small-scaled species, like trout (Salmonidae) and eels (Anguil1idae), are easier to collect--than thick-scaled species. There are also species that are adapted to staying on the bottom. These species have hi h speci- fic-gravity, may lack a swim bladder, or have a rudimentary ,one like some darters ~Percidae) and sculpins (Cottidae), have fewer scales,and tend to roll over and become lodged in rock CAL-NEVA WILDLIFE TRANSACTIONS 1984 60