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I ~ '00 . 90 BO 70 ~ 60 .~ ~ 50 ::i: 40 30 o - 1.6 pulses!s C - B.B pulses!. '" - 16.0 pulses!s '" 30 60 120 1BO Duration of Exposure (5) Fig. 2. The relation of pulse frequency to the effect of duration of exposure on mortality of bluegills. Tests run at 4.0 Vlcm. to be electrofished, and use of the results as a guideline, should enable workers to adjust the electroshocker so that mortality can be minimized and effective shock- ing be maintained. Acknow ledgments '" c Robert F. Raleigh and Carl B. Schreck, formerly rnembers of the Department of Fisheries and Wildlife Sciences at Virginia Polytechnic Institute and State University, gave critical suggestions during this study. The Oklahoma and Virginia Cooperative Fishery Re- search Units provided support for the study. The Ok- lahoma Unit is jointly sponsored by the U.S. Fish and Wildlife Service, Oklahoma State University, and the Oklahoma Department of Wildlife Conservation; and the Virginia Unit by the U.S. Fish and Wildlife Service, Virginia Polytechnic Institute and State University, and the Virginia Cornmission of Garne and Inland Fisheries. o References Collins, G.B., C.D. Volz, and P.S. Trefethen. 1954. Mortality of salmon fingerlings exposed to pulsating direct current. U.S. Fish Wild. Servo Fish. Bull. 56:61-81. Everhart, W.H. 1972. Fishery science: Its principles and appli- cation. Colorado State University. 231 pp. Miller, J.G. 1962. The use of electricity in fish management in Pennsylvania. Pa. Angler 31(10):1-6. Northrop, RB. 1961. Design of a pulsed-dc/ac shocker. Conn. Dep. Fish Game, Job Completion Rep., Proj. F-25-R , 1967. Electrofishing IEEE [Inst. Electr. Electron. Eng.] Trans. Bio-Med. Eng. 14:191-200. Schreck, C.B., RA. Whaley, M.L. Bass, O.E. Maughan, and M. Solazzi. 1976. Physiological reponses of rainbow trout (8almo gairdneri) to electroshock. J. Fish. Res. Board Can. 33(1):76-84. Vincent, R 1971. River electrofishing and fish population es- timates. Prog. Fish-Cult. 33(3):163-169. Accepted 8 June 1978 New Fluorescent Fish Scale Marker Various tetracycline compounds have been successfully used as fluorescent markers in fish (Weber and Ridgway 1962,1967). These cornpounds (periodically presented in food or by injection) are readily absorbed by bone-like materials and often produce fluorescent yellow rings. Unfortunately, internal bones (such as ribs) are rnost reliably rnarked, and exarnination for rnarks norrnally involves destruction of fish. Cornpounds with similar properties rnay also be de- posited in the growing scales of fish. The compound 2,4 VOL. 40, NO.4, OCTOBER 1978 bis (N, N' di-carboxyrnethyl-arninomethyl) fluorescein (DCAF), from Nutritional Biochernicals, Cleveland, Ohio, which has been successfully used as a fluorescent marker in studies of rnarnrnalian bone developrnent (Krook et al. 1970), was administered in food to rnixed populations of guppies (Poecilia reticulata) to evaluate its potential for use as a fluorescent marker in fish. A stock solution of 100 rng DCAF and 10 mg glu- cosamine hydrochloride in 0.2 N potassium hydroxide was added to thawed frozen brine shrirnp (Artemia 163