JOURNAL. OF THE Vol. 20, No. 4 WORLD AQUACULTURE SOCIETY December, 1989

Winterkill Syndrome in Largemouth Bass Produced Under Laboratory Conditions

ANDRIES J. ROEMI Fisheries Research Laboratory, Southern Illinois University, Carbondale, Illinois 62901 USA

ROBERT R. STICKNEY School of Fisheries, University of Washingfon, Seattle, Washington 981 95 USA

Fish farmers in the north central United States have reported several incidences of winterkill in largemouth bass, Micropterus salmoides, when the fish were reared on pel- leted rations (Brown, personal communi- cation). Affected fish exhibited fatty livers, but there were no indications of pathology which might be associated with diseases or parasites. The relationship between the feed and incidence of winterkill seemed to im- plicate a sinking ration more than it did one which floated. The present preliminary study was undertaken to determine if the syn- drome could be induced in the laboratory and if feed formulation was related to fatty liver incidence and mortality.

Materials and Methods Three diets were compared in the labo-

ratory: a commercial floating catfish feed (Ralston Purina, St. Louis, Missouri), a commercial salmon feed (Rangen, Buhl, Idaho) and a combination of a trout starter (Silver Cup, Murray, Idaho) mixed in a 1: 1 wet weight ratio with ground gizzard shad, Dorosoma cepedianum, and fortified with a vitamin premix. The latter combination (developed by B. Tetzlaff, at Southern Illi- nois University) has been effectively uti- lized on a variety of fish species.

Three tanks (200 L) were stocked with 20 largemouth bass each. Charcoal-filtered municipal water was continuously provided

I Corresponding author's present address: Juneau Center for Fisheries and Ocean Sciences, University of Alaska-Fairbanks, 1 1 120 Glacier Highway, Juneau, Alaska 9980 1 USA.

at 1.5 L/min. Temperature was initially maintained at 25 C with aquarium heaters. The diets were each offered to fish in a sep- arate tank to satiation, three times a day. Fish were weighed every two weeks to mon- itor growth and feed conversion efficiency (FCE = wet weight fish gain/dry weight of feed offered x 100).

After nine weeks of feeding, the water temperature was gradually reduced over a three week period to 3 C with water chillers. Temperature was maintained at the lower level for eight weeks to simulate local winter conditions. Feeding activity ceased during the simulated winter period. Next, the water temperature was slowly brought up to 18 C over a period of several days. Feeding re- sumed for a two week period, after which the experiment was terminated. Samples of three fish were taken at the end of the cold exposure period and at the end of the ex- periment for liver lipid analysis by the method of Folch et al. (1 957).

Results Growth and FCE of fish fed the catfish

diet were poor compared with the other two diets (Table 1). Four weeks into the cold exposure period, fish fed the catfish and combination diets began to experience mor- tality. Moribund fish lost their equilibrium and lay upside down on the bottom of the tank up to a week before dying. After eight weeks, winter simulation was discontinued because all fish in the two adversely affected groups could be characterized as moribund. As a result ofthe temperature increase, most of those fish recovered. No differences in

0 Copyright by the World Aquaculture Society 1989

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278 ROEM AND STICKNEY

TABLE 1. Initial andjinal mean weights, feed conversion efiiency, and survival data for largemouth bass.

Choline Feed supplementation Initial mean Final mean conversion

Diet (mg/kg) weight (g) weight (g) efficiency (96) Survival (%)

Catfish 0 45.0 55.1 32 90b Combinationa 25 41.0 62.3 71 85b Salmon ca. 200 41.4 68.1 90 I00

a Combination = 1 : 1 mixture of trout starter and ground shad. The majority of these fish were moribund at the end of cold exposure period.

liver lipid percentage were found among treatments. However, liver lipid percent- ages during the cold exposure period were 8% higher than at the end of the study (Table 2).

Discussion Winterkill syndrome in largemouth bass

was induced in the laboratory and seemed to be related to feed formulation. Based on survival at low temperature, fish fed the salmon diet were protected, although those fish had liver lipid percentages comparable to the other fish. High liver lipid levels can be a result of lipid mobilization impair- ment, due to a lack of methyl donors such as methionine and choline (Maynard et al. 1979). All fish contained higher liver lipid levels during the cold exposure period (no feeding) than at the end of the study (feeding resumed). This may indicate that large- mouth bass have a high choline demand in the winter and that feeding diets with cho- line supplementation before the onset of winter could be beneficial. This corresponds well with the fact that the vitamin premix in the catfish diet contained no choline sup-

TABLE 2. Liver lipid percentages during the cold ex- posure period and at the end of the feeding trial of largemouth bass.

Liver lipids Liver lipids during cold at the end of

Diet exposure (%) the study (%)

Catfish 14.4 6.1 Com bination 13.4 6.5 Salmon 14.4 5.1

plementation. The combination diet con- tained only 25 mg choline supplementation per kg of diet. The salmon diet was supple- mented with choline estimated at several hundred milligrams per kg of diet (NRC 1981). This could have been sufficient to prevent mortalities, but not enough to pre- vent fatty livers. However, caution is ad- vised because choline occurs naturally in substantial amounts in most practical diet ingredients (NRC 1983). Thus, other di- etary factors, such as vitamin E or fatty acid composition, may have affected largemouth bass in this study. Further research in this area is recommended and will be conduct- ed.

Acknowledgments The authors wish to thank Larry Brown,

Manager of the Fountain Bluff Fish Farm, for donating the fish used in this study. Funding for the work was provided by the Fisheries Research Laboratory of Southern Illinois University in Carbondale.

Literature Cited Folch, J., M. Lees and G. H. Sloane Stanley. 1957.

A simple method for isolation and purification of total lipids from animal tissues. Journal of Bio- logical Chemistry 226:497-509.

Maynard, L. A., J. K. Loosli, H. F. Hintz, and R. G. Warner. 1979. Animal nutrition. McGraw-Hill, New York, New York, USA.

National Research Council. 198 1 . Nutrient require- ments of coldwater fishes. National Academy of Sciences, Washington, D.C., USA.

National Research Council. 1983. Nutrient require- ments of warmwater fishes and shellfishes. Na- tional Academy of Sciences, Washington, D.C., USA.