Introduction
Yellow perch, Perca flavescens (Michill, 1814), is one of the most commercially important species of the Percidae family, most abundant in the inland waters of the northern regions of the North American continent. However, the reduction in the wild catch due to overfishing (Saoud et al, 2004), and the devastation of the natural habitat (Clapp and Dettmers, 2004) has resulted in an increased interest in research of the biology of this species, and the development of appropriate culture technologies. The slow growth rate ascribed to yellow perch usually evidenced in open pond culture conditions. Controlled conditions in a closed recirculation rearing system should prevent most of the problems which are common in outdoor cultivation systems and provide for optimal growth of yellow perch (Westers and Weeks, 2002). Perch proved to be very timid (compared to sea bass, trout and tilapia grown in the same culture systems) reacting to any activity above and inside the tanks. It was important to provide optimal tank design and hydrodynamic water flow characteristics while not disturbing the fish, so that they could maintain efficient positioning while feeding. The aim of the study was to observe feeding and behavior of yellow perch in two different tank designs, rectangular and round, and quantify the effect of these conditions on their growth.
Materials and Methods
15 g yellow perch fingerlings were stocked in two round and two rectangular 12000-liter tanks at maximum density of 50 kg/mᵌ. The final size was set at 160 g. As the fish grew, they were transferred to more tanks of the same design to maintain a constant stocking density. The round tanks had a tangential water inlet below the water surface, and a central effluent port, thus creating a circular flow. The “crossflow” rectangular tanks had a series of water inlets at one side, and drains on the other side of the bottom creating a tubular cross tank current. Fish were raised at constant temperature of 24-25˚C. Dissolved oxygen level was maintained at saturation using oxygen injection. Calcium hardness was maintained between 80-150 mg/l using calcium chloride, and pH was maintained between 7.2 and 7.4 with the addition of sodium bicarbonate. Chlorides were adjusted to 300 - 400 ppm using sodium chloride and calcium chloride. During the entire experimental period perch were fed continuously with commercial slow sinking pelletized feed (“Ziegler Brothers”, PA), contained 45% of crude protein and 12% fat, using automatic belt feeders. Fish were fed 5% of the body weight per day in the first month, 4% in the second, 3% in the third and fourth, 2.5% in the fifth and sixth month, and 1.5% in the last four months of the growing period (Jug-Dujakovic and Van Gorder, 2002). A sample of 50 fish was taken every month from the tanks to obtain a measure of average weight. Dead fish were counted every day to calculate the survival at the end of the experiment. The analysis of variance followed by the SNK multiple comparison procedure was used to test variations in growth rate.
Results and Discussion
When not feeding, the fish would swim indiscriminately, not always orienting against the flow of the water, a posture which was more pronounced in the rectangular tanks. In round tanks the administered pelleted feed would follow the current while slowly sinking, and the fish would position themselves to consume the moving food pellets. This pattern was less noticeable with crossflow tanks. Instead of maintaining perpendicular position to the water flow direction, fish would swim more randomly. More feed would reach the bottom of the tank and be removed uneaten.
At the beginning of the study, the 120-day old fish stocks had an average wet weight ranging between 14.7 and 18.1g. Eight months later, at the end of the experiment, the mean weight of fish was 176.1 g in the round tanks, and 160.1g in the rectangular tanks (table 1).
The difference of 16 grams of growth between tank designs represents an approximately 10% weight difference, or expressed in time, a month of growth under comparative rearing conditions. The grow-out phase of yellow perch (10g to 150g) in recirculating systems has been successfully demonstrated by Jug-Dujakovic and Van Gorder (2002). Effects of tank wall color and positive up-welling water flow on growth and survival of Eurasian perch larvae (Perca fluviatilis) was recorded by Jentoft (2006).
Conclusions
The circular flow of the water in round tanks elicited a natural swimming and schooling behavior with a common orientation by the fish populations (into the flow). This positioning and swimming behavior, and the accompanying effective feeding behavior resulted in increased feed conversion efficiencies, and improved overall growth and survival characteristics compared to those of populations cultured within cross-flow raceways.
References
Clapp, D. F., and Dettmers J. M., 2004. Yellow perch research and management in Lake Michigan: Evaluating progress in a cooperative effort, 1997-2001. American Fisheries. 29(11):11-19.
Jentoft AH, Oxnevad S, Aastevit A, Andersen Q (2006) Effects of tank wall color and up-welling water flow on growth and survival of Eurasian perch larvae (Perca fluviatilis). J World Aquac Soc 37:313–317.
Jug Dujaković, J, and S. D. Van Gorder., 2002. Pilot production of yellow perch (Perca flavescens) in a commercial closed recirculation system. Proceedings: 4th International Conference on Recirculating Aquaculture, July 25-27, Roanoke, VA
Saoud, I.P., Rodgers, L., Chappell, J., and D.A. Davis. 2004. Overwintering yellow perch fry in Alabama. North American Journal of Aquaculture. 66(3):217-221.
Westers, H. and C.T. Weeks., 2002. The Status of Yellow Perch Aquaculture in the USA. Proceedings from the Fourth International Conference on Recirculation Aquaculture. July 18-21, 2002. pp.117-124.