WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 2015 59 can be used to create feeding tables and estimate channel catfish growth at regular time intervals, for fish grown at optimum water temperatures. Power and natural log functions had the best fit for the data in Table 1, where x = individual fish body weight and y = FCR or % BW fed (Figs. 1 and 2). Correlations were high for both FCR and % BW fed (R2 = 0.97-0.98). Power equations for FCR and % BW fed FCR = 0.6742x 0.1694 (R2 = 0.98) % BW fed = 16x-0.378 (R2 = 0.97) Natural log equations for FCR and % BW fed FCR = 0.2803ln(x) + 0.0293 (R2 = 0.98) % BW fed = -0.866ln(x) + 7.0132 (R2 = 0.98) All that is needed to use these equations is an inexpensive scientific calculator. Sample Calculation Assume a catfish producer has stocked 45-g channel catfish and wants to feed fish accurately, adjusting % BW fed for growth at 14-d intervals. The farmer has selected the power equations to calculate % BW fed daily and FCR. To begin, % BW fed and FCR for a 45-g fish must be determined for the first 14-day period (W is weight). For a 45 g fish, FCR = 0.6742 W 0.1694 = 1.28 and % BW = 16 W-0.378 = 3.79 %. Fish are fed 3.8 % of BW daily for 14 days. Then, growth during the first 14 days is calculated. Feed Fed = % BW × Fish weight × Days = 0.038 × 45 g × 14 days = 24 g. Weight gained = Feed fed/FCR = 24/1.3 = 18.46 g. New fish weight at the end of day 14 = 45 g + 19 g = 64 g. The same sequence of calculations is performed for the next 14-d feeding period, day 14 to day 28. For a 64 g fish, FCR = 0.6742 W 0.1694 = 1.36 and % BW = 16 W-0.378 = 3.34 %. Fish are fed 3.34 % of BW daily for the next 14 days. Next, growth from day 14 to day 28 is calculated. Feed Fed = % BW x Fish Weight x Days = 0.0334 x 64 g x 14 days = 30 g. Weight Gained = Feed Fed/FCR = 30/1.36 = 22 g. New fish weight at the end of day 28 = 64 g + 22 g = 86 g. These calculations are repeated for each new 14-d feeding period until the desired fish size is reached. Table 2 shows the results of the calculations for this example through day 84. Selecting the Appropriate Model Equations There are four possible combinations of the power (pwr) and natural logarithm (ln) equations for FCR and % BW fed, discussed previously: pwr FCR + pwr % BW, ln FCR + ln % BW, pwr FCR + ln % BW and ln FCR + pwr % BW. The discussion and the growth data in Table 2 were based on the power equations for % BW and FCR only. This combination yields the most conservative growth results for all possible power and/or natural logarithm equation combinations. Calculating weight increase with the natural logarithm equations for both % BW fed and FCR produces estimates of weight gain that are the greatest possible for any combination of FCR and % BW. (CONTINUED ON PAGE 60) FIGURE 1. Feed conversion ratio (FCR) as a function of individual channel catfish body weight. FIGURE 2. Percent body weight (% BW) fed daily as a function of individual channel catfish body weight. FIGURE 3. A comparison of weekly weight gains calculated using the Power FCR + Power % BW fed vs. natural logarithm FCR + natural logarithm % BW fed, both combinations derived from the data in Table 1.
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