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RESEARCH PRODUCT

Indirect estimation of stomach volume of rainbow trout Oncorhynchus mykiss (Walbaum)

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The main hypothesis of this study was that if stomach volume is correlated with food intake it can be estimated without laborious and destructive direct measurement. Rainbow trout, Oncorhynchus mykiss (Walbaum), ca. 500^1300 g, were starved for 1, 4, 8 or16 days at151C after which they were fed in excess with dry pellets containing known amounts of X-raydense markers. Immediately after feeding the ¢sh were killed, X rayed and weighed. Then the stomach was dissected, its contents removed and weighed, and stomach volume was measured. X-ray plates were developed and feed intake was estimated based on the amount of marker. All measured variables correlated positively with stomach volume. The best ¢t for linear regression models was obtained for ¢sh starved for 4 days, where stomach content (dry mass) explained 94%, food intake (based on X-ray measurement) 77% and ¢sh mass 62% of the variation in stomach volume. However, as stomach content measurement can be a lethal, or at least very stressful, event for the ¢sh, the accuracy of food intake measurement (X-ray) could be increased using multiple regression. In multiple linear regressions, R 2 -values varied between 0.79 (16-day starvation) and 0.91 (1-day starvation) with food intake and ¢sh mass as explanatory variables for stomach volume. These results indicate that the stomach volume in rainbow trout can be estimated satisfactorily using indirect methods, which are not detrimental to the ¢sh, although feeding history may aiect the accuracy of the estimates.