VARIATION IN DIGESTIVE PERFORMANCE BETWEEN GEOGRAPHICALLY DISJUNCT POPULATIONS OF ATLANTIC SALMON - COUNTERGRADIENT IN PASSAGE TIME AND DIGESTION RATE

European Atlantic salmon (Salmo salar) populations inhabit rivers from northern Portugal to northern Norway across a wide spectrum of enviro nmental variability. To address whether single physical factors might lead to genetic divergence of isolated populations, we compared the di gestive performances - total digestibility, relative nitrogen digestib ility, passage time, and digestion rate (g dry matter.h(-1))- of north ern (Scotland) and southern (Asturias, northern Spain) populations at three temperature regimes (5, 12, and 20 degrees C). Total dry matter digestibilities increased directly with temperature but were similar f or both populations at each of the three trials. Relative nitrogen dig estibility did not differ between populations nor among temperature re gimes. In contrast, passage time was significantly longer for low- tha n for high-latitude fish at both 5 and 20 degrees C. When the percenta ge of food digested and the passage time were integrated as digestion rates (food digested per unit time), a significant population x temper ature interaction consistent with a genotype x environment interaction was detected in addition to the population and temperature effects. T his implies that not only is the digestive performance of the high-lat itude population higher throughout the range of temperatures examined, but moreover the difference is reinforced at high temperatures, where the digestion rate of high-latitude fish was 1.6 times greater. Taken together, these two results provide preliminary evidence for counterg radient variation in digestive rates of salmonids in response to varia tion in growth opportunity. The data support our previous work on the same two populations showing differences in growth rates, and underlie one of the possible mechanisms leading to more rapid growth of the hi gh-latitude fish when both populations are reared in a common environm ent.