European Atlantic salmon (Salmo salar) populations inhabit rivers from northern Portugal to northern Norway across a wide spectrum of environmental variability. To address whether single physical factors might lead to genetic divergence of isolated populations, we compared the digestive performances total digestibility, relative nitrogen digestibility, passage time, and digestion rate (g dry matter · h) - of northern (Scotland) and southern (Asturias, northern Spain) populations at three temperature regimes (5, 12, and 20° C). Total dry matter digestibilities increased directly with temperature but were similar for both populations at each of the three trials. Relative nitrogen digestibility did not differ between populations nor among temperature regimes. In contrast, passage time was significantly longer for low-than for high-latitude fish at both 5 and 20° C. When the percentage of food digested and the passage time were integrated as digestion rates (food digested per unit time), a significant population × temperature interaction consistent with a genotype × environment interaction was detected in addition to the population and temperature effects. This implies that not only is the digestive performance of the high-latitude 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 countergradient variation in digestive rates of salmonids in response to variation 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 mechnisms leading to more rapid growth of the high-latitude fish when both populations are reared in a common environment.

Both in foraging groups and in a sequential prey encounter context, learning had a visible effect on the pattern of selection for three live prey types (Ecdyonurus larvae, Hydropsyche larvae, and Gammarus) by juvenile Atlantic salmon Salmo salar. Compared to wild-caught fish, naive, hatchery-reared fish that had not been exposed to natural prey ate Hydropsyche larvae in a remarkably low proportion, and consumed a higher proportion of Gammarus. Ecdyonurus experienced a high and rather steady predation rate across the experience gradient, but after a short period of experience with live prey the consumption rate for Hydropsyche increased drastically, and that of Gammarus decreased, matching the selection pattern exhibited by wild fish. Individual fish offered prey in a sequential encounter context increased consumption rates of all the prey types as they gained experience, but the improvement was higher for the prey that were less consumed initially. Fish became more selective as they approached satiation, conforming to the prediction of optimal foraging theory that higher predator's energy requirements, as well as low food availability, result in reduced selectivity. The results also suggest that fish from distinct populations can differ in the degree of diet selectivity according to their energetic requirements for growth. The fast learning response of Atlantic salmon parr towards novel prey probably allows fish to maintain a high foraging efficiency when faced with frequent changes in the availability of different prey types. 1998 The Fisheries Society of the British Isles