Ag. Nicieza et al., VARIATION IN DIGESTIVE PERFORMANCE BETWEEN GEOGRAPHICALLY DISJUNCT POPULATIONS OF ATLANTIC SALMON - COUNTERGRADIENT IN PASSAGE TIME AND DIGESTION RATE, Oecologia, 99(3-4), 1994, pp. 243-251
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.