Abstract
Ivlev’s (1960) study of young bleak (Alburnus alburnus) is reanalyzed to determine whether or not the routine swimming speeds of planktivorous fishes could have evolved to maximize some obvious measures of biological performance – namely the growth rate, the growth efficiency, or the growth efficiency arising from a particular food ration. Each criterion had a maximum value at a different swimming speed. But for bleak the observed speed of 107 m h−1 was closest to the one, 111 m h−1, which maximizes growth. The possible evolutionary significance of this finding is discussed.New calculations based on Ivlev’s work indicate that the field metabolism of young, actively growing fish may be closer to 3 times the standard metabolic rate, rather than the factor of 2 suggested by Winberg (1956).

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