The number of wild Atlantic salmonSalmo salarthat return to Spanish rivers has steadily decreased because of environmental degradation and overfishing. To alleviate this decline, juvenile salmon obtained from adults returning to Asturian rivers are used in a supplementation program. Gametes from no more than 35 adults are used to create groups of juveniles that are released back into the wild. We examined the effect of parental numbers on the chromosomal patterns found in juveniles produced by this protocol. When less than 20 parental fish were used, the chromosomal patterns found in their offspring were significantly different (P< 0.001) from those found in naturally produced fish. Consequently, in supplementation endeavors, at least 20 parents should be used to create groups of juveniles.