Comparative aspects of the biology of naturally produced and hatchery-reared Atlantic salmon smolts ( Salmo salar L.)

Abstract

The age composition of naturally produced (wild) smolts in the Burrishoole river system has given a consistent proportion of > 90% as 2+, with the remainder divided equally between 1+ and 3+. Hatchery-reared smolts produce 40% as 1+ and the remainder as 2+; the proportion of 1+ smolts can be increased by the use of warmed water in early stages of the life history. The size and condition of wild smolts has also proved to be consistent each year but they are considerably smaller than hatchery-reared smolts of comparable age. Grilse derived from both wild and hatchery-reared 1+ smolts are smaller than those from 2+ smolts. The survival rate from ova to wild smolts has averaged 0.61% over the past 10 years, compared with almost 50% for reared smolts. Returns to fresh water at the grilse stage are almost four times better for wild than reared smolts. Up to 81% exploitation of grilse from reared smolts by coastal nets has been calculated from coded-wire tagging results. No significant straying has occurred from either wild or reared smolts but the latter make repeated attempts to rediscover their homing point when upstream of it. Smolt rearing has given rise to abnormalities such as “giant” smolts and vertebral compaction, not reported in wild stocks. From 20 to 50% of the reared parr population are precocious males in their second freshwater winter.