Effects of rearing density on sexual maturation and growth in sea-cage reared Atlantic salmon, Salmo salar L.

Abstract. Paired subgroups of fish were derived in January from each of two parent sea-cages of Atlantic salmon, Salmo salar L. The proportion of fish which later became sexually mature in each parent group, after one winter (as grilse) and under commercial rearing density, was determined. Maturity rates in the subgroups, reared in sea-cages at lower density, were significantly greater than in the parent groups. Rearing at reduced density was associated with increased growth in some, hut not all, comparisons. Periodic anaesthesia, handling and sampling of blood for steroid hormone determinations did not consistently affect maturation rate or growth among fish in one of each pair of subgroups. Sexual development was assessed by determining levels of the steroid hormones 11-oxotestosterone or 17β-oestradiol in samples of blood serum taken monthly from individually marked fish in one of each pair of subgroups. Between February and April specific growth rates in maturing male grilse were significantly higher than in fish subsequently shown to have remained immature. Growth rates in maturing female grilse differed similarly between March and April. Between July and August, however, growth rates of non-maturing salmon exceeded those of grilse.