Family × photoperiod interaction on growth in juvenile Atlantic salmon, Salmo salar

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Abstract
A factorial experiment was used to study genotype .times. environment interaction on growth of underyearling Atlantic salmon parr (Salmo salar). Thirty-six families, each represented by 200 randomly sampled hatchery-reared parr, the progeny of single-pair matings, were subjected to two photoperiod treatments. These were 16 h light:8 h dark (16L:8D) and a simulated natural photoperiod decreasing from 13L:11D on September 8 to 9L:15D on December 15, 1986. Families with small starting mean length or weight showed a greater difference between photoperiod treatments (16 h minus natural) for absolute gain in length and weight, resulting in family .times. photoperiod interaction without reranking of families between treatments. A family''s length or weight distribution was generally bimodal. The expected (based on a general exponential model) and actual November and December upper mode proportions of families in the 16-h photoperiod were larger than those in the natural photoperiod treatment, for length and weight. Family .times. photoperiod interaction for upper mode size resulted from the reranking of families for the expected but not the actual values. In general, the 16-h photoperiod produced the biggest fish and the largest upper mode proportion. Photoperiods interacted with families. The lack of significant reranking of families in the two environments, when based on absolute gain or actual upper mode proportion, does not indicate an impediment to correlated genetic gains from selection in the alternate environment.