A decrease in photoperiod shortly after first feeding influences the development of Atlantic salmon (Salmo salar)

Abstract Four groups of Atlantic salmon fry ( n  = 2000) were exposed to continuous light (LD24 : 0) from first feeding on 18th April 2001, after which they were exposed to either an 8 or 12 week period of short days (LD10 : 14) starting on either the 21st May or the 18th June. Each group was then returned to LD24 : 0 until the conclusion of the experiment the following March. In August 200 fish per treatment were individually PIT tagged. All groups were maintained under an ambient temperature regime. The highest levels of sexual maturation in 0+ male parr were recorded in the 12 week/May group (> 11% of the entire male and female population), with the lowest levels (  6%) in the 8 week/May and 8 week/June groups ( P + , K + -ATPase activity, indicative of smoltification. At the conclusion of the experiment, fish showing signs of silvering were most prevalent (30%) in the 12 week/June group. It is concluded that the initiation of maturation can be influenced by an 8 or 12 week period of short days (LD10 : 14) applied from mid May or mid June in the first growing season. The duration and timing of a stimulatory short day photoperiod during early development may also influence whether a fish undergoes smoltification in the coming year or whether it delays the parr–smolt transformation for at least a further year.