A report on overripe female and spent male Atlantic salmon (Salmo salar) in August

Abstract This report is based on a finding of spent males and overripe females of Atlantic salmon on 30 August 2013. The studied fish population had been reared in tanks from seawater transfer in May 2011 to April 2013, and then in a sea-cage under natural light. From May 2011 until August 2012, and then again from February until April 2013, the fish were under natural light and temperature. In between these periods, the fish were reared under continuous light and a temperature ranging between 3 and 18 °C in order to study temperature effects on food consumption and oxygen consumption rate. In February 2013, a dissection of 33 fish showed that 29% of the females and 79% of the males were maturing. In August 2013, all remaining fish ( n  = 80) were inspected by dissection and measured for gonad weight; males were either spent ex-spawners (50%), maturing (25%) or immature (25%), while females were either overripe (11%), maturing (29%) or immature (61%). The fish that were classified as maturing in August had gonads that resembled the gonads of the fish that were classified as maturing 5 months earlier. The females that were overripe and the spent males in August were estimated to have been ready for spawning sometime between April and June. This is the first indication of summer ovulation of female Atlantic salmon and shows that a relatively short period of temperature manipulation combined with continuous light may be used as a tool to prolong the spawning period in salmon to all year round. This would have a major impact on salmon farming.