Changes in condition factor and gonadosomatic index in maturing and non-maturing Atlantic salmon (Salmo salar L.) in Bay of Fundy sea cages, and the effectiveness of photoperiod manipulation in reducing early maturation

Abstract

Changes in condition factor (cf) and gonadosomatic index (GSI) in maturing and non-maturing Atlantic salmon (Salmo salar L.) cultured in Bay of Fundy sea cages was investigated under control (natural) and continuous light conditions. All experimental cages contained salmon stocked as smolts in April 2001. Six control cages received only natural illumination, three received continuous light from 3 November, 2001 until 31 May, 2002, and three received continuous illumination from 15 February, 2001 until 31 May, 2002. In mid-July 2002, each cage was sampled, and sex, round weight, fork length (FL), mean muscle lipid content and gonad weight were recorded from each sampled fish. At harvest (August 2002 to February 2003), sex, maturation state, round weight and FL were measured from a sample of fish from each cage. The frequency distributions of GSIs from fish sampled in July indicated for both sexes a GSI>0.3 was indicative of early maturation. The relationship between cf and GSI suggested that salmon of both sexes had to have a cf greater than 1.3 in early summer for early maturation to develop. Continuous illumination from November greatly reduced the number of males with GSI>0.3 in mid-July (0.8%, compared with 50% for control males); 17% of males from cages lit in February had GSIs exceeding 0.3. The percentage of control females exceeding 0.3 was 7%, compared with 0% and 4% for females from cages receiving continuous illumination from November and February respectively. For salmon with GSI of 0.3 or less, the percent muscle lipid increased linearly with cf. A multiple regression of cf and FL on GSI (GSI=−0.973+0.35 cf+0.013 FL) correctly identified 88% of maturing vs. non-maturing salmon from control cages in July.