Proximate Composition, Reproductive Development, and a Test for Trade-Offs in Captive Sockeye Salmon

Abstract

Energy limitations during reproduction should lead to the evolution of adaptive patterns of energy use and should cause trade-offs in the expression of different traits. We addressed these issues by measuring secondary sexual development, gonad investment, and proximate composition for sockeye salmon Oncorhynchus nerka maturing in captivity. Each of the last 3 months before maturity was characterized by a different pattern of reproductive development and energy use. From June to July, gonad mass increased (1.1% to 5.2% of male body mass; from 1.3% to 2.7% of female body mass), muscle fat decreased (15.1% to 8.6% sex-specific values averaged), and viscera fat decreased (23.9% to 16.7%). From July to August, male gonad mass did not change appreciably, but female gonad mass nearly doubled (to 5.5% of body mass). Muscle fat and viscera fat continued to decrease (to 6.0% and 8.8%, respectively), but muscle protein remained relatively constant. From August to maturity (September–October), female gonad mass more than tripled (to 18.6% of body mass) and secondary sexual characters increased in linear dimension by as much as 20.0% (male snout length). Viscera fat continued to decline (to 3.3%), but muscle fat did not decrease appreciably. The conservation of muscle protein until after fat was depleted may postpone reductions in performance that would accompany muscle degeneration. Mass-specific energy decreased between June and maturity in muscle (9.5–5.6 kJ · g⁻¹) and viscera (11.2–4.9 kJ · g⁻¹). We found no evidence for trade-offs in allocation between stored somatic energy, the size of secondary sexual characters, and gonad investment. An important area requiring further research is the effect of variation in energy stores prior to maturity on reproductive development at maturity. This prebreeding energy variation may obscure phenotypic trade-offs.