Condition Dependence and Adaptation-by-Time: Breeding Date, Life History, and Energy Allocation within a Population of Salmon

Abstract

Correlations between breeding date and the life history or energy stores of individuals might stem from a combination of two different mechanisms. The conventional view is that individual size and condition influence breeding date (i.e., condition dependence). A different view is that heritable maturation schedules allow temporally separated population components to adapt to selective regimes associated with particular breeding times (i.e., adaptation-by-time). Considering each of these hypotheses, we examined a population of sockeye salmon (Oncorhynchus nerka) for evidence of correlations among life history traits (age, body size, egg size, and reproductive life span), patterns of energy allocation (somatic energy stores and gonadal investment), and breeding date. Life history traits were measured for 705 individually tagged adult salmon monitored from the onset of breeding until death. Energy stores were quantified for 60 fish collected when they entered the stream and 46 fish collected at death. Multiple regression models revealed that most of the variation could be explained by simple linear relationships among traits; older fish were larger, and larger lish had larger gonads, larger eggs, and more available energy when they started breeding. Condition dependence did not appear relevant to breeding date because fish that started breeding early were similar in size and did not have more stored energy than fish that would breed later. Similarly, adaptation-by-time had little influence on variation in body size or egg size (predicted relationships were significant but very weak). In contrast, adaptation-by-time appeared very important to variation in reproductive life span and patterns of energy allocation. Early-breeding fish lived considerably longer than late-breeding fish (females, R 2 =0.525; males. R 2 =0.533). This pattern arose because late-breeding fish expended more energy before breeding, and because late-breeding females invested more energy into egg production and retained less for metabolism during breeding. Adaptation-by-time may play an important role in life history evolution within some species, particularly those with breeding systems characterised by semelparity, capital breeding, and heritable breeding times.