Reproductive energetics of Pacific Salmon: strategies, tactics, and trade-offs

Hendry, Andrew

Sockeye salmon (Oncorhynchus nerka) stored energy in their somatic tissues before maturation, and then mobilized that energy efficiently. The early stages of maturation were coincident with upstream migration and initial gonad development, for which fat was the primary energy source. The later stages of maturation were coincident with the continuing development of ovaries, and secondary sexual characters, for which protein was used. Preferential use of fat early in maturation conserves the structure of muscles needed for swimming. Delayed use of protein may postpone secondary sexual development until just before breeding. Reproduction was energetically costly; somatic energy stores were depleted by 76.8% (females) and 68.6% (males). Accordingly, reproductive trade-offs were evident. First, females that invested more energy into egg production retained less energy for metabolism during breeding. Second, egg size was negatively correlated with egg number (after correction for body size). Egg number increased much more than egg size with increasing body size, and with increasing gonad investment for a given body size. This pattern could be explained by an egg size/offspring fitness function that peaked at an intermediate egg size, and by the tendency for larger females to obtain high quality nesting sites. The date that breeding commenced influenced life history and energy allocation. Late breeding fish expended more energy prior to breeding, invested more energy into eggs. and retained less for metabolism. Accordingly, early-breeding fish lived considerably longer than late-breeding fish. This pattern may arise because selection favors longer life in early-breeding fish and greater gonad investment in late-breeding fish. Such "adaptation-by-time" may play an important role in life history evolution in semelparous, capital-breeding species with heritable breeding times. The first females to enter the stream settled in sections (patches) where the water was deeper and slower than average. These preferred patches then attained the highest densities. Smaller fish occupied shallower sites, and small females settled in patches with higher densities. Intensity of competition was negatively correlated with reproductive life span for males. These patterns were consistent with theoretical predictions that the choice of a breeding location reflects a trade-off between the location's intrinsic quality and the level of competition.