Diadromy and the Life History of Sockeye Salmon: Nature, Nurture, and the Hand of Man
Thomas P. Quinn, Katy Doctor, Neala Kendall, and Harry B. Rich, Jr.
Abstract.—All diadromous fishes share some common themes in their life histories that are linked to the two migratory transitions between freshwater and marine habitats that define this group. We explore these linkages using Pacific salmon and specifically the sockeye salmon Oncorhynchus nerka populations from Bristol Bay, Alaska as a model system to illustrate concepts that are broadly applicable to diadromous fishes. In general, large size is beneficial for survival at sea or reproduction, but because the seasonal migration window is often narrow, the main option in many cases is to go in 1 year or delay until the next. The age at which fish make this transition is related to growth, but it reflects population-specific norms of reaction rather than species-wide rules. Common growing conditions have different effects on discrete populations that are shaped by the trade-offs between the benefits of large size in the new environment and the risks of mortality in the present environment associated with another year of growth. Within-season variation in timing is also related to body size (larger fish generally migrating earlier), reflecting a balance between the need to grow as large as possible before making the transition and the need to migrate at the optimal date. These complex life history connections are affected by natural variation in growing conditions driven by biotic and abiotic processes. In addition to the natural evolutionary and ecological processes affecting life history transitions, humans can also have profound effects. Fishing affects the abundance of prespawning adults, hence the levels of intraspecific competition and growth of their juvenile offspring in freshwater prior to seaward migration. In addition, fishing is often selective with respect to the body size and timing of migration, and the linkage between these traits further complicates the effects of selection.