Incorporating Life-History Diversity into Estimates of Skagit River Chinook Salmon Production

Understanding how various habitat features influence production and carrying capacity of watersheds, and how life history variation of outmigrants responds to these habitat factors, is critical for setting escapement levels and meeting population recovery goals. Here we illustrate how these concepts can be applied to improve conservation efforts using Skagit River Chinook salmon, a population with a history of extensive monitoring at multiple life stages and complementary watershed habitat data. Long-term monitoring reveal five juvenile life history types (LHTs) are expressed in proportion to available LHT-specific rearing habitat (i.e. density-dependent migration) and migrate to sea at different ages and sizes. We integrate estimates of LHT-specific habitat capacity with size-specific variation in marine survival to predict outmigrant production and adult returns under a range of spawner abundance. We show that current juvenile rearing capacity is met at moderate spawner abundance, above which there is an increasing expression of fry migrants, the small fish that bypass the riverine and estuarine habitats and consequently experience low marine survival. Thus, fry migrants increasingly reduce adult returns as spawner abundance increases, yet the reduction in return rates are not predicted by outmigrant production when life history variation is ignored.