Juvenile Abundance, Survival, and Diversity of Puget Sound Chinook: A Cross-Watershed Evaluation of Density-Dependence in the Freshwater Environment

What conservation actions should increase the productivity of Puget Sound Chinook? Many of the approaches to answer this question have focused on habitat capacity models. This study takes an alternate approach and uses juvenile and adult abundance data to identify whether Puget Sound Chinook are limited by factors in the freshwater or marine environment. The premise of this approach is that the freshwater environment limits productivity only if spawner escapements are large enough to maximize natural-origin juvenile production. This study compiles results from Puget Sound watersheds where juvenile and adult abundances are monitored simultaneously. The objectives of this study are to determine whether the number of juvenile migrants is an increasing function of spawner abundance, whether the juvenile-adult relationship is linear (density-independent) or non-linear (density-dependent), and whether the juvenile migrant type is a density-dependent function of total juvenile migrants. Three juvenile migrant types (i.e., fry, parr, and yearling) differ in the length of their freshwater rearing period. Juvenile abundance was an increasing function of spawner abundance in 4 of the 5 populations. Egg-to-migrant survival differed among watersheds and ranged from 1% to 21.5%. Among the types of juvenile migrants, parr migrants were the least variable among years. The number of fry per parr ranged from 0.04 to 11.29 and was a positive function of total juvenile migrants in 4 of 5 populations. Our results suggest that, over the years included in this analysis, the number of spawners has limited natural-origin juvenile production in multiple populations of Puget Sound Chinook but that the composition of juvenile migrants has been limited by freshwater habitat. If survival in the marine environment differs among juvenile migrant types, the composition of juvenile migrants may be an important variable that contributes to overall stock productivity.