Report on Lessons Learned from the Co-Managers Workshop on Age and Size at Maturity in Pacific Salmon

Pacific salmon (Oncorhynchus sp.) show a wide range of ages and sizes when they return to freshwater as sexually-maturing adults. Many hatchery managers and fishery biologists in the Columbia River Basin have suggested that the mean size of adult Chinook salmon has decreased in recent years, indicated most notably by increasing proportions of “jacks” among adult fish trapped annually for broodstock.  [“Jacks” are males that mature and return to freshwater one year earlier and at a significantly smaller size than females.]   Several reports in the scientific literature have also suggested that the mean size of Pacific salmon has decreased relative to historical values. Many factors have been hypothesized for those purported changes including size-selective fisheries, hatchery practices, decadal changes in marine ocean conditions, etc.  In the Fall of 2009, record numbers of jacks for both spring Chinook and fall Chinook salmon were reported from throughout the Columbia River basin.  At exactly the same time, the results of an empirical model were published that predicted decreases in the mean age and size at maturity of a hatchery population of Chinook salmon over multiple generations – even in the absence of any fishery – when adult fish are selected and mated randomly for broodstock each year (Hankin et al 2009; CJFAS 66:1505-1521). Those modeling results contradict genetic principles and suggest an unidentified form of selection outside the hatchery environment may significantly favor early age at maturity, particularly for hatchery-propagated populations.  To address these questions and scientific uncertainties, the U.S. Fish and Wildlife Service organized a workshop of regional managers and biologists in Portland, Oregon on May 17-19, 2011. The intent of the workshop was to establish a common understanding of the most current scientific information and use that knowledge as a foundation for subsequently addressing specific management issues and questions (e.g., hatchery spawning protocols, selection and passage of hatchery-origin fish upstream of weirs, forecasting run size abundance of harvestable fish, etc.).  A summary of the results and conclusions of that workshop will be presented.