Individual and Environmental Factors Influencing Survival for Exploited Populations of Northern Pikeminnow in the Columbia and Snake Rivers

In order to reduce predation on anadromous salmonid juveniles, a sport-reward fishery on piscivorous northern pikeminnow Ptychocheilus oregonensis has been implemented in the Snake and Columbia rivers since 1991.  While this program was developed based on a strong research foundation, the initial analyses used to set the programmatic exploitation goal, which is to annually remove 10-20% of the predatory (≥250 mm fork length) population segment, were based largely on coarse demographic data derived from individuals studied within a single reservoir.  It is uncertain whether or not these early findings still apply after 20 years of sustained exploitation and/or whether there are new opportunities to refine management goals in space and time.  To help fill these information gaps, we analyzed an extensive northern pikeminnow mark-recapture data set which contained information on more than 7,000 PIT (passive integrated transponder) tagged individuals.  Fish included in our study were sampled and released over seven years (2003-2009, with recaptures into 2010) and more than 500 km of riverine and reservoir habitat.  Using these data in combination with harvest information, we estimated natural and fishing mortality using tag-return models, an analysis approach that has been widely applied in waterfowl monitoring and management.  Using this modeling framework, we also assessed relationships between individual (length and age at tagging) and environmental (location and year effects) characteristics and estimated survival components.  Lastly, we explored the utility of ancillary fish detections made at passive PIT-tag detection sites in the study area for estimating survival using models that accomodate both live and dead recapture information, as well as for acquiring insight on fish movement patterns.  Our findings update the initial demographic assumptions of the Northern Pikeminnow Management Program and, more generally, provide insight on the utility of survival models for understanding the population biology of exploited fish populations.