Use of contradictory data sources in stock assessments

Abstract Model predictions can vary greatly when different data sources are used for input. Here, I develop the likelihood function for a composite model based on data from a lingcod ( Ophiodon elongatus ) stock off Vancouver Island, British Columbia, Canada. The composite model includes two components, a fishery and a tag model. The fishery model incorporates historical data on commercial catch, effort, and the size-frequency distribution of the catch. The tag model summarizes a mark-recapture experiment on the same stock. Both component models predict natural and fishing mortality rates. However, as the fishery and tag components lead to statistically distinct estimates of lingcod mortality, I demonstrate that a composite model is unwarranted for the lingcod data. In addition, I show how the use of likelihood weights in composite models can be misleading. Moderate values of a likelihood weight result in parameter estimates that are exterior to the confidence regions for either component model. Finally, I suggest guidelines for evaluating composite models.

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