The next installment of the winter series will feature:
Mark Maunder, Ph.D.
Head of the Stock Assessment Program
Inter-American Tropical Tuna Commission (IATTC)
Kevin Piner, Ph.D
Fishery Research Ecologist
Southwest Fisheries Science Center
“Integrated Analysis: The Worst Thing that Happened to Fisheries Stock Assessment”
Thursday, January 22, 2015 at 11:00 AM in the Northwest Fisheries Science Center Auditorium: 2725 Montlake Blvd. E., Seattle WA 98112
Contemporary fisheries stock assessment models often use multiple diverse data sets to extract as much information as possible about all model processes. This has led to the mindset that integrated models can compensate for lack of good data (e.g. surveys and catch-at-age). However, models are, by definition, simplifications of reality, and model misspecification can cause degradation of results when including additional data sets. The process, observation, and sampling components of the model must all be approximately correct to minimize biased results. Unfortunately, even the basic processes that we assume we understand well (e.g. growth and selectivity) are misspecified in most, if not all, stock assessments. These misspecified processes, in combination with composition data, result in biased estimates of absolute abundance and abundance trends, which are often evident as “data conflicts”. This is further compounded by over-weighting of composition data in many assessments from misuse of data weighting approaches. The law of conflicting data states that since data is true, conflicting data implies model misspecification, but needs to be interpreted in the context of random sampling error, and down weighting or dropping conflicting data is not necessarily appropriate because it may not resolve the model misspecification. Data sets could be analyzed outside the integrated model and the resulting parameter estimates for population processes and their uncertainty used in the integrated model (e.g. as a prior), but these analyses typically involve more assumptions, implicit or explicit, that are potentially misspecified leading to biased results. Model misspecification and process variation can be accounted for in the variance parameters of the likelihoods (observation error), but it is unclear when this is appropriate. The appropriate method to deal with data conflicts depends on if it is caused by random sampling error, observation model misspecification, or system dynamics model misspecification. Diagnostic approaches are urgently needed to test goodness of fit and identify model misspecification. We recommend external estimation of the sampling error variance used in likelihood functions, including process variation in the integrated model, and internal estimation of the process error variance. The required statistical framework is computationally intensive, but practical approaches are being developed.
Dr Maunder is recognized internationally as a leader in the development of methodology for fisheries stock assessment and population dynamics modeling. He is currently the head of the Stock Assessment Program at the Inter-American Tropical Tuna Commission and has served as a consultant for several national and international governmental and non-governmental organizations. Dr Maunder is co-founder and former president of the ADMB Foundation, and co-founder of the Center for the Advancement of Population Assessment Methodology (CAPAM). He organizes and chairs the successful Fall workshop series on stock assessment methodology held at the Inter-American Tropical Tuna Commission for over 10 years, which has now transitioned into the CAPAM workshop series. Dr Maunder has been involved in the stock assessments of a variety of different species (e.g. marine mammals, sea birds, crustaceans, sharks, large pelagics, groudfish) and has been a main developer of several general stock assessment models including A-SCALA and Coleraine.
Dr. Piner is the leader of the Highly Migratory Species stock assessment group at NOAA’s Southwest Fisheries Science Center in La Jolla Ca. He is formerly of the Northwest and the Pacific Island Science Centers. Currently he is a member of the International Scientific Committee’s Albacore tuna, Pacific Bluefin tuna, and Shark working groups as well as the Pacific Fishery Management Council’s Science and Statistical Committee. His research ranges from basic biological and ecological research to methods of model structuring and diagnostics. When not working, he likes watching classic Scooby Doo cartoons, but only seasons 1 and 2 after which Scrappy doo and other co-stars were added. He is no fan of the butternut squash but has grown to appreciate the eggplant.
Maunder, M.N. and Piner, K.R. (2015) Contemporary fisheries stock assessment: many issues still remain. ICES Journal of marine Science doi: 10.1093/icesjms/fsu015
Piner, K.R., H.H Lee,.M. N. Maunder, and R. D. Methot. (2011). A simulation-based method to determine model misspecificaton: Examples using natural mortality and population dynamics models. Mar. Coast. Fish.3:336-343.
Lee, H.H., M.N. Maunder, K.R. Piner, R. D. Methot (2011). Estimating natural mortality within a fisheries stock assessment model: an evaluation using simulation analysis based on twelve stock assessments. Fish. Res. 109:89-94
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