Northwest Fisheries Science Center

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Document Type: Journal Article
Center: NWFSC
Document ID: 7423
Title: Accounting for space-time interactions in index standardization models
Author: James T. Thorson, E. J. Ward
Publication Year: 2013
Journal: Fisheries Research
Volume: 147
Pages: 426-433
Keywords: index standardization

 Interpretation of fishery-dependent and independent-survey data requires accounting for changes in the proportion of local individuals that are caught by fishing gear (catchability).  Catchability may be influenced by measured characteristics of fishing gear, and even standardized fishing techniques may experience changing catchability over time due to changes in fishing vessel characteristics, personel, and other reasons.  The importance of vessel power has long been recognized in the analysis of fishery dependent catch per unit effort data, but less-studied in the analysis of fishery independent data collected by research vessel surveys. Here we demonstrate how differences in catchability among vessels (vessel effects), as well as random variation in vessel-specific catchability over time (vessel-year effects) may be incorporated into generalized linear mixed models through their treatment as random effects.  We apply these methods to data for 28 groundfish species caught in a standardized survey using contracted fishery vessels and personel in the Northeast Pacific.  Model selection shows that vessel, vessel-year, and both effects simultaneously are supported by available data for at least a few species.  However, vessel-year effects generally have a much larger effect on catch rates than vessel-effects and hence abundance indices estimated using both vessel- and vessel-year effects are generally similar to estimates when using just vessel-year effects.  Simultaneously, models indicate vessel characteristics such as length and displacement have an insignificant impact on survey catch rates.  Finally, inclusion of vessel- or vessel-year effects generally results in wider estimates of credible intervals for resulting indices of abundance.  This increased credible interval width is consistent with statistical theory, because vessel effects will result in non-independence of different sampling occasions, thus decreasing effective sample sizes. 

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Theme: Recovery and rebuilding of marine and coastal species
Foci: Characterize the population biology of species, and develop and improve methods for predicting the status of populations.