Modelling Pollock egg counts from the western Gulf of Alaska by a zero-inflated Bayesian hierarchical space-time model

Data from egg sampling surveys often contain a large proportion of zeros. We examine the time and location of collected walleye pollock egg counts from the western Gulf of Alaska, from Kodiak Island to Unimah Pass, in the years 1978-2000. We take the excessive number of zeros in the data into accoun...

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Bibliographic Details
Main Authors: Tvete, Ingunn Fride, Storvik, Geir, Natvig, Bent
Format: Report
Language:English
Published: Matematisk Institutt, Universitetet i Oslo 2006
Subjects:
Online Access:http://hdl.handle.net/10852/10354
http://urn.nb.no/URN:NBN:no-28475
Description
Summary:Data from egg sampling surveys often contain a large proportion of zeros. We examine the time and location of collected walleye pollock egg counts from the western Gulf of Alaska, from Kodiak Island to Unimah Pass, in the years 1978-2000. We take the excessive number of zeros in the data into account by taking a two stage modelling approach, resulting in a zero-inflated hierarchical space-time model. The binary (presence/absence of zeros) process is directly linked to the underlying intensity process. Hence, the latter process influences both the presence of zeros and the amount of eggs in non-zero observations. We fit our model for each year, and compare the underlying intensities over the years within regions of the sampled area.