Trends in poaching for West Coast rock lobster from modelling the “old” and the “new” databases simultaneously.

Brandão, Anabela; Butterworth, Doug S

Trends in poaching for West Coast rock lobster from modelling the “old” and the “new” databases simultaneously.

Report

2019

Abstract

Results are given for an approach which simultaneously models the “old” and the “new” databases as recommended by the Panel for the 2018 International Fisheries Stock Assessment Workshop. These results are compared to two previous GLM methods reported by Brandão and Butterworth (2018a); a) a GLM applied to the “new” compliance database, and b) a GLM method applied to the “old” database in which estimates of relative effort efficiencies obtained from the “new” database are used to link effort to the number of confiscations (“old”-linked). By simultaneously modelling the “old” and the “new” databases, information on the trend in poaching, as well as on the relative efficiencies of different effort types from the “new” database, influence the estimates of poaching trends obtained from the combined model. However, due to the fact that the “old” database is a summation of operations which produced a number of instances of confiscations (including cases of zero confiscations), while the “new” database reports individual incidents with non-zero confiscations, undue weight is being given to the information available in the “new” database if data from each are equally weighted. We therefore advocate that an analysis which upweights the data in the “old” database should be used in the model which combines use of both the ”old” and “new” database values, and that this should serve as the primary basis for inferring poaching trends.

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