Three SNA 8 stock productivity-change scenarios were investigated: the first assumed an upward shift in productivity after 2000; the second assumed a downward productivity shift after 2000; the third had no productivity shift.

 

Various SNA 8 stock assessment models were run under these productivity shift scenarios including one that explicitly allowed for a post-2000 productivity shift. Assessment bias was investigated specific to two important management metrics: current-stock-biomass; current-stock-status (being the ratio of current-stock-biomass to stock virgin (unexploited) biomass).

 

All assessment models produced unbiased estimates of current-stock-biomass under the no-regime-shift scenario. Only the post-2000 productivity shift model produced unbiased current-stock-biomass estimates under increasing and decreasing productivity scenarios.

 

All model current-stock-status estimates were biased under increasing and decreasing productivity scenarios. Although the post-2000 productivity shift model current-stock-status estimates were markedly less biased that those of the other models. An important finding from the study was all models were substantively less biased in their estimates of current-stock-biomass than current-stock-status.

 

An important conclusion from the simulation work was that we should not be using model predicted stock-status ratios as stock assessment measures when it is suspected that stock productivity is likely to have changed. Instead, we should be placing more ‘faith’ in assessment model predictions of current-stock-biomass and therefore be measuring sustainability solely against these estimates.

Document date
Tuesday, 7 May 2024

Document type
V 1.3

File format
Adobe PDF

File size
2.0 MB

Reference number
2024/24

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FAR: 2024/24;
AUTHOR: Marsh, C.; McKenzie, J.R.; Langley, A.D.;
ISSN: 1179-5352;
ISBN: 978-1-991285-45-4;

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