This report describes two separate pieces of work done in 2023–24 to improve the way scampi surveys are included in stock assessment models.

 

Scampi build and live in burrows made from the sediment in their environment. One of the ways scampi stocks have been assessed historically is with a series of photo surveys where the number of burrow entrances in a series of photos are counted by photo ‘readers’. There can be variability between readers in interpretation of bottom features as burrows. An individual reader’s interpretation can also ‘drift’ over time. A way to test for differences in interpretation between readers and for ‘reader drift’ was developed so that the burrow count indices could be properly calibrated.

 

In 2019, this calibration methodology was updated but when it was applied to later surveys there were large and unexpected adjustments which undermined confidence in the method. This report outlines the design for a simulation framework that could test whether the calibration method is performing as intended. 

 

When an abundance index is used in a stock assessment model a catchability (labelled q) converts the numbers in the index to numbers in the population. These q values are estimated within the model, but the model uses ‘prior’ information about what values of q are considered reasonable. These prior distributions have historically been estimated by combining survey results with results from acoustic tag experiments. These have been used to estimate what proportion of the time scampi spend outside of their burrows and can therefore be caught by a trawl or seen in a photograph.

 

It has been found that the assessment results are sensitive to the specification of the q priors. A workshop was held to document how the q-priors had been calculated in the past and to agree on the best practise going forward. This report documents the results of this workshop.

Document date
Friday, 14 February 2025

Document type
V 1.3

File format
Adobe PDF

File size
910.7 kb

Reference number
2025/02

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AUTHOR: Holmes, S.J.; McGregor, V.L.; Underwood, M.J.; Wieczorek, A.M.;

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