Report of the Second Workshop on Age Determination of Salmon (WKADS2)

A second Workshop on Age Determination of Salmon (WKADS 2) was held in Derry - Londonderry, Northern Ireland from September 4th to 6th, 2012. The meeting was attended by 12 people from six countries, representing nine laboratories.  The meeting addressed recommendations made in the WKADS report (2011) (ICES CM 2011/ACOM:44) to review, assess, document and make recommendations for ageing and growth estimations of salmon (Salmo salar) using digital scale reading, with a view to standardisation.  Available tools for measurement, quality control and implementation of inter-laboratory QC were considered.
Information on scale reading errors and inaccuracies was presented, including:

• possible scale deformation from jewellers press; 
• differences in circuli number and spacings, on scales from different locations on smolts;
• measurements of smolt and adult scales made by different scale readers;
• measurements of adult scales made by the same scale reader. 

The image collection gathered during WKADS (ICES CM 2011/ACOM:44) was augmented by addition of scale images showing complexities in their growth, including scales with growth checks and repeat spawners.
It was recommended that an inter-laboratory calibration exercises should be held.  This should be implemented remotely, beginning with a video-conference meeting to detail the exercise. A two week period should be given for participants to read scales, followed by a week for the co-ordinator to produce an initial report. A further video-conference meeting should be held to review the exercise and report and allow for feedback. A final report should then be produced.
The meeting reviewed “Web services for support of Growth and Reproduction Studies” (WebGR), a server distributed PC application for organising scale reading comparisons and considered its application in supporting an inter-laboratory salmon scale reading exercise.  While the application could support such an exercise, it is aimed at marine species, which do not exhibit the same diadromous growth (fresh-water and marine) and so would not be directly applicable unless i. it was further developed or ii. two files per scale were created.  
Open source software “Image-J”, an image processing and analysis application that enables measurements through microscope mounted cameras was reviewed.  This could be a useful tool, able to measure and export inter-circuli distances.
The back calculation of lengths from scale features was re-examined and different models presented. This practice is used to check age estimates from scale readings by estimating fish size at the specified age. Size estimates that do not appear realistic for the estimated age indicate that a scale needs further examination.
The processes of fish scale preparation, reading (microfiche, microscope and digital reading) and storage are all well documented; however references and details specific to salmon scale reading are disparate. Available material was reviewed and itemised so that references detailing the best practice pertinent to Atlantic salmon are available in one place.