skate length conversion

[kellijohnson-NOAA]

@taylori mentioned that lengths for skates are sometimes disk width or inter-spiracle distance rather than total length, depending on the agency and year, but the type of measurement is not necessary well recorded, so it would be good for the code to have an optional step to apply a length conversion factor at some stage. Vlada has been working on this and can provide more information.

[kellijohnson-NOAA]

@chantelwetzel-noaa do you know how length is recorded for the survey?

[chantelwetzel-noaa]

The way skate length have been take by the NWFSC WCGBT survey has not been entirely consistent across time. If there is a value in the "Length_cm" column that would be total length of the skate. Alternatively, if there is a value in the "Width_cm" column this would indicate disc width. I found the picture below on the internet illustrating what a disc width is:

[kellijohnson-NOAA]

Fantastic picture, thanks for adding it to the issue.

Does nwfsc-assess/nwfscSurvey have code to convert disc width to total length? Vlada gave me this code based off of survey data that I use to convert the measurement. If you don't have code already, maybe we can move this to the survey package and I can just reference the function? Whatever code we end up using we should include links to references and check the literature for other methods if you don't already have this information?

[chantelwetzel-noaa]

If there is a standard conversion between disc and total length for all skates, then yes I think we should add that to the code. We should check with Vlada and @iantaylor-NOAA to see if the code already in the PacFIN.Utilities package is A) could be applied across all skate species (not species specific), and B) whether there are alternative conversions in the literature.

[shcaba]

I recommend checking with Joe Bizzarro @.***) of the SWFSC on any elasmobranch data or conversion questions.

On Fri, Oct 8, 2021 at 7:40 AM Chantel Wetzel @.***> wrote:

If there is a standard conversion between disc and total length for all skates, then yes I think we should add that to the code. We should check with Vlada and @iantaylor-NOAA https://github.com/iantaylor-NOAA to see if the code already in the PacFIN.Utilities package is A) could be applied across all skate species (not species specific), and B) whether there are alternative conversions in the literature.

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[iantaylor-NOAA]

I think that conversions like these should always be species-specific and chosen with input from the user as the best source of information may evolve over time. I do think it would make a lot of sense to provide optional inputs for a slope and intercept of the conversion factor from disk width or inter-spiracle width. Also, for those species where a conversion has been made as part of a west coast groundfish assessment (e.g. Big Skate and Longnose Skate), I think it would be OK to include the conversion values as defaults in this package.

For what it's worth, here is text and figure from Big Skate (2019):

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