FR-CGFS

[AquaAuma]

how to document better the 2015 change in gear & modify the data product accordingly?

[AquaAuma]

from Esther: "I guess we can do two things:

• Make it more obvious, as you suggest, and provide the corresponding intercalibration report (see attached) so that users can decide for themselves how to proceed. E.g. only use the data up to 2014 or only from 2015 onwards, or use the correction factors from the report so that they can use all data simultaneously. Making it more obvious may also mean for instance having two gear categories for the survey, and presenting the plots on for example most abundant species for the periods before and after the vessel change separately.
• Or: We incorporate the correction factors from the intercalibration report into the data cleaning process. I don’t think it’s unfortunately that straight forward (e.g. the factors are for CPUE in no./km2, so how then to correct things when they are in biomass and/or in per hour?), so this will require some work.

By the way, I had a very hard time finding a copy of the report! I took it from the Annex of an IBTSWG report"

[AquaAuma]

@Esther would you have the link to that report by any chance? Easier to have it here than in an email

[eshdb]

Here is a link to an ICES report to which the report on the intercalibration was attached in Annex 7, Working Document 4: https://ices-library.figshare.com/articles/report/Report_of_the_International_Bottom_Trawl_Survey_Working_Group_IBTSWG_/18614291?file=33391769

[eshdb]

To elaborate on the issue (based on new insights and discussion with Daniel and Marcel): both the old (up to 2014) and new (from 2015 onward) vessel use the GOV gear, but the old vessel used a smaller GOV. The wingspread of this smaller GOV was however never really reported in DATRAS. Wingspread is only reported for later years when the bigger GOV was used. However, the wingspread of the big GOV is within FishGlob (I assume) now used to estimate the wingspread of the small GOV used up to 2014 - this results in an overestimation of the wingspread and thus a subsequent overestimation of swept area for hauls up to 2014.

A solution for this particular issue is to take the average wingspread for the old gear up to 2014 when estimating swept area, as reported in the historical survey description in Annex 1 of the Manual of the IBTS North Eastern Atlantic Surveys (https://ices-library.figshare.com/articles/report/SISP_15_-_Manual_of_the_IBTS_North_Eastern_Atlantic_Surveys/19051037) p. 68:

"From 1988 to 2014, the FR-CGFS survey was conducted on the RV Gwen drez, a stern trawler of 24.5 m long, with a load displacement of 249 t and 440 Kw for propulsion. The gear was the GOV 19/25 with 19.7 m footrope and 25.9 m headline. Doors had a surface of 3.58 m² and weighted 650 kg. During this period, the survey was already following a fixed stratified sampling design, with 88 stations planned by the protocol covering the eastern English Channel (ICES Division 7d) and the south of the North Sea (ICES Division 4c) up to 51°15’N. The net opening was about 3 m and wing spread was 10 m in average."

This may not solve the catchability changes for several species as found in the previously mentioned intercalibration report by Auber et al., but it at least improves the cpua estimations of hauls up to 2014 to some extent.

[AquaAuma]

should we do anything about this soon @Esther? Otherwise we can leave it as an existing issue until we have more time