The survey indices by age group for bottom trawl and acoustic estimates for cod from the winter survey (January-March) in the Barents Sea are calculated as described in Mehl et al. 2016 (bottom trawl) and Mehl et al. 2018 (acoustic). Mean length and weight at age is also calculated. Updated data series are provided in the annual survey reports (e.g. Mehl et al. 2019). The series go back to 1981, but StoX estimates go back only to 1994. The survey has been a Joint Norwegian-Russian one from 2000 onwards except in the years 2006, 2007 and 2017.
The survey area is divided into eight main areas and 26 strata. In 2014, the investigated area was enlarged by three new strata in northwest, 24-26. These strata are, however, not included in the standard calculations of indices. Note also that prior to 1994, the area covered was smaller and no adjustments have been made for that. In the early period (1981-1995) there were also some gear changes, for which adjustments have been made, these changes are also described in the reports referred to.
In 1997, 1998 and 2007 only the Norwegian EEZ (NEZ) and parts of the Svalbard area (S) was covered. The indices for cod and haddock have therefore been raised to also represent the Russian EEZ (REZ) (Mehl et al. 2016, 2018).
In 2006, there was not a complete coverage in southeast due to restrictions. The observations in the partially covered strata 7 were extrapolated to the full strata, and the observations in the partially covered strata 13 were extrapolated to the same area as covered in 2005. In 2012 the coverage was incomplete in the eastern areas, and the cod and haddock swept area estimates within the covered area were raised by the “index ratio by age” observed for the same area in 2008-2011 (ICES 2012). The scaling factor (“index ratio”) for estimating adjusted total from <Total – area D’> was the average ratio by age for Total/(Total – area D’) in the years 2008-2011 (Aglen et al. 2012).
In 2017, the Norwegian vessel was not allowed to operate south of 70º 10’ N and west of 41º 00 º E, and no Russian vessel participated in the survey. Only a small part of strata 7 was covered, and strata 13, 15, 17 and 20 were not covered. The cod, haddock, Greenland halibut and beaked redfish swept area estimates and cod and haddock acoustic estimates within the covered area were raised following the same procedure as for 2012. The scaling factor for estimating adjusted total from <Total –strata 7 > was the average ratio by age for Total/(Total – (strata 7+13+15+17+20)) swept area indices in the years 2014-2016.
All these adjustments are included in the StoX projects used for the calculations.