Historical growth of Eastern Baltic cod (Gadus morhua): Setting a baseline with international tagging data

Multidecadal changes in fish growth rates estimated from tagging data: A case study from the Eastern Baltic cod (Gadus morhua, Gadidae)

“…The historical tagging database of Baltic cod compiled in Mion et al. (2020) covering the period 1955–1970 was extended by digitizing and collating additional historical data from tagging experiments between the 1970s and the 1990s by Sweden, Poland, Latvia, Finland, Denmark and Germany within ICES Subdivisions (SDs) 23–32 (Figure 1). Data for a total of 10,143 recaptured cod were available, covering a release period between 1955 and 1993 (Table 1).…”

“…Around 75% of the recaptures had information on the tag type used. Most of the recaptured tags were Lea´s hydrostatic tags (54%), followed by T‐bars (15%), and Carlin tags (5%), while less than 1% of fish were tagged with Peterson discs (for description of the tags used in the historical Baltic cod tagging experiments see Mion et al., 2020). Around 59% of the recaptures contained information about the recapture gear (43% active gears, i.e.…”

“…Before undertaking growth analyses, some data filters were applied in a stepwise approach following Mion et al. (2020; Figure S1). In the Baltic, two cod stocks occur, that is EBC and Western Baltic cod (WBC), located in ICES SDs 24–32 and 22–24, respectively (Figure 1), with a main mixing area in SD 24 (Hüssy et al., 2016).…”

“…These historical tagging data were mainly used to analyse cod movements over the Baltic seascape (reviewed in Aro, 1989 and Aro, 2002), but they have been underutilized for growth analyses. Although some estimates of growth based on the historical tagging data exist, they are mainly limited to single national surveys and cover limited periods (Draganik & Netzel, 1966; Mion et al., 2020; Sjöblom et al., 1980).…”

“…In this study, we extended the historical database used in Mion et al. (2020), which is based on tagging data collected during 1955–1970, by collating and digitizing the existing archived data from the countries that have performed tagging experiments in the Baltic Sea between the 1970s and the 1990s. In addition, data from international tagging experiments carried out in the southern Baltic Sea in the 2000s and in recent years (2016–2019) have also been integrated with the collated historical data.…”