Baltic Sea Cod Reproductive Volume from Reanalysis

'''DEFINITION'''

The cod reproductive volume ocean monitoring indicator was introduced in Copernicus Marine Service Ocean State Report, Issue 3 (Raudsepp et al, 2019) and is derived from regional reanalysis modelling results for the Baltic Sea BALTICSEA_MULTIYEAR_PHY_003_011 and BALTICSEA_MULTIYEAR_BGC_003_012. The volume has been calculated taking into account the three most important influencing abiotic factors of cod reproductive success: salinity > 11 g/kg, oxygen concentration > 2 ml/l and water temperature over 1.5°C (MacKenzie et al., 1996; Heikinheimo, 2008; Plikshs et al., 2015). The daily volumes are calculated as the volumes of the water with salinity > 11 g/kg, oxygen content > 2 ml/l and water temperature over 1.5°C in the Baltic Sea International Council for the Exploration of the Sea subdivisions of 25-28 (ICES, 2019).

CONTEXT

Cod (Gadus morhua) is a characteristic fish species in the Baltic Sea with major economic importance. Spawning stock biomasses of the Baltic cod have gone through a steep decline in the late 1980s (Bryhn et al., 2022). Water salinity and oxygen concentration affect cod stock through the survival of eggs (Westin and Nissling, 1991; Wieland et al., 1994). Major Baltic Inflows provide a suitable environment for cod reproduction by bringing saline oxygenated water to the deep basins of the Baltic Sea (BALTIC_OMI_WMHE_mbi_bottom_salinity_arkona_bornholm and BALTIC_OMI_WMHE_mbi_sto2tz_gotland). Increased cod reproductive volume has a positive effect on cod reproduction success, which should reflect an increase of stock size indicator 4–5 years after the Major Baltic Inflow (Raudsepp et al., 2019). Eastern Baltic cod reaches maturity around age 2–3, depending on the population density and environmental conditions. Low oxygen and salinity cause stress, which negatively affects cod recruitment, whereas sufficient conditions may bring about male cod maturation even at the age of 1.5 years (Cardinale and Modin, 1999; Karasiova et al., 2008). There are a number of environmental factors affecting cod populations (Bryhn et al., 2022).

CMEMS KEY FINDINGS

Typically, the cod reproductive volume in the Baltic Sea oscillates between 200 and 400 km³. There have been two distinct periods of significant increase, with maximum values reaching over 1200 km³, corresponding to the aftermath of Major Baltic Inflows (BALTIC_OMI_WMHE_mbi_bottom_salinity_arkona_bornholm and BALTIC_OMI_WMHE_mbi_sto2tz_gotland) from 2003 to 2004 and from 2016 to 2017. Following a decline to the baseline of 200 km³ in 2018, there was a rise to 800 km³ in 2019. The cod reproductive volume hit a second peak of 1000 km³ in 2022, stabilized at 600 km³ and increased to 800 km3 by the end of 2024. However, Bryhn et al. (2022) report no observed increase in the spawning stock biomass of the eastern Baltic Sea cod.

'''DOI (product):''' https://doi.org/10.48670/moi-00196