New insights into the recent collapse of Eastern Baltic cod from historical data on stock health

Abstract

The Eastern Baltic cod (Gadus morhua) stock is currently in a very poor state, with low biomass and adverse trends in several life history and demographic parameters. This raises concern over whether and to what level recovery is possible. Here, we look for new insights from a historical perspective, extending the time series of various stock health indicators back to the 1940s, i.e. to the beginning of intensive exploitation of the Eastern Baltic cod. The historical data confirm that the stock deterioration in recent years is unprecedented, as all indicators are presently in their worst states on record. Cod body condition and energy reserves were equally low in the 1940s–1950s, accompanied by high parasitic liver worm infection, comparable to that measured in recent years. However, other stock parameters (size structure, size at maturity, stock distribution) are currently in their worst states over the past 80 years. In contrast, the state of cod in the 1970s to early 1990s that is often perceived as a desirable target, was exceptional, with the most favorable indicator levels in the time series. Long-term observation data reveal concurrent or asynchronous trends in different indicators of stock health and to what extent these have coincided with changes in possible external drivers. In this way, the extended time series contribute to ongoing research on understanding the collapse of the cod and its recovery potential.

Trophic lengthening triggered by filamentous, N2‐fixing cyanobacteria disrupts pelagic but not benthic food webs in a large estuarine ecosystem

“…In contrast to cod, the TP flounder was not affected, thus the position of flounder in a trophic pyramid (Figure 9 ) and the energy transfer (Figure 10 ) is not changed by FNC‐caused trophic lengthening. Therefore, the missing or massive effect of trophic lengthening on the benthic or pelagic food web, respectively, could explain the stable spawning stock biomass (SSB) of flounder (ICES, 2023 ) and the historic low SSB of cod, respectively, in the central Baltic Sea (Eero et al., 2023 ).…”

“…Analyses of sediment cores revealed that massive blooms of Nodularia spumigena and Aphanizomenon flos‐aquae already occurred at the beginning of the 20th century in the central Baltic Sea (Kaiser et al., 2020 ). A potential lengthening of the food web due to unpalatable primary producers and accompanied detrimental effects for cod production in this environment may, therefore, have played a role even before the historic SSB peak of the eastern Baltic cod (EBC) stock in the early 1980s (Eero et al., 2023 ) since the SSB of EBC also showed remarkable lows in the 1960/70s. Interestingly, the SSB peak of the EBC in the early 1980s coincided with lower sea surface temperatures and a lower frequency of FNC accumulations (Kahru et al., 2020 ; Kaiser et al., 2020 ).…”

“…stable spawning stock biomass (SSB) of flounder (ICES, 2023) and the historic low SSB of cod, respectively, in the central Baltic Sea(Eero et al, 2023).…”

Aerobic metabolic scope mapping of an invasive fish species with global warming

“…The Eastern Baltic cod stock is in distress and potentially on the verge of collapse ( Eero et al , 2015 , 2023 ). This collapse may be caused by an interplay of multiple drivers including unfavourable climatic conditions, reduced quantity and quality of food, and an increasing grey seal population, which besides predation transmit parasites to the cod ( Casini et al , 2016 ; Sokolova et al , 2018 ; Neuenfeldt et al , 2019 ; Ryberg et al , 2020 ).…”

“…Atlantic cod has an endemic population in the Baltic Sea where it coexists with round goby. Historically, Atlantic cod constitutes a commercially important resource for the area, but recently it has been considered in a critical state due to overfishing, increasingly unfavourable oxygen conditions, limited food availability and increasing infection loads with parasites ( Eero et al , 2015 , 2023 ).…”