On the decline of eastern Baltic cod: we need to take more holistic views into account. Reply to Brander (2022) comment on Svedäng et al. (2022)

Abstract

The eastern Baltic cod (EBC) stock has gone through drastic changes over the last century, in parallel to developments in its environment, the Baltic Sea itself. The most dramatic part of these transformations has occurred since the 1970s. Naturally, there may be links between these changes; the question is: which link(s) and how?

Brander (2022) explains that the recent demise of EBC can be attributed mainly to mild hypoxia, which has expanded in the water column and on the seafloor since the mid-1990s. Brander (2022) correctly points out that the issue of the recent decline of EBC is contentious; several possible drivers of the decline have been proposed. Brander (2022) asserts that some of the answers are found in Svedäng et al. (2022), specifically, that a negative trend in oxygen concentration exists in the present core area of EBC, SD 25. In an additional argument, Brander (2022) asserts that juvenile cod is increasingly exposed to hypoxia because their depth distribution has deepened, and that the physiological cost of living under mild hypoxia may offset the risk of being eaten by diving seals and cormorants in shallower water with more oxygen.

We find that the reasoning of Brander (2022) minimizes the significance of the outburst of production at the end of the 1970s/beginning of the 1980s,  as well as the present decline in growth and survival since the mid-1990s (ICES, 2021). Both contrasting periods occurred at almost similar levels of hypoxia. Hence, it is difficult to reconcile this long-term development with the hypothesis that a minor decrease in oxygen content can explain the current strong and uniform decline in growth.

Brander (2022) argues that our findings support the idea that the return of hypoxia since the stagnation period ended (see Svedäng et al., 2022 for further details) has led to growth stunting in EBC. Brander (2022) highlights the fact that there is a general trend towards lower oxygen content at depths of 50 and 60 m, however weak, even in SD 25 (Bornholm Basin), as reported by Svedäng et al. (2022). However, this change in oxygen content is minuscule (see Figure 8 in Svedäng et al., 2022). The intra-annual variability in oxygen is much larger than the inter-annual changes; A theoretical fish staying below 50 m would have experienced variable oxygen conditions in any year of choice. At this stage of reasoning, the hypothesis of Brander (2020, 2022) should also be contrasted with observations: Why would the entire cod stock in the southern Baltic Sea have reacted in such a uniform manner, as shown by its ubiquitous truncated size distribution (Svedäng and Hornborg, 2017), when large parts of the stock are also found at shallower depths than 50 m?

Furthermore, the present oxygen conditions at a depth of 50–65 m are not detrimental for EBC health: “There is no visual shallowing of the 4.3 ml/l level [the threshold below which an effect on condition and growth starts to be observable (Chabot and Dutil, 1999)], merely seasonal variations (Supplementary Figure S6). Ignoring sharp vertical gradients and averaging oxygen dynamics in the water layer 50–65 m, Brander (2020, Figure 2) gives an impression of almost detrimental deep-water oxygen concentrations, fluctuating in the recent decades between about 4 and 5 ml/l, i.e. about 50–60% of the oxygen saturation. Meanwhile, in the appropriately depth-wise differentiated approach (Figure 8), the oxygen concentration of about 6–7 ml/l (70–80% saturation) is typical for 50 m depth and is certainly higher at the shallower depths” (Svedäng et al., 2022).

Regarding the proposed deepening of juvenile cod as a consequence of balancing predator risk with hypoxia-related reduction in growth, our main point is that it is highly speculative at this stage. It builds on one single distribution modelling study (Orio et al., 2019), who found a deepening of juvenile cod occurrence in the Baltic Sea over time. We have not been able to find such deepening of juvenile cod (15–30 cm) in data from trawl surveys in SD 25 (www.ices.dk DATRAS: downloaded 5 November 2018). The effect of predator presence on depth distribution of juvenile cod has not been studied. Furthermore, it should be observed that cormorants and grey seals mostly feed close to the coast and at depths <40 m (Sjöberg and Ball, 2000), and that seals were not common in the southern Baltic Sea in the 2000s (Anon., 2012) when growth and survival declined (ICES, 2021).

Explaining the decline of EBC is challenging to the scientific community as is the loss of cod production to society. We must preserve our ability to get a better understanding of the threats which lie before us by scrutinizing ideas that have won general acceptance.

Data availability statement

Fishery data referred to in the text are freely available on the ICES data portal (http://www.ices.dk).

References