Western Baltic cod in distress: decline in energy reserves since 1977

Abstract

The western Baltic Sea cod (WBC) stock is at historically low levels, mainly attributed to high fishing pressure and low recruitment. Stable stock assessment metrics suggested recovery potential, given appropriate fisheries management measures. However, changing environmental conditions violate stability assumptions, may negatively affect WBC, and challenge the resource management. The present study explored 42 years of changes in WBC biological parameters. WBC body condition gradually decreased over the last decades for juveniles and adults, with a rapid decrease in recent years when a single cohort dominated the overfished stock. The hepato-somatic index and the muscle weight decreased by 50% and 10% in the last 10 years, respectively, suggesting severely decreasing energy reserves and productivity. The changes in energy reserves were associated with changes in environmental conditions (increase in bottom water temperature, expansion of hypoxic areas during late summer/autumn), and changes in diet composition (less herring). A key bottleneck is the warming and longer-lasting summer period when WBC, trapped between warmed shallow waters and hypoxic deeper waters, have to mobilize energy reserves to account for reduced feeding opportunities and thermal stress. Our results suggest that stock recovery is unlikely to happen by fisheries management alone if environmental trajectories remain unchanged.

Four Decades of Spatiotemporal Variability of Per- and Polyfluoroalkyl Substances (PFASs) in the Baltic Sea

“…Another explanation could be deterioration of the condition of the cod stock in the Southern Baltic Proper over the past decades. 74,75 This has resulted in decreasing Fulton's condition factors (100 × weight (g)/[length (cm)] 3 ) for cod in our study from ∼1.1 in the 1990s to ∼0.9 in the past decade, 20 mirroring that of other studies, 76,77 and alterations in the feeding strategy and physiology of cod 78 that could impact the PFAS concentration and possibly the in situ transformation of precursors. An increase in concentration in cod has also been observed for another fairly stable contaminant, mercury, in the basin (100% increase since the 1990s), 20 showing the importance of other environmental factors compared to water concentrations.…”

A climate vulnerability assessment of the fish community in the Western Baltic Sea

“…In contrast to other highly sensitive species, the sensitivity of cod is not related to any complex life-cycle attributes, but to its dependence on higher salinities 56 . Generally, cod is considered to suffer from the effects of climate change in the Western Baltic Sea, especially warming, while the exact process is still unclear 18 , 57 – 59 . In contrast, herring is tolerant to a wide range of salinity changes 60 , 61 , but belongs to the group of climate sensitive species because of its rather complex life cycle.…”

Tracing growth patterns in cod (Gadus morhua L.) using bioenergetic modelling

“…Weight losses during summer may lead to an overall decreased condition, a reduced growth rate and a decreased build‐up of reproductive products for the upcoming spawning season (Kjesbu et al., 1991; Lambert & Dutil, 2000). Especially the latter may have drastic consequences for the reproductive success of WBC, which can be already considered to be in a critical state (Möllmann et al., 2021; Receveur et al., 2022).…”