Deoxygenation impacts on Baltic Sea cod: Dramatic declines in ecosystem services of an iconic keystone predator

The intensified expansion of the Baltic Sea’s hypoxic zone has been proposed as one reason for the current poor status of cod (Gadus morhua) in the Baltic Sea, with repercussions throughout the food web and on ecosystem services. We examined the links between increased hypoxic areas and the decline in maximum length of Baltic cod, a demographic proxy for services generation. We analysed the effect of different predictors on maximum length of Baltic cod during 1978–2014 using a generalized additive model. The extent of minimally suitable areas for cod (oxygen concentration ≥ 1 ml l−1) is the most important predictor of decreased cod maximum length. We also show, with simulations, the potential for Baltic cod to increase its maximum length if hypoxic areal extent is reduced to levels comparable to the beginning of the 1990s. We discuss our findings in relation to ecosystem services affected by the decrease of cod maximum length.

Changes in population depth distribution and oxygen stratification are involved in the current low condition of the eastern Baltic Sea cod (<i>Gadus morhua</i>)

During the past 20 years, hypoxic areas have expanded rapidly in the Baltic Sea, which has become one of the largest marine “dead zones” in the world. At the same time, the most important commercial fish population of the region, the eastern Baltic cod, has experienced a drastic reduction in mean body condition, but the processes behind the relation between deoxygenation and condition remain elusive. Here we use extensive long-term monitoring data on cod biology and distribution as well as on hydrological variations to investigate the processes that relate deoxygenation and cod condition during the autumn season. Our results show that the depth distribution of cod has increased during the past 4 decades at the same time of the expansion, and shallowing, of waters with oxygen concentrations detrimental to cod performance. This has resulted in a progressively increasing spatial overlap between the cod population and low-oxygenated waters after the mid-1990s. This spatial overlap and the actual oxygen concentration experienced by cod therein statistically explained a large proportion of the changes in cod condition over the years. These results complement previous analyses on fish otolith microchemistry that also revealed that since the mid-1990s, cod individuals with low condition were exposed to low-oxygen waters during their life. This study helps to shed light on the processes that have led to a decline of the eastern Baltic cod body condition, which can aid the management of this population currently in distress. Further studies should focus on understanding why the cod population has moved to deeper waters in autumn and on analyzing the overlap with low-oxygen waters in other seasons to quantify the potential effects of the variations in physical properties on cod biology throughout the year.

Is analysis of otolith microstructure a valid method for investigating early life history of Western Baltic cod?

Daily formation of fish otolith micro-increments is frequently assumed, however applying inferences about timing of life history events and formation of otolith macro-structures requires further validation of the periodicity of micro-increment formation. We analysed micro-increments from Western Baltic cod (WBC, Gadus morhua) otoliths marked with tetracycline-hydrochloride as part of an age validation study to test the assumption of daily formation of micro-increments. We found that the number of counted micro-increments consistently underestimated the age of cod aged 1 and older. Time at liberty was also underestimated, especially for fish at liberty during winter. In contrast, micro-increment counts of otoliths from wild-caught young-of-the-year (YOY) cod could be used to realistically estimate timing of hatch and translucent zone formation. Under ambient conditions, settlement did not correspond to any visible pattern within the otoliths, but could be inferred from the prey switch observed from stomach content analyses. We therefore conclude that micro-increments can be assumed to form on a daily basis until the first winter, and can therefore be used to investigate early life history of YOY WBC. However, the periodicity of micro-increment formation appears to vary seasonally in older individuals, with the number of micro-increments formed during the winter period being particularly low.

It’s elemental, my dear Watson: validating seasonal patterns in otolith chemical chronologies

Accurate age data is essential for reliable fish stock assessment. Yet many stocks suffer from inconsistencies in age interpretation. A new approach to obtain age makes use of the chemical composition of otoliths. This study validates the periodicity of recurrent patterns in 25Mg, 31P, 34K, 55Mn, 63Cu, 64Zn, 66Zn, 85Rb, 88Sr, 138Ba, and 208Pb in Baltic cod (Gadus morhua) otoliths from tag-recapture and known-age samples. Otolith P concentrations showed the highest consistency in seasonality over the years, with minima co-occurring with otolith winter zones in the known-age otoliths and in late winter/early spring when water temperatures are coldest in tagged cod . The timing of minima differs between stocks, occurring around February in western Baltic cod and one month later in eastern Baltic cod; seasonal maxima are also stock-specific, occurring in August and October, respectively. The amplitude in P is larger in faster-growing western compared to eastern Baltic cod. Seasonal patterns with minima in winter/late spring were also evident in Mg and Mn, but less consistent over time and fish size than P. Chronological patterns in P, and to a lesser extent Mg and Mn, may have the potential to supplement traditional age estimation or to guide the visual identification of translucent and opaque otolith patterns used in traditional age estimation