Eastern Baltic Cod Research Articles

Centurial Variation in Size at Maturity of Eastern Baltic Cod (Gadus morhua) Mirrors Conditions for Growth.

The status of Eastern Baltic cod (EBC) Gadus morhua has remained poor despite low fishing mortality for over a decade, including a fishing ban since 2019. Although the decline in productivity can be explained by lower individual growth and survival rates, other aspects of life-history changes such as maturation patterns for EBC has so far not been sufficiently explored. According to current stock assessments, the median size at maturity (L 50) has halved from 40 to around 20 cm in total length since the 1990s, while the overall size distribution has become increasingly truncated. It has previously been suggested that changes in L 50 can be attributed to both fishing-induced evolution and phenotypic plasticity induced by growth rates. However, since L 50 is currently occurring around 20 cm, the maturation process must have been initiated at much smaller sizes, that is, long before the fish could be caught in the dominant trawl fishery at around 35 cm. In this study, we aimed to further investigate what drivers may have led to reduced productivity in EBC by determining variations in size at sexual maturity in longer time series than has been done before (1930s to 1980s) and include prey productivity and quality. We found that L 50 declined already in the 1930s and thereafter remained stable at around 40 cm up to the 1990s. On a centurial perspective, L 50 has been positively correlated to growth potential (L 95), length diversity, total stock biomass, total catch and yield per recruit, while Fulton's condition factor was not related to L 50. Our results suggest that the links between life-history parameters and external drivers are complex, but the present unprecedented early onset of maturity and hence decline in L 50 since the 1990s signals a decline in growth potential, which also has hampered the productivity of EBC.

Annual, seasonal and spatial differences in the growth rate of Baltic cod larvae and early juveniles in relation to zooplankton biomass fluctuations in 2006-2014

It is an acknowledged theory that a faster growth rate, determined by various environmental drivers, may boost the survival of larval and juvenile fish. In this study we examined the differences in the growth rate of larval and juvenile cod (age 2 – 136 d; SL: 4.1 – 39.2 mm) between the years 2006 and 2014, sub-areas of the Baltic Sea (Bornholm Basin, BB; Słupsk Furrow, SF; Gdańsk Basin, GB), and seasons (spring and summer). The average growth rate for all specimens was 0.25 mm/d, with significantly lower values between 2012 and 2014 than between 2006 and 2011. A reduction in zooplankton biomass, especially the large zooplankton fraction, was observed after the 2006–2008 period, which was related to the prevailing temperature conditions in the surface layers. The reduction in zooplankton biomass was accompanied by changes in the structure of zooplankton: less T. longicornis, C. hamatus, and Pseudocalanus spp., and more Acartia spp. The results suggest that the inter-annual differences observed in zooplankton biomass and structure are likely responsible for the observed reduction in the growth rate of cod larvae between 2012 and 2014 compared to the years between 2006 and 2011. The growth rate reduction could be one of the reasons for the decline in the recruitment of eastern Baltic cod between the years 2013 and 2015, after the high recruitment years of the period 2011-2012.

Marked recent declines in boron in Baltic Sea cod otoliths – a bellwether of incipient acidification in a vast hypoxic system?

Abstract. Ocean acidification is spreading globally as a result of anthropogenic CO2 emissions, but the Baltic Sea has until recently been thought to be relatively well-buffered by terrigenous inputs of alkalinity from its watershed. We discovered a 3- to 5-fold decline in boron (as B : Ca) in otoliths of eastern Baltic cod (EBC) between the late 1990s and 2021. Examining a time series of EBC otoliths, we found varying levels of B : Ca starting in the 1980s, with the most recent years showing an all-time low for this period. This trend correlates with declines in pH and dissolved oxygen but not with changes in salinity. We examined possible physiological influences on B : Ca by including a collection of Icelandic cod as an “out-group”. Icelandic cod otoliths showed strongly positive correlations of B : Ca with physiologically regulated P : Ca; this was not the case for EBC. Finally, B : Ca in EBC otoliths is negatively correlated, to some extent, with Mn : Mg, a proposed proxy for hypoxia exposure. This negative relationship is hypothesized to reflect the dual phenomena of hypoxia and acidification as a result of decomposition of large algal blooms. Taken together, the otolith biomarkers Mn : Mg and B : Ca in cod suggest a general increase in both hypoxia and acidification within the Baltic intermediate and deep waters in the last decade.

Evidence of hybridization between genetically distinct Baltic cod stocks during peak population abundance(s).

Range expansions can lead to increased contact of divergent populations, thus increasing the potential of hybridization events. Whether viable hybrids are produced will most likely depend on the level of genomic divergence and associated genomic incompatibilities between the different entities as well as environmental conditions. By taking advantage of historical Baltic cod (Gadus morhua) otolith samples combined with genotyping and whole genome sequencing, we here investigate the genetic impact of the increased spawning stock biomass of the eastern Baltic cod stock in the mid 1980s. The eastern Baltic cod is genetically highly differentiated from the adjacent western Baltic cod and locally adapted to the brackish environmental conditions in the deeper Eastern basins of the Baltic Sea unsuitable for its marine counterparts. Our genotyping results show an increased proportion of eastern Baltic cod in western Baltic areas (Mecklenburg Bay and Arkona Basin)-indicative of a range expansion westwards-during the peak population abundance in the 1980s. Additionally, we detect high frequencies of potential hybrids (including F1, F2 and backcrosses), verified by whole genome sequencing data for a subset of individuals. Analysis of mitochondrial genomes further indicates directional gene flow from eastern Baltic cod males to western Baltic cod females. Our findings unravel that increased overlap in distribution can promote hybridization between highly divergent populations and that the hybrids can be viable and survive under specific and favourable environmental conditions. However, the observed hybridization had seemingly no long-lasting impact on the continuous separation and genetic differentiation between the unique Baltic cod stocks.

Corrigendum: Compensatory feeding in Eastern Baltic cod (Gadus morhua): recent shifts in otolith growth and nitrogen content suggest unprecedented metabolic changes

Corrigendum: Compensatory feeding in Eastern Baltic cod (Gadus morhua): recent shifts in otolith growth and nitrogen content suggest unprecedented metabolic changes

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

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.

Comparative histopathology of livers from Baltic cod infected with the parasitic nematode Contracaecum osculatum

Infection levels with the parasitic nematode Contracaecum osculatum in Eastern Baltic cod have increased in the last decades. Eastern Baltic cod is transport host for this parasite that has a high affinity for the liver of the fish. The liver is a highly vital organ and damage to the liver tissue can result in reduced functionality of the organ. Previous studies have revealed that cod with high infections loads reveal impaired physiological performance, reduced nutritional condition and show signs of having a liver disease. Yet, little is known about the pathological changes and inflammatory reactions of the cod liver related to the infections. In this study, we performed histological examinations on 30 Baltic cod livers caught in the eastern part of the Baltic Sea (length; 38 ± 0.9cm, weight; 454 ± 34.8gram) and three Sound cod livers (length; 63 ± 2.9cm, weight; 3396 ± 300.2gram) to categorize the degree of inflammation and its relation to pathological changes in infected cod livers. We further investigated how C.osculatum infection levels varied with intensity of inflammation and co-infections. We found that high infection loads with C.osculatum caused severe inflammation in the liver tissue of cod and reduced fat content of the hepatocytes. Conspicuous amounts of glycogen were found in the muscle and intestinal epithelial cells of the nematodes and parasitic co-infections occurred more frequently in the most heavily infected livers.

Bioenergetics modelling of growth processes in parasitized Eastern Baltic cod (Gadus morhua L.).

Changes in physiological processes can reveal how individuals respond to environmental stressors. It can be difficult to link physiological responses to changes in vital rates such as growth, reproduction and survival. Here, bioenergetics modelling can aid in understanding non-intuitive outcomes from stressor combinations. Building on an established bioenergetics model, we examine the potential effects of parasite infection on growth rate and body condition. Parasites represent an overlooked biotic factor, despite their known effects on the physiology of the host organism. As a case study, we use the host-parasite system of Eastern Baltic cod (Gadus morhua) infected with the parasitic nematode Contraceacum osculatum. Eastern Baltic cod have during the past decade experienced increasing infection loads with C. osculatum that have been shown to lead to physiological changes. We hypothesized that infection with parasites affects cod growth negatively as previous studies reveal that the infections lead to reduced energy turnover, severe liver disease and reduced nutritional condition. To test this, we implemented new variables into the bioenergetics model representing the physiological changes in infected fish and parameterized these based on previous experimental data. We found that growth rate and body condition decreased with increased infection load. Highly infected cod reach a point of no return where their energy intake cannot maintain a surplus energy balance, which may eventually lead to induced mortality. In conclusion, parasite infections cannot be ignored when assessing drivers of fish stock dynamics.

Современное состояние восточнобалтийской популяции трески Gadus morhua callarias Linnaeus, 1758

A review of scientific literature describing the state and dynamics of the Eastern Baltic cod population over the past 30 years has been conducted. The Eastern Baltic cod is one of the most important commercial species in the Baltic Sea. An ecological regime shift in the early 1990s, resulting from a reduction in the frequency of large North Sea inflows, led to a de­crease in salinity and oxygen levels in the bottom layers of the deep basins of the Baltic Sea. This caused a sharp deterioration in the spawning conditions. Factors influencing the Eastern Baltic cod population and changes in the population under the influence of these factors are described. The decisive impact of North Sea inflows and their prolonged absence on the popu­lation's development, the influence of oxygen saturation levels on all life stages of cod, and the negative consequences of hypoxia for the feeding of Eastern Baltic cod are noted. Among the changes in the population, a decrease in the number of eggs in spawning areas during the peak spawning period, a reduction in the average individual sizes and weights of specimens, and a shift in the spawning peak to later dates are highlighted. The conclusion is drawn about the unsatisfactory state of the population and the impossibility of its recovery in the medium term, regardless of commercial catch rates.

Correction to: Re-thinking the “ecological envelope” of Eastern Baltic cod (Gadus morhua): conditions for productivity, reproduction, and feeding over time

Correction to: Re-thinking the “ecological envelope” of Eastern Baltic cod (<i>Gadus morhua</i>): conditions for productivity, reproduction, and feeding over time

Living apart together: Long-term coexistence of Baltic cod stocks associated with depth-specific habitat use.

Coexistence of fish populations (= stocks) of the same species is a common phenomenon. In the Baltic Sea, two genetically divergent stocks of Atlantic cod (Gadus morhua), Western Baltic cod (WBC) and Eastern Baltic cod (EBC), coexist in the Arkona Sea. Although the relative proportions of WBC and EBC in this area are considered in the current stock assessments, the mixing dynamics and ecological mechanisms underlying coexistence are not well understood. In this study, a genetically validated otolith shape analysis was used to develop the most comprehensive time series of annual stock mixing data (1977–2019) for WBC and EBC. Spatio-temporal mixing analysis confirmed that the two stocks coexist in the Arkona Sea, albeit with fluctuating mixing proportions over the 43-year observation period. Depth-stratified analysis revealed a strong correlation between capture depth and stock mixing patterns, with high proportions of WBC in shallower waters (48–61% in <20m) and increasing proportions of EBC in deeper waters (50–86% in 40-70m). Consistent depth-specific mixing patterns indicate stable differences in depth distribution and habitat use of WBC and EBC that may thus underlie the long-term coexistence of the two stocks in the Arkona Sea. These differences were also reflected in significantly different proportions of WBC and EBC in fisheries applying passive gears in shallower waters (more WBC) and active gears in deeper waters (more EBC). This highlights the potential for fishing gear-specific exploitation of different stocks, and calls for stronger consideration of capture depth and gear type in stock assessments. This novel evidence provides the basis for improved approaches to research, monitoring and management of Baltic cod stocks.

Support for the hypothesis that growth of eastern Baltic cod is affected by mild hypoxia. A comment on Svedäng et al. (2022)

Abstract Stock biomass and size composition of eastern Baltic cod (EBC) has declined, with low recent stock productivity attributed to loss of major spawning areas, reduced growth rates, and possibly increased natural mortality. Svedäng et al. present valuable new data for evaluating the causes of these changes. However, contrary to their own conclusions, those new data actually provide both direct and indirect support for the hypothesis that the reduced growth rate observed may be due to a decline in ambient oxygen for small cod in the Bornholm Basin area (SD25). Their long time series of reproductive volume (RV) extends our knowledge of historic changes in the limits to spawning imposed by salinity and oxygen. However, the causes and consequences of such spawning area changes are well established. Therefore, this Comment only addresses the contentious debate about causes of decline in growth.

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 Explaining the recent decline of eastern Baltic cod (EBC) remains scientifically challenging. Brander proposes in a comment to Svedäng et al. that the observed trend in oxygen in SD 25 supports the idea that juvenile cod are balancing the physiological cost of living under mild hypoxia by offsetting the risk of being eaten by diving seals and cormorants in shallower water with more oxygen. There are a number of objections to this conjecture, besides the fact that supporting observations are missing. Hence, it is difficult to reconcile the long-term development of EBC under varying oxygen conditions with the hypothesis that a small reduction in oxygen content can explain the current strong and uniform decline in growth observed in the entire southern Baltic Sea.

Re-thinking the “ecological envelope” of Eastern Baltic cod (Gadus morhua): conditions for productivity, reproduction, and feeding over time

Abstract Hypoxia is presently seen as the principal driver behind the decline of the former dominating Eastern Baltic cod stock (EBC; Gadus morhua). It has been proposed that both worsening conditions for reproduction and lower individual growth, condition, and survival are linked to hypoxia. Here, we elucidate the ecological envelope of EBC in terms of salinity stratification, oxygen content, and benthic animal biomasses, and how it has affected EBC productivity over time. The spawning conditions started deteriorating in the Gotland Deep in the 1950s due to oxygen depletion. In contrast, in the Bornholm Basin, hydrographic conditions have remained unchanged over the last 60 years. Indeed, the current extent of both well-oxygenated areas and the frequency of hypoxia events do not differ substantially from periods with high EBC productivity in the 1970s–1980s. Furthermore, oxygenated and therefore potentially suitable feeding areas are abundant in all parts of the Baltic Sea, and our novel analysis provides no evidence of a reduction in benthic food sources for EBC over the last 30 years. We find that while reproduction failure is intricately linked to hydrographic dynamics, a relationship between the spread of hypoxia and the decline in EBC productivity during the last decades cannot be substantiated.

Is Diversity the Missing Link in Coastal Fisheries Management?

Fisheries management has historically focused on the population elasticity of target fish based primarily on demographic modeling, with the key assumptions of stability in environmental conditions and static trophic relationships. The predictive capacity of this fisheries framework is poor, especially in closed systems where the benthic diversity and boundary effects are important and the stock levels are low. Here, we present a probabilistic model that couples key fish populations with a complex suite of trophic, environmental, and geomorphological factors. Using 41 years of observations we model the changes in eastern Baltic cod (Gadus morhua), herring (Clupea harengus), and Baltic sprat (Sprattus sprattus balticus) for the Baltic Sea within a Bayesian network. The model predictions are spatially explicit and show the changes of the central Baltic Sea from cod- to sprat-dominated ecology over the 41 years. This also highlights how the years 2004 to 2014 deviate in terms of the typical cod–environment relationship, with environmental factors such as salinity being less influential on cod population abundance than in previous periods. The role of macrozoobenthos abundance, biotopic rugosity, and flatfish biomass showed an increased influence in predicting cod biomass in the last decade of the study. Fisheries management that is able to accommodate shifting ecological and environmental conditions relevant to biotopic information will be more effective and realistic. Non-stationary modelling for all of the homogeneous biotope regions, while acknowledging that each has a specific ecology relevant to understanding the fish population dynamics, is essential for fisheries science and sustainable management of fish stocks.

New perspectives on Eastern Baltic cod movement patterns from historical and contemporary tagging data

Knowledge of the movement patterns and area utilisation of commercially important fish stocks is critical to management. The Eastern Baltic cod Gadus morhua, one of the most commercially and ecologically important stocks in the Baltic Sea, is currently one of the most severely impacted fish stocks in Europe. During the last 2 decades, this stock has experienced drastic decreases in population size, distributional range, individual growth and body condition, all of which may have affected the movements between different areas of the Baltic Sea. In this study, we investigated the seasonal movement patterns of Eastern Baltic cod by re-analysing historical tagging data collected by the countries surrounding the Baltic Sea (1955-1988) and compared historical patterns with contemporary data from a recent international tagging experiment (2016-2019). Our re-analyses of historical data showed the presence of different movement behaviours, i.e. resident or seasonally migratory, with larger distances moved by cod released in the northern and central Baltic areas compared to cod released in the southern Baltic areas. Furthermore, trends from the recent tagging experiment indicate a persistent resident strategy in the southern Baltic area. These findings present additional information on general movement patterns and area utilisation of Eastern Baltic cod that could inform future management actions and aid stock recovery.

Fisheries management and tipping points: Seeking optimal management of Eastern Baltic cod under conditions of uncertainty about the future productivity regime

AbstractHistorical patterns of the Eastern Baltic cod stock recruitment show a shift from a regime with high reproductive potential before the early 1980s to a regime with low reproductive potential since then. This shift can be attributed to increasingly unfavorable environmental conditions for cod reproduction at that time: critical salinity and oxygen levels, needed for successful egg and larval development, deteriorated. Yet, significant inflows of salt‐ and oxygen‐rich water from the North Sea or improved eutrophication management might trigger a shift back to a more productive regime. Coupling a statistical recruitment model to a state‐of‐the‐art, age‐structured bio‐economic model of the Eastern Baltic cod fishery, we study how optimal management depends on the uncertainty about the future productivity regime. We extend the predominantly theoretical literature on optimal management of a natural resource with a potential regime shift by analyzing an empirical model of age‐structured population dynamics and by allowing for the possibility of a back‐shift from a “bad” into a “good” regime. We find that with a higher probability of a shift back to the more productive regime the optimal management of the fishery becomes more conservative in the short run. We conclude that these benefits for the fishery warrant strong action reducing eutrophication to increase the probability of a regime shift back to high reproductive potential of the Eastern Baltic cod fishery.

Changes in population depth distribution and oxygen stratification are involved in the current low condition of the eastern Baltic Sea cod (&amp;lt;i&amp;gt;Gadus morhua&amp;lt;/i&amp;gt;)

Abstract. 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.

Improving species and size selectivity in the Baltic cod trawl fishery with two simple codend designs

The eastern Baltic cod (Gadus morhua) stock is at a critically low level and recruitment of the western Baltic cod stock is at the lowest observed level in over 30 years. The EU (European Union) landing obligation was introduced in 2015 for cod and in 2017 for plaice (Pleuronectes platessa). Size selective codend designs have been used in the Baltic cod trawl fishery for 25 years; however, there are still relatively high discard rates for cod, plaice and European flounder (Platichthys flesus; henceforth flounder) in the trawl fishery and landings of individuals below the minimum conservation reference size (MCRS). These problems call for additional improvements of species and size selectivity in the cod trawl fishery. In this study, two trawl codend designs were developed and tested to improve species and size selectivity in the Baltic cod trawl fishery. The codends were tested onboard a commercial trawler and the test codends were compared to the conventional Bacoma codend, which was fished simultaneously in a twin trawl rig. New codend designs were based on the Bacoma design, where the first test codend had a bottom net panel made of larger diamond meshes (nominal 130 mm mesh size) to improve selectivity in a cod fishery with high catch rates of flatfish. This codend showed a reduction of plaice and flounder, and cod below 45 cm, in the catches. The other test codend was a full square mesh codend (nominal 120 mm mesh size) developed for a cod directed fishery with low flatfish catches, and showed a marked reduction in the catch of cod below 50 cm, with no differences for flounder and plaice catches. With cod stocks at critically low levels and relatively high discard rates of cod, plaice and flounder, the simple codend designs tested in this study could benefit the sustainability of the Baltic cod trawl fishery, but with some loss of commercially targeted sizes.

Multidecadal changes in fish growth rates estimated from tagging data: A case study from the Eastern Baltic cod (Gadus morhua, Gadidae)

AbstractLong time series of reliable individual growth estimates are crucial for understanding the status of a fish stock and deciding upon appropriate management. Tagging data provide valuable information about fish growth, and are especially useful when age‐based growth estimates and stock assessments are compromised by age‐determination uncertainties. However, in the literature there is a lack of studies assessing possible changes in growth over time using tagging data. Here, data from tagging experiments performed in the Baltic Sea between 1971 and 2019 were added to those previously analysed for 1955–1970 to build the most extensive tagging dataset available for Eastern Baltic cod (Gadus morhua, Gadidae), a threatened stock with severe age‐determination problems. Two length‐based methods, the GROTAG model (based on the von Bertalanffy growth function) and a Generalized Additive Model, were used to assess for the first time the potential long‐term changes in cod growth using age‐independent data. Both methods showed strong changes in growth with an increase until the end of the 1980s (8.6–10.6 cm/year for a 40 cm cod depending on the model) followed by a sharp decline. This study also revealed that the current growth of cod is the lowest observed in the past 7 decades (4.3–5.1 cm/year for a 40 cm cod depending on the model), indicating very low productivity. This study provides the first example of the use of tagging data to estimate multidecadal changes in growth rates in wild fish. This methodology can also be applied to other species, especially in those cases where severe age‐determination problems exist.