Studies identifying the impacts of long-term shifts in temperatures and weather patterns affecting the Baltic Sea ecosystem are prominent in the analysis. The consensus among the 31 identified papers is that the Baltic Sea and its ecosystem have already exhibited clear evidence of change in response to rising temperatures. For example, Voss et al. (2019) demonstrates how human-induced ocean warming of +1.7°C drastically decreases cod fishing opportunities in the region, whilst a +2°C eliminates cod’s recovery chances due larvae preferring colder temperatures. Lindegren et al. (2010); Margonski et al. (2010) and Voss et al. (2012) further demonstrates how temperature change negatively influences cod and herring stock-recruitment abilities.

Thøgersen et al. (2015) forecasts the management difficulties and economic impacts of climate change on Baltic Sea fisheries using management scenarios based on age-structured models. Hinrichsen et al. (2011) underlines that reducing fish catch mortality is insufficient, emphasising how climate-driven long-term temperature trends in oxygen concentrations will be key contributors to the downfall of Western and Eastern cod populations. Hiddink and Coleby (2012) reveals that the effect of temperature rise on marine fish biodiversity is most pronounced in areas with low population connectivity, such as the Baltic Sea, because species in these areas have limited opportunities to migrate to more favourable conditions, leading to a higher risk of local extinctions.

Neumann (2010) calculates that the most serious impact of temperature change will be alterations in the physical environment, leading to changes in habitat conditions and distribution patterns of species. This includes shifts in temperature, salinity, and oxygen levels, which in turn affect the biological parameters of species such as growth rates, reproductive success, and survival. Furthermore, Neumann (2010) emphasises how increased warming will favour cyanobacteria growth, leading to longer lasting and more widespread blooms. Studies conducted a decade later by Cederqvist et al. (2020) and Meier et al. (2021) reinforce those earlier findings.

Brander (2010) discusses the implications of climate cycles on fish stocks, suggesting that temperature increases could exacerbate existing pressures on marine life. Meier et al. (2011b, 2011c, 2011d) and Nowicki et al. (2016) uses simulation models to predict future temperature scenarios, highlighting potential shifts in species distributions. Andersson et al. (2015a) explores future ecosystem management strategies against warming trends. Meanwhile, Hordoir and Meier (2012) investigates the effects of thermal stratification on the Baltic Sea, predicting significant changes in water column stability and nutrient cycling. Hägg et al. (2014) and Meier et al. (2012a, 2012c) study future nutrient loading scenarios, projecting that warming temperatures would likely increase the frequency and intensity of algal blooms.

Other notable research includes Barbosa and Donner (2016), who examine the interplay between temperature rise and marine ecosystem health; Philippart et al. (2011) on the long-term ecological impacts of temperature rise; Lehmann et al. (2011) on species adaptation mechanisms; Friedland et al. (2012) on fisheries management challenges; Niiranen et al. (2013) on ecosystem resilience; Rutgersson et al. (2014) on climate modelling; and Viitasalo and Bonsdorff (2022) on future biodiversity projections. Overall, rising sea temperatures have been identified as a major driver of ecological change in the Baltic Sea, influencing deoxygenation, eutrophication and saltwater inflow events.

The phenomenon of deoxygenation in the Baltic Sea and the creation of hypoxia or dead zones is the focus of intense research, with 29 articles highlighted in this review. The collective findings underscore the expansion of dead zones in the Baltic Sea. For example, Conley et al. (2011) identifies 115 hypoxic sites between 1955 and 2009, underscoring how the Baltic Sea contains over 20% of all known hypoxic sites worldwide. Modelling studies by Hansson and Gustafsson (2011); Gustafsson (2012) and Savchuk (2013) consistently reveal a worsening trend in the long-term development, volume and dynamics of hypoxic areas in various Baltic sub-regions. Meier et al. (Meier et al., 2011a, 2017) reinforce this, demonstrating how accelerated mean sea level rise will exacerbate the spread of dead zones. Bendtsen and Hansen (2013) find that a 3°C temperature rise on the seafloor will reduce oxygen production by 30%. Other studies indicating a significant expansion of hypoxic areas include Almroth-Rosell et al. (2011, 2021); Bange et al. (2010); Gilbert et al. (2010); Rabalais et al. (2010); Hansson and Gustafsson (2011); Reed et al. (2011); Vaquer-Sunyer and Duarte (2011); Lehmann et al. (2014); Breitburg et al. (2018) and Zilí and Conley (2010) suggest that a worst-case scenario for the extent of hypoxic areas in the Baltic Sea has been reached.

Academics also research the effects of increasing hypoxia ranges and durations on the local ecosystem of the Baltic Sea (Meier et al., 2019c). Díaz and Rosenberg (2011) discusses the potential environmental and economic consequences of deoxygenation, highlighting how hypoxia increases fish vulnerability to diseases and reduces mobility, survivability, growth and reproductive success. Similar impacts are discussed by Vaquer-Sunyer and Duarte (2010); Hansson et al. (2011); Carstensen et al. (2014) and Villnäs et al. (2012), emphasising how increasing bottom-water hypoxia is the main stressor impacting benthic communities, including cod, whiting, flounder and plaice, creating barren wastelands on the seafloor.

Casini et al (Casini et al., 2016 and Casini et al., 2021) detail the adverse consequences of hypoxia on cod, in relation to their size, physiology, behaviour, and trophic interactions. Statistical evidence indicates hypoxia-induced restrictions for cod, forcing them to crowd and compete for fewer resources. Moreover, Norkko et al. (2012) emphasises how increased abundances of polychaetes Marenzelleria spp. in the Baltic Sea have helped mitigate the spread of hypoxia. Dietze and Löptien (2016); Neumann et al. (2017) and Stigebrandt and Kalén (2013) highlight the potential of major Baltic inflows that could oxygenate the central Baltic Sea. This oxygenated water could settle in oxygen-deprived areas, potentially revitalising them (Stigebrandt and Kalén, 2013). However, the absence of a major saltwater inflow since 2015 has created a prolonged period without new oxygenated water mixing in the Baltic Sea, causing further hypoxic expansion.

Nutrient enrichment, often resulting in eutrophication, is a primary driver of harmful algal and bacterial blooms in the Baltic Sea. This phenomenon is mainly fuelled by excessive nitrogen and phosphorus inputs, as highlighted by 67 studies emphasising its trends and ecological risks. One of the most visible signs of eutrophication is increased algal growth, which reduces water clarity and blocks sunlight from penetrating as deeply as before (Zilí and Conley, 2010).

Bergström et al. (2019) find that species exhibit varied responses to cyanobacteria blooms; for example, carnivorous fish are more negatively affected, while herbivores benefit from the increased algae. This adversely impacts cod, whiting, plaice and flounder, which prey on other fish. Benthic habitats housing Gadidae and Pleuronectidae species are expected to be under the biggest pressure, with cyanobacteria blooms contributing to the spread of hypoxia on the seafloor (Dzierzbicka-Głowacka et al., 2011a; Legrand et al., 2015). Additionally, Suikkanen et al. (2010, 2013) show how plankton communities in the Baltic Sea have shifted the food-web structure of smaller-sized organisms, leading to decreased energy available for zooplankton and consequently for herring, which rely on zooplankton as a food source (Adam et al., 2016). The decomposition of cyanobacteria releases nitrogen and depletes oxygen, creating uninhabitable environments for many fish species (Ferreira et al., 2011; Karlson et al., 2015; Kahru et al., 2020, Meier et al., 2012b, Walve and Larsson, 2010; Viitasalo and Bonsdorff, 2022).

Phytoplankton composition and growth trends indicate that large-scale climatic shifts will likely increase harmful algal blooms (Klais et al., 2011; Mazur-Marzec et al., 2013; Kahru and Elmgren, 2014; Kaiser et al., 2020, Meier et al., 2018b, Olenina et al., 2010; Ploug et al., 2010; Wasmund et al., 2011; Varjopuro et al., 2014; Munkes et al., 2021). Cyanobacteria life cycle models by Hense et al. (2013) demonstrate that cyanobacteria thrive in higher water temperatures, enabling them to benefit from climate change. Neumann et al. (2012) predict that cyanobacteria blooms contribute up to 50% of the total nitrogen load in the Baltic Sea. Studies by El-Shehawy et al. (2012), Wasmund (2017), Meier et al. (2018a, 2018b, 2019a, 2019b), Murray et al. (2019), Voss et al. (2011a) and Śliwińska-Wilczewska et al. (2019) anticipate record-breaking cyanobacteria bloom events in the region. Long-term temporal and spatial trend studies by Andersen J. H. et al. (2015), Fleming-Lehtinen et al. (2015) and Gustafsson et al. (2012) reveal that the Baltic Sea eutrophication problem is expanding.

The Baltic Sea has long been polluted by excessive nutrients from agricultural runoff, with over 250 rivers discharging into the sea. Nutrient enrichment detrimentally impacts the health of commercially valuable stocks, which frequently spawn in coastal lagoons and estuaries (Dodson et al., 2018, Kuss et al., 2020). Excess nutrients promote earlier and more intense cyanobacteria blooms, creating longer and more damaging cycles (Andersson et al., 2015b, Mort et al., 2010; Ploug et al., 2010; Raateoja et al., 2011; Viktorsson et al., 2012; Stigebrandt et al., 2014; Olofsson et al., 2016; Yli-Hemminki et al., 2016; Gustafsson et al., 2017). Projections by Pihlainen et al. (2020) indicate that if current conditions persist, by 2100 the overall nutrient loading in the Baltic Sea will increase from 52% to 115%. Räike et al. (2020) note that despite water protection measures, nutrient exports from Finnish rivers into the Baltic Sea have not decreased.

Contrarily, Kuss et al. (2020) observe a gradual 30% decrease in anthropogenic nitrogen input in the Western Baltic Sea since 1995, with phosphorus input declining by 25% between 1995 and 2000. Hägg et al. (Hägg et al., 2010, 2014) suggest that improved agricultural waste management could lead to better conditions for the Baltic Sea, despite warming trends. Simultaneous reductions in nitrogen and phosphorus loads could significantly improve oxygen conditions on the seafloor, combating hypoxic areas and allowing marine life to return to benthic regions (Gustafsson et al., 2012; Riemann et al., 2016). However, other studies underscore that climate change will only exacerbate the eutrophic dangers associated with riverine nutrient enrichment and limit the recovery success for many commercially valuable fish species, notably Gadidae and Pleuronectidae species (Carstensen et al., 2011; Arheimer et al., 2012; Meier et al., 2012a; Bring et al., 2015). While direct studies linking nutrient enrichment effects on top commercial fish species are lacking, research by Hongisto (2011; Hong et al. (2012)), Korpinen et al. (2012); Wikner and Andersson (2012); Savchuk (2018) and Wulff et al. (2014) offer insights into the increased pressures of nutrient cycling dynamics, interactions with algal blooms, and nutrient transport pathways in the Baltic Sea, which disrupt the food-web and habitat of commercial fish stocks.

Salinity levels and their impacts on marine life are highly researched in the Baltic Sea, with 20 articles highlighted in this review. The Baltic Sea has the lowest salinity level of any sea in the world. Key studies include Kniebusch et al. (2019); Maar et al. (2011) and MohHrholz et al. (2015) on the changing salinity gradients in the Baltic Sea. They highlight how unfavourable weather patterns for saltwater inflows from the North Sea, coupled with freshwater inflow from rivers, have resulted in significantly reduced salinity (Fu et al. 2010; Kniebuschet al., 2019). If significant inflow events do not occur in this century, the Baltic Sea could potentially become a lake (Mohrholz et al., 2015, Reusch et al., 2018). Lehmann et al. (2022) underscores how ecosystems adapted to the current salinity level may face ecological stress from sudden changes. Consequently, salt-tolerant species may struggle to adapt to shifting salinity levels, leading to reduced distribution and challenges in larval survival in oxygen-poor depths (Lehmann et al., 2022).

Miethe et al. (2014) suggests that the distribution pattern of herring is influenced by saltwater inflow events into the Baltic Sea. Despite herring’s tolerance to varying salinities and temperatures, central herring stocks are observed to avoid water layers with the lowest oxygen saturation and salinity levels. Nissling and Dahlman (2010) and Vuorinen et al. (2015) highlight how declining salinity levels in the Baltic Sea affect crucial marine benthic foundation species such as Eelgrass (Zostera marina) and Wrack (Fucus radicans). These species, integral in providing food and shelter for other marine organisms, see their distributions altered, potentially impacting commercially important species like cod and flounder.

Kijewska et al. (2016) examine the stress response of Eastern and Western Baltic cod populations to varying salinities. Their results show how cod accustomed to higher salinity conditions in the Kiel Bight are unlikely to migrate to less saline regions like Gdansk Bay, maintaining both physical and genetic separations among cod stocks. Berg et al. (2015) demonstrates how cod expanded into the Central Baltic Sea when the surface salinity is higher. Consequently, the presence of low-saline waters significantly impedes the spawning success of cod in the Baltic Sea (Berg et al., 2015). Additionally, low salinity promotes harmful bacterial blooms, creating a favourable environment for Dolichospermum spp (Brutemark et al., 2015). Similar experiments are conducted by Engström-Öst et al. (2011) and Rakko and Seppälä (2014), with their results showing the positive effect that low salinity has on the growth rates of various cyanobacteria species contributing to eutrophication and hypoxia.

The marine CO₂ system is a key focus in marine research aimed at understanding ocean acidification and the consumption or production of CO₂ in the Baltic Sea. This region is especially vulnerable to ocean acidification due to its considerably lower alkalinity levels compared to the ocean (Omstedt et al., 2014). Thirty-three studies focus on the distribution and spread of CO₂ within this review. Beldowski et al. (2010) mapped the distribution pattern of CO₂ and alkalinity in the region. They calculate that an annual increase of CO₂ by 1.2–1.5 ppm will decrease pH by 0.002 per year. Other studies measure the unfavourable impacts of organic carbon, nitrogen and phosphorus cycling the Baltic Sea, including Brutemark et al. (2011); Dzierzbicka-Glowacka et al. (2011b); Skoog et al. (2011); Omstedt et al. (2012); Szczepańska et al. (2012); Gustafsson et al. (2014); Müller et al. (2016); and Hammer et al. (2017). These papers discuss how the accumulation of CO₂ is influenced by organic matter from rivers, saltwater renewal, and atmospheric warming. Consequently, Kuliński and Pempkowiak (2011) and Kuliński et al. (2014) notes how the Baltic Sea has 3–5 times higher organic carbon concentrations than the North Sea.

Melzner et al. (2020) underscores how ocean acidification amplifies coastal hypoxia. Higher concentrations of methane also exacerbate warming and acidification in the Baltic Sea (Gülzow et al., 2011, Reindl and Bolałek 2013). This has serious consequences for marine life that are unable to tolerate the more acidic environment (Hammer et al., 2014, 2017). Moreover, Melzner et al. (2011) emphasise that calcifying organisms, such as blue mussel (Mytilus edulis), will be particularly vulnerable to pH alterations, leading to decreases in abundance and size. This observation is confirmed by Fitzer et al. (2012); Thomsen et al. (2017); Sanders et al. (2018), and Wahl et al. (2018). Graiff et al. (2017) notes how ocean acidification and warming could negatively impact the reproduction of rocky seaweeds, such as Fucus vesiculosus, which are pivotal in forming expansive underwater forests and providing habitat and nurseries for many fish in the Baltic Sea. They find that elevated CO₂ and warming accelerated the spring maturation of Fucus vesiculosus (Graiff et al., 2015, 2017). However, Takolander et al. (2019) observe that CO₂ increases did not affect the growth rates of F. vesiculosus, except for seasonal variations. Additionally, Wahl et al. (2020) reports that ocean warming has a more significant impact than acidification, causing structural and functional shifts in coastal communities of F. vesiculosus.

This destruction of blue mussels and seaweed beds could cascade up the food chain and negatively affect cod, plaice, and flounder who rely on these species as nursery grounds (Seitz et al., 2013). There is also evidence to show that ocean acidification negatively affects cod and herring larvae survival (Frommel et al., 2012, 2014; Stiasny et al., 2016).

Fishing and environmental changes significantly alter the food-web of the Baltic Sea, affecting species composition, population dynamics and inter-species interactions (Tomczak et al., 2012, 2013). Compared with other regions, the brackish Baltic Sea exhibits the distinctive characteristic of low biodiversity, resulting in simpler predator-prey relationships and a higher level of interdependence among the existing species (Ojaveer et al., 2010). There are 44 articles under the food-web dynamics grouping, with 35 articles highlighting the impact of other marine predators and prey on the status of commercial fish.

Bivalves such as clams, oysters and mussels are among the most important taxonomic groups in benthic habitats (Zaiko et al., 2010). Studies by Koivisto and Westerbom (2010); Koivisto and Westerbom (2012); Darr et al. (2014) and Löfstrand et al. (2010) highlight the impact of blue mussels on wider population structures, namely Pleuronectidae that depend on mussels as a food source. Blue mussels also help mitigate eutrophication by filtering the water (Koivisto and Westerbom, 2010, 2012). Westerbom et al. (2019) note how blue mussel abundance is related to salinity, winter severity, wave exposure and depth, with peak densities in high saline areas. Their study highlights how blue mussel biomass has fallen by 80% in the Gulf of Finland since 1998, raising concerns for benthic stocks as the Baltic Sea continues to become less saline.

Studies on the community structures and dynamics of zooplankton and copepods are also crucial in determining the health of herring and sprat, which feed on these organisms (Vehmaa et al., 2013; Otto et al., 2014; Gorokhova et al., 2016; Labuce et al., 2021). Otto et al. (2014) investigates the long-term dynamics of three dominant zooplankton in the central Baltic Sea: Acartia spp., Temora longicornis and Pseudocalanus acuspes. They highlight these species’ behavioural and physiological adaptations over time, including preferred water depths, feeding habits and diel vertical migration patterns. Notably, Otto et al. (2014) highlights how Acartia spp. and Temora longicornis exhibits the highest rates of egg production as temperatures increased up to 18°C. Furthermore, bloom-forming cyanobacteria promote higher copepod reproduction, providing a greater food source for adult herring and sprat (Hogfors et al., 2014; Otto et al., 2014).

Moreover, Ojaveer and Kalejs (2010) and Ojaveer et al. (2018) analyses the stomach contents of herring and sprat, revealing a positive correlation between predators and prey, with the stomach fullness of Clupeidae fish increasing in areas with higher zooplankton. Considering the dietary dynamics of predators targeting commercially valuable fish is also critical. Lundström et al. (2010) observes how grey seals (Halichoerus grypus) predominately prey on herring, followed by sprat in the South, and whitefish in the North, with juveniles consuming small non-commercial species. However, MacKenzie et al. (2011) emphasise that seal predation has a lower impact on cod recovery compared to the effects of environmental drivers and fisheries. Roos et al. (2012) underscores that, despite the increasing number of grey seals, they do not impact stock dynamics. Similarly, Edrén et al. (2010) underscore that harbour porpoises (Phocoena Phocoena) prey on herring, cod and sprat, but their small population does not negatively affect stock dynamics. Other predators, such as the Northern pike (Esox Lucius), prey on smaller fish, but their interactions with sprat and herring are poorly understood (Engstedt et al., 2010; Nilsson et al., 2014; Larsson et al., 2015; Olsson, 2019).

Additionally, Järv et al. (2011) illustrate how the recent introduction of round gobies (Neogobius melanostomusin) in Mugga Bay may negatively affect flounder. They highlight a significant overlap in diet composition between these species, with gobies shown to have a larger diet variability and fuller stomach contents, potentially outcompeting flounder in the area (Järv et al., 2011). Likewise, three-spined stickleback (Gasterosteus aculeatus) populations have increased in many coastal areas of the Baltic Sea due to reduced predation and nutrient enrichment (Olin et al., 2022). Their presence can alter the food-web dynamics as they prey on their larvae (Lefébure et al., 2014; Jakubavičiūtė et al., 2017).

Furthermore, great cormorants (Phalacrocorax carbo) in the Eastern and Northern Baltic Sea are increasing in number, putting pressure on food-web dynamics (Veneranta et al., 2020; Van Eerden et al., 2022). So far, studies on cormorants primarily focus on their consumption of perch (Perca fluviatilis), with Veneranta et al. (2020) highlighting how high cormorant densities can locally reduce perch catches. Van Eerden et al. (2022) note how interactions between cormorants and fisheries are unlikely in the Gulf of Finland, although local effects on perch may exist. Also, moon jellyfish (Aurelia aurita) and lion’s mane jellyfish (Cyanea capillata) serve as competitors and predators for commercially valuable species, as both consume cod larvae and compete with sprat and herring for zooplankton resources (Janßen et al., 2013). Stoltenberg et al. (2021) notes large outbreaks of moon jellyfish overlapping in time with late spawning cod in the Bornholm basin. They conclude that both species heavily prey on Eastern cod eggs, highlighting that cod eggs are found in 49.3% of lion’s mane jellyfish and in 5.5% of moon jellyfish guts (Stoltenberg et al., 2021).

Despite this, Stoltenberg et al. (2021) mentions that adult whiting and herring prey on moon jellyfish, although no studies in the Baltic Sea have recorded this behaviour. In the Western Baltic, the invasive warty comb jelly (Mnemiopsis leidyi) was first recorded in 2006 (Schaber et al., 2011). Although the species cannot sustain itself due to lowering salinity, the comb jelly travels with currents and is often found preying in the spawning grounds of top fish stocks in the Baltic Sea (Jaspers et al., 2011; Schaber et al., 2011). Brulińska et al. (2016) highlights that increased jellyfish outbreaks are caused by climate change and nutrient enrichment and are likely to become more frequent in the Baltic Sea. Therefore, understanding the population dynamics and feeding preferences of these predators could inform broader fisheries management strategies.

Parasitic presence in the alimentary tract of fish pose a universal problem, leading to organ, physiological, behavioural and reproductive damage in many species. This review highlights nine papers on parasitic infections in top commercial fish species. Ryberg et al. (2021) reveal that in the Eastern Baltic, the probability of cod being in a critical condition increased when the parasitic nematode (Contracaecum osculatum) is detected. Horbowy et al. (2016); Podolska et al. (2016) and Polak-Juszczak (2017) shows how infected individuals in the Eastern Baltic cod stock has a 20% lower body condition compared to those free of Contracaecum parasites. This is further reinforced by Mohamed et al. (2020), who illustrates that parasitic presence slows down the growth regulation in cod. Furthermore, adult cod measuring 70-80 cm exhibited the highest prevalence and intensity of infection.

Unger et al. (2014) observe differences in parasite distribution among Central, Western and Gulf of Finland herring stocks, with a decreasing presence towards the East Baltic. Skrzypczak and Rolbiecki (2015) reported a low overall prevalence (3.2%) of parasites in sprat. This is reinforced by Kleinertz et al. (2011) who highlight that the Baltic Sea contains sprat with a lower number of parasite infections compared to the North Sea. Additionally, Kuciński et al. (2023) notes a significant decline in the fitness and catch volume of Pleuronectidae species, particularly on Slupsk Bank, attributing it to prevalent parasitic presence, including Glugea stephani, affecting 42% of investigated flounder. Despite these findings, the impact of parasitic fauna on commercial fish species in the Baltic Sea remains unclear.

The Gadidae family, including whiting (M. merlangus) and the Atlantic cod (G. morhua), is extensively researched in the Baltic Sea. Cod, divided into Eastern and Western stocks, have seen a steep population decline in the last four decades, leading to the subsequent closure of targeted cod fisheries in 2019 (ICES, 2019). Scientists have been intensely studying the species to understand the reasons for this decline, with 41 papers highlighted in this review. Stiasny et al. (2016) estimates Western cod larvae mortality under ocean acidification, showing a recruitment reduction by 8%. Frommel et al. (2012) demonstrates how exposure to increased CO₂ can lead to lethal tissue damage in many internal organs of cod larvae, contributing to early-life mortality. Hüssy (2011) emphasises how salinity and oxygen play a key role in cod egg buoyancy, influencing survival success in oxygen-poor depths. Petereit et al. (2014) observes that cod larvae have the highest chances of survival in April and May, but no cohorts that drifted into lower saline and oxygen levels in the Central Baltic Sea survived. Similar results are published by Ljunggren et al. (2010); Margonski et al. (2010); Ojaveer et al. (2011); Hinrichsen et al. (2012); Maneja et al. (2013), and Voss et al. (2012).

Mion et al. (2018) notes how the disappearance of larger female individuals also reduced the spawning capacity for cod because larger and older individuals produce a higher number of eggs. Consequently, the remaining smaller females are producing fewer eggs, limiting their ability to repopulate. ICES (2022a) cite reduced spawning capacity as one of the key reasons that led to a population decline in cod stocks, prompting a targeted ban on cod fisheries in 2019.

Research on population distribution, feeding and growth of commercially valuable fish stocks has also been extensive. Neuenfeldt et al. (2020) identifies factors behind the reduction in cod size over the last two decades, with decreased food availability leading to higher densities of smaller cod competing for scarce resources, stunting their growth. Pachur and Horbowy (2013) reinforced this by studying Eastern cod catches in the Polish Exclusive Economic Zone and noting shifts in their diet composition since the 1980s, with sprat replacing herring as their primary food source. Gårdmark et al. (2015) also underscores the changes in cod’s diet, emphasising the consequential impacts on the spatial-temporal dynamics of other structured populations. Casini et al. (2016) investigates the body conditions of cod, revealing how alterations are linked with food limitations. Furthermore, Casini et al. (Casini et al., 2012, 2016) demonstrates how hypoxic areas further deteriorate cod’s health through mechanisms related to physiology, behaviour and trophic interactions.

Moreover, Schaber et al. (2012) study cod distribution patterns in the Bornholm Basin and observe that both salinity and oxygen concentration are key parameters affecting cod distribution. Consequently, increased hypoxia and eutrophication forced cod out of their previously favoured habitats. Similarly, Nielsen et al. (2013) observe changes in cod abundance linked with variations in the environment, noting differences in juvenile cod behaviour compared to their North Sea counterparts, as Baltic Sea cod did not form dense schooling patterns. Other notable cod-related studies that look at the age structure and spatial-temporal dynamics correspondingly suggest that there are fewer large adults (Hüssy et al., 2010a, 2010b; Mackenzie and Gislason, 2011; Hüssy et al., 2012; Kristensen et al., 2014, Nielsen et al., 2014, Orio et al., 2017a, 2019). As cod biomass decreased, sprat and herring emerged as key species in the Baltic Sea. Tomczak et al. (Tomczak et al., 2012, 2013) proposed that the decline of cod resulted in trophic cascades affecting Clupeidae fish, who lost their primary predator.

Whiting, although commercially fished in the Western Baltic Sea, holds lesser economic value compared to cod. Reflecting its less prominent status, there is a scarcity of published research on whiting, with two papers highlighted in this review from the same leading author. These papers examine the diet of whiting in the Western Baltic Sea, finding that clupeids making up 90% of adult diets, while gobies, brown shrimps and polychaetes as primary prey for juvenile whiting (Ross and Hüssy, 2013; Ross et al., 2016). The authors emphasise whiting’s role as a top predator in the Western Baltic Sea, stressing the importance of investigating its ecology and population dynamics. Comprehensive population assessments of whiting have only been conducted in the North Sea, and not much is known about the impact of environmental divers on their population.

Herring and sprat are key marine fish species with the highest commercial value in the Baltic Sea. ICES began managing Baltic herring in the 1970s, assessing them in four stocks: Bothnian, Western, Central and Riga herring. Their status varies, with Bothnian and Central herring SSB stabilising over the last six years, while Riga herring populations continue to grow. In contrast, Western herring faces collapse, having experienced substantial SSB declines over the last decade (ICES, 2022b-e). Extensive research is carried out to closely monitor herring, with 22 papers highlighted in this review.

Raid et al. (2010) examine the recruitment dynamics of herring populations in the Gulf of Riga, revealing extensive geographical variability, with all 12 local populations adapting to the highly variable environmental conditions in the region, allowing stocks to grow. Atmore et al. (2022) shows varied spawning success across different stocks, with autumn spawners making up 90% of Riga herring landings. However, MacKenzie and Ojaveer (2018) underscores how, despite high reproductive rates and rapid growth, herring can still undergo drastic population declines over time. Dodson et al. (2018) find that the optimal temperatures for healthy yolk-sac herring are between 9°C and 13°C, with rising temperatures resulting in increasing larval deformities. Polte et al. (2021) notes how Western Baltic herring has reduced reproductive success during warmer winters, reducing the herring reproduction index by 3% per day. Frommel et al. (2014) also shows that exposure to increased CO₂ can cause lethal tissue damage and increased mortality in herring larvae.

Herring stocks in SD 25-32 are more abundant, but this does not necessarily indicate a healthy population. Dziaduch (2011) finds that out of the 1,615 analysed herring, no stomach was full of food. On the contrary, in the Bornholm Basin, 90% of all herring had empty stomachs due to changes in prey composition (zooplankton) and increased competition with sprat (Dziaduch, 2011). Livdane et al. (2016) reinforces this point by highlighting zooplankton availability as a major factor affecting herring fitness. Additionally, Casini et al. (2010) demonstrates that herring growth is positively influenced by the abundance of sprat, resulting in lower growth in areas with high sprat density. Dippner et al. (2019) observe a decreasing trend of 3-year-old central Baltic herring, with their mean weight dropping from 50-70 g in the 1970s to 25-30 g today, attributing this to increased precipitation, habitat reduction and changes in the prey composition.

Atmore et al. (2022) highlight the vulnerability of herring stocks to overfishing, nutrient enrichment, climate change, low salinity and competition from expanding sprat populations. Soerensen et al. (2022) observe a decline in selenium levels in Baltic herring, linking this to external source loads and changing atmospheric emissions. Miethe et al. (2014) notes that environmental conditions, specifically marine inflow events, strongly influence the distribution patterns of Western Baltic spring-spawning herring during autumn migrations. Rajasilta et al. (2019) attributes declining lipid content of Baltic herring and stock size to decreasing salinity. These studies underscore the significance of environmental factors in shaping herring fitness, abundance and distribution in the Baltic Sea.

Sprat is the smallest fish species in the Clupeidae family. There is less research published on sprat compared to herring, with 10 studies highlighted in this review. Sprat is monitored as a single stock covering SD 22-32, currently the largest stock in the Baltic Sea with the largest TAC quotas (ICES, 2022h). Voss et al. (2011b) observe significant population fluctuations in sprat due to rising temperatures, enhancing recruitment success during their early life stages. In addition, higher temperatures lead to faster larval development, resulting in decreased egg mortality as their most vulnerable life stage passes more quickly. MacKenzie et al. (2012) identifies the greatest positive impact of rising temperatures on sprat. Using a climate-ocean-fish population model, they projected that sprat spawning biomass could increase to 1.5 million tonnes with a 2-5°C sea temperature rise (MacKenzie et al., 2012). Therefore, sprat is among the few commercial fish species that could benefit from the effects of ocean warming.

Sprat experiences varying levels of success depending on the region, with 75% of the total sprat stock caught in the Northern Baltic and the Gulf of Finland (Casini et al., 2011). Kulke et al. (2018) find higher mean stomach contents in sprat from the Bornholm Basin compared with the Aroka and Gotland Basin, with a preferred feeding depth of 26 m, where light intensity is higher and plankton is visible. Ojaveer et al. (2010) highlights how the most stable environment for sprat is in the southwestern region, attributing freshwater discharge variations as the main factor affecting their abundance. During periods of high river runoff, sprat habitat expands and their stocks increase (Ojaveer et al., 2010). Voss et al. (2012) observe that the survival time of sprat larvae increased by an additional four days (~45%), indicating a reproductive advantage under higher rainfall.

Correspondingly, Eero (2012) notes how sprat has positively reacted to the changing environment as well as reduced cod populations and is outperforming herring in many categories. For example, Ojaveer et al. (2018) find that sprat have fewer empty stomachs compared to herring, indicating that they are more successful at finding and consuming zooplankton. Similar conclusions are made by Margonski et al. (2010); Casini et al. (2011); Heikinheimo (2011) and Tomczak et al. (2012), who study the relationships between cod, herring and sprat, noting how sprat shows the best adaptation in more hostile environments.

Since the decline of cod, Pleuronectidae have become one of the most economically important species for small-scale fisheries in the Central Baltic Sea (Dabrowska et al., 2017). Among these, the European flounder (P. flesus) is the focus of most scientific studies. The European plaice (P. platessa) is the only Pleuronectidae species with imposed TAC limits, which have increased annually due to high recruitment. ICES (ICES, 2022f, g) monitors plaice as two stocks, in SD 21-23 and 24–32. Meanwhile, the CFP monitors plaice as a single unit (European Commission, 2022). Flounder has no catch restrictions due to its perceived healthy status with 17 studies highlighted in this review.

Haase et al. (2020) observe that flounder populations have increased in the past two decades, hypothesising that they have deprived cod of benthic food resources. Orio et al. (2017a) illustrates how both flounder and cod have migrated to shallower depths because of expanded hypoxia in deep waters, leading to increased competition. This has resulted in a general decline in size and weight for both species, with smaller individuals progressively dominating the demersal ecosystem (Orio et al., 2017a). Flounders in the Baltic Sea exhibit two reproductive behaviours: pelagic and demersal. Orio et al. (2017b) notes that pelagic spawning flounder negatively react to rising temperatures, leading to a reduction in their spawning areas in the Bornholm Basin despite favourable salinity levels. Further studies by Orios et al. (Orio et al., 2019, 2020) show additional spatial contraction of cod and flounder populations in the last 40 years due to significant environmental and hydrological changes.

Momigliano et al. (2018) attributes poor genetic diversity as a major threat to flounder adaptability, using DNA samples, they show how reduced genetic diversity in the Åland Sea and the Gulf of Finland made flounder more vulnerable to changing environmental conditions (Momigliano et al., 2019). Jokinen et al. (2019) supports this, revealing that in 1983, the fishery unknowingly targeted a single gene pool of flounder, depleting the genetic diversity of those stocks. Jokinen et al. (2016) emphasises that shallow habitats for juvenile Pleuronectidae in the North Baltic Sea are most vulnerable to coastal eutrophication, showing a negative correlation between juvenile flounder abundance and vegetation density. They also reported a 40-time decline in juvenile flounder densities since the 1980s.

Furthermore, Nissling and Dahlman (2010) notes how low salinity negatively affects the ability of flounder to reproduce in SD 25, 27, 28 and 29. Ustups et al. (2013) reinforced this by showing that flounder’s recruitment success depends on producing eggs that float in low-salinity waters. Borg et al. (2014) highlights significant year-to-year variations in flounder recruitment and distribution, with differences in diet and habitat preferences between males and females. Florin and Lavados (2010) and Momigliano et al. (2018) notes how flounder in the Baltic Sea display contrasting reproductive behaviours and diet preferences, which affect their resilience in different locations.

Despite the TAC allowance and its regional significance, plaice studies in the Baltic Sea are limited, with two highlighted in this review. Ulrich et al. (2013) examine the variability and connectivity of plaice populations from the Eastern North Sea and Western Baltic Sea. They find that plaice in Skagerrak, Kattegat, the Belts and the Sound Baltic Sea regions are closely linked with plaice in the North Sea (Ulrich et al., 2013). Spawning in the Kattegat occurs between February and March at depths of 30-40 m, with temperatures around 4°C (Ulrich et al., 2013). Many eggs and larvae settling in the shallow waters of Skagerrak are believed to originate from the North Sea. This drift is reinforced in winter when stronger winds prevail. Free-floating eggs are also found in the deeper basins of the South Baltic Sea, with spermatozoa and eggs well adapted to low salinity conditions, allowing plaice to thrive there (Ulrich et al., 2013).

Ulrich et al. (2013) find that plaice stocks have strong genetic diversity, indicating a healthy population. However, tagging data reveals limited movements for both adults and juveniles in the Belts, Western Baltic, Kattegat and the North Sea, with up to 90% of plaice recaptured in the same area, indicating non-migratory behaviours that make them more susceptible to habitat loss. Cardinale et al. (2010) reports a 60% reduction in adult plaice biomass and a 10 cm decrease in average maximum length in the Kattegat region since the 1960s. Their study linked these declines to increased fishing mortality. However, a recent ICES (2022f) report shows a positive shift, with relative fishing mortality declining, suggesting improving trends for plaice stocks.