Many fish species are a human food source, but fish are also prey for marine mammals and sea birds. Fish themselves feed on benthic species, zooplankton, and smaller fish, and are thereby a link between different parts of the food web. When migrating, they also have an ecological role in connecting different areas of the sea. The assessment of fish from a biodiversity perspective indicates good status for coastal fish in about half of the assessed areas. The migrating species salmon and sea trout show overall not good status. In the open sea, three out of eight currently assessed commercial stocks show good status.
Coastal and open sea areas are characterised by different species groups, and there are also clear differences in species composition among sub-basins due to the gradient in salinity. About 230 fish species are recorded in the Baltic Sea (HELCOM 2012).
Marine species are the most common in the southwest and in open sea areas. Coastal areas are key habitats for freshwater species, such as perch and cyprinids, as well as providing spawning and feeding areas for many marine species, such as cod, flounder, and herring. Most of the migrating species, including salmon, sea trout, sea lamprey and some populations of whitefish, are born and spawn in rivers but spend most of their growth phase in the Baltic Sea. The eel of the Baltic Sea is a highly migrant species and belongs to the same population as all other European eels (Box 5.3.1).
Indicators included in the assessment
The integrated assessment of coastal areas includes core indicators representing characteristic Baltic Sea coastal fish species, the ‘Abundance of key coastal fish species’ and ‘Abundance of key coastal fish functional groups’ (Core indicator reports: HELCOM 2017x-y).
The open sea assessment was based on results for internationally assessed commercial fish stocks, using information on spawning stock biomass and fishing mortality based on ICES (2016a; see Chapter 4.6 Species removal by fishing and hunting for detailed assessment results for commercial fish).
The migrating species salmon and sea trout are assessed by core indicators (HELCOM 2015c, 2017z), but were not included in the integrated assessment at this time, due to inconsistencies in the input data. HELCOM work is ongoing to develop indicators to represent the demographic characteristics of fish communities (for example size distribution) as an important complement to the assessment in the future.
Integrated status assessment of fish
The integrated status of coastal fish was good in about half of the twenty-one assessed coastal areas (Figure 5.3.1). Differences among the areas likely reflected the influence of local factors on reproduction, growth and mortality. The assessment covered around 75% of the coastal area of the region, but the density of monitoring sites within each assessment unit was low.
The integrated status in the open sea was assessed as not good for both pelagic and demersal fish (Figure 5.3.1). Demersal fish were only included for the southern Baltic Sea. Separate results for each fish stock are given in Table 4.6.1, and are also described further below. Assessment results for additional stocks, including also demersal fish in the eastern parts of the Baltic Sea, are foreseen to be included by the end of 2017.
|ICES SD||BQR score|
The core indicator ‘Abundance of key coastal fish species’ is based upon changes over time in perch (Perca fluviatilis) or flounder (Platichtys flesus), with the species chosen depending on the natural distribution of these species. Perch is assessed in the eastern and northern coastal areas, and flounder in the southeast. Thirteen out of twenty-one assessed areas achieved the threshold value (Figure 5.3.2, Core indicator report: HELCOM 2017x).
he core indicator ‘Abundance of key coastal fish functional groups’ combines information on two aspects of the food web: the abundance of predatory fish and of fish feeding at lower trophic levels. The indicator is only assessed in the eastern and northern coastal areas.
Low values in the component on predatory fish indicates disturbed food webs. Fishing is one key pressure potentially influencing the indicator, but it may also be influenced by pressures affecting recruitment and growth, for example (HELCOM 2017z). This component achieved the threshold value in thirteen of sixteen assessed areas (Figure 5.3.2).
The lower trophic level component was most often measured as abundance of fish from the taxonomic family cyprinids, for which high values are associated with eutrophication. Cyprinids do not occur naturally in more saline areas, and in those cases total abundances of coastal lower trophic level fish species are used. This component achieved the threshold value in seven of sixteen assessed areas (Figure 5.3.2, Core indicator report: HELCOM 2017y).
Overall, a continuously deteriorating status has predominated during the past three decades according to trends in both cyprinids and coastal predatory fish, and a slight increase in the share of areas with improving status has only been seen during the years of the current assessment period (Bergström et al. 2016).
Salmon (Salmo salar) and sea trout (Salmo trutta) spend the first few years of their life cycle in the river as parr. After this, they become smolt and start their feeding migration to the sea. Many Baltic rivers have lost their original wild salmon populations due to damming of rivers for hydropower and dredging. The species are also affected by targeted fishing as well as by being incidental by-catch in fisheries targeting other species.
The core indicator ‘Abundance of salmon spawners and smolt’ was assessed for the Gulf of Bothnia, Gulf of Finland, Gulf or Riga, and the Gotland Basin, indicating not good status in all these areas except for the Northern Quark (Core indicator report: HELCOM 2017z). Assessment results are lacking for many assessment units (Figure 5.3.2).
The core indicator ‘Abundance of sea trout spawners and parr’ was last updated in 2014. At that time, the indicators showed not good status in most of the assessed area, except for the most western parts of the Baltic Sea (Core indicator report: HELCOM 2015c).
The restoration of river habitats and management of river fisheries to strengthen Baltic Sea salmon and sea trout is a BSAP regional commitment (HELCOM 2011, update of reporting is currently ongoing).
The internationally assessed commercial fish in the Baltic Sea encompass twenty-two fish stocks, representing twelve species. The stocks are assessed in relation to the objective of the fisheries management; that the spawning stock biomass and the fishing mortality should be kept at levels that are consistent with long term sustainability (see Chapter 4.6 Species removal by fishing and hunting).
Out of the assessed stocks (assessed for the years 2011–2015) five were in not good status, three showed good status, and fourteen lacked assessment results (Figure 5.3.3). The demersal sole (Solea solea), Western Baltic cod (Gadus morhua), as well as herring spring spawners in the Western Baltic and Kattegat (Clupea harengus), did not achieve good status with respect to spawning stock biomass. The Gulf of Riga herring stock and sprat in the Baltic Sea failed good status with respect to fishing mortality (See also Table 4.6.1 in Species removal by fishing and hunting).
Eastern Baltic cod was not assessed due to lack of quantitative biomass estimates and reference points in later years, but survey data indicate that its biomass has been reduced since a peak in 2010–2011 and has reached a stable lower level during the period from 2013 to 2016.
The long term development in spawning stock biomass of some of the main stocks in the Baltic Sea, sprat, central Baltic Sea herring and Western Baltic cod, are shown in figure 5.3.4.
Figure 5.3.4. Temporal development in the spawning stock biomass of sprat and central Baltic Sea herring (1974–2015; upper graph) and of Western Baltic cod (1994–2015; lower graph), based on data from stock assessment models (ICES 2016a). Sprat covers ICES subdivisions 22–32, central Baltic Sea herring covers 25–29 and 32, while Western Baltic cod covers 22–24.
Size structure of fish
In addition to abundance and biomass, changes in individual size and condition of fish are important measures of the overall status of fish populations. The proportion of larger individuals of Eastern Baltic cod has declined sharply since 2013, and the condition factor of Eastern Baltic cod shows a declining trend (Figure 5.3.5).
There are many potential reasons for the decline, including changes in fishing patterns, natural mortality, and reduction in growth, for example, but so far no conclusive explanation has been identified. The declining condition of Eastern Baltic cod has also been related to changes in feeding opportunities and the spread of hypoxic areas in the Baltic Sea, and possibly other factors such as increased parasite infestation coupled to increased abundance of grey seals and fisheries selectivity (Eero et al. 2015, Casini et al. 2016).
For pelagic fish, the condition and mean weight declined substantially in the 1990s to a stable lower level (Casini et al. 2011).
Red-listed species of fish and lamprey
Fourteen species of fish and lampreys have been evaluated as threatened according to the HELCOM Red List (HELCOM 2013b). The American Atlantic sturgeon (Acipenser oxyrinchus) which used to be common in the Kattegat and more rarely occurring in the Sound is considered regionally extinct.
The list of critically endangered species includes the European eel (Box 5.3.1), as well as grayling (Thymallus thymallus) in coastal areas of the Bothnian Sea. The sharks porbeagle (Lamna nasus) and spurdog (Squalus acanthias) in the Kattegat are also listed in this category, likely reflecting impacts of pressures occurring outside of the Baltic Sea region to a large extent, as the species are represented by populations that are widely distributed in the Northeast Atlantic.
The list has three further species listed as endangered and seven as vulnerable, including sea lamprey (Petromyzon marinus). Further, all shark and ray species in the Kattegat and western Baltic Sea are included on the red list. As they are at the border of their distribution in the Kattegat, the status of the shark and ray stock and their return to this area is also dependent on management outside of the HELCOM region.
Impacts and potential future changes
The status of fish is potentially affected by several pressures in the ecosystem. Where overfishing occurs, this is typically connected with reduced fish population sizes. Further, targeted fishing on certain species and size classes often leads to a shortage of large predatory fish, and an overrepresentation of smaller fish and fish of lower trophic levels (Pauly et al. 1998). Fish are also strongly affected by climate change, as well as many other factors, such as eutrophication, habitat loss and disturbance. Climate change affects fish directly, with effects on recruitment and growth. It also influences the distribution range of species, as well as prey availability and species interactions (MacKenzie et al. 2007).
In coastal areas and river mouths, a gradual but continued deterioration of essential recruitment habitats is a concern, as these often coincide with areas that are attractive for coastal development and construction, and habitat quality is also affected by eutrophication (Seitz et al. 2014). In the open sea, the most important spawning area for Eastern Baltic cod (currently), the Bornholm Basin, is only a fraction of its historical area due to increasing oxygen deficiency. The Gdansk Basin and the Gotland Basin have had a very limited contribution to cod recruitment since the 1990s (Köster et al. 2017).
Climate change is likely to increase in importance over time, by affecting the physiology of fish and the availability of zooplankton, which fish depend on during their early life stages. A foreseen increased temperature and decreased salinity would also affect how fish species are distributed within the Baltic Sea, so that marine species will be disadvantaged and habitats of freshwater species will likely expand.