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 and migrating fish in about half of the evaluated assessment units. Three out of eight currently assessed commercial stocks show good status. The status of eel continues to be critical.
Coastal and open sea areas are characterized by different species of fish, and there are also clear differences in species composition among sub-basins due to the salinity differences. About 230 fish species are recorded in the Baltic Sea (HELCOM 2012a).
Marine species are the most common in the southwestern Baltic Sea and in open sea areas. Coastal areas are the key habitats for freshwater species, such as perch (Perca fluviatilis) and cyprinids (Cyprinidae), and are also spawning and feeding areas for many marine species, such as cod (Gadus morhua), flounder (Platichtys flesus), and herring (Clupea harengus). The anadromous migrating species, such as salmon and sea trout (Salmo salar, Salmo trutta), but also sea lamprey (Lampetra fluviatilis) and some populations of whitefish (Coregoniidae), are born and spawn in rivers but spend most of their growth phase in the Baltic Sea. The European eel (Anguilla anguilla) is a diadromous migrating species spawning in the Sargasso Sea, with Baltic Sea eel being part of the same population as all other European eels.
Indicators for assessing fish
The integrated assessment of fish in coastal areas included core indicators representing characteristic Baltic Sea coastal fish species and functional groups (Core indicator reports: HELCOM 2018aj-ak).
- The ‘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 used in the eastern and northern coastal areas, and flounder in the south. Good status is achieved when the abundance is above a site-specific threshold value (HELCOM 2018aj).
- ‘Abundance of coastal fish key functional groups’ evaluates the abundance of selected functional groups of coastal fish in the Baltic Sea: piscivores and a lower trophic level component (cyprinids/mesopredators). Low values in the core indicator component on ‘piscivores’ indicates disturbed food webs. The ‘lower trophic level’ component is most often measured as the abundance of fish from the taxonomic family cyprinids, for which high values are associated with eutrophication. Good status is achieved when the abundance of piscivores is above a site-specific threshold value, and the abundance of cyprinids or mesopredators is within an acceptable range for the specific site (HELCOM 2018ak).
The open sea assessment was based on results for internationally assessed commercial fish stocks, using information on spawning stock biomass and fishing mortality from ICES (2017a-b). Data for cod (Gadus morhua), sole (Solea solea), plaice (Pleuronectes platessa), herring (Clupea harengus) and sprat (Sprattus sprattus) were included in the integrated assessment, as these were the ones for which assessment results in relation to both spawning stock biomass and fishing mortality were available (For more information on the indicators and reference points to define good status for open sea fish, see Figure 4.6.2 in Chapter 4.6 Species removal by fishing and hunting).
Further, the two core indicators on migrating fish, ‘Abundance of salmon spawners and smolt’ and ‘Abundance of sea trout spawners and parr’ represent species which migrate between freshwater and sea areas: salmon (Salmo salar) and sea trout (Salmo trutta; see also Box 1.2 in Chapter 1 Our Baltic Sea).
- ‘Abundance of salmon spawners and smolt’ is based on the production of smolt in rivers with wild salmon stocks. It is applicable in all HELCOM countries except Denmark, Germany, Poland and Russia. The estimated smolt production is compared to an estimated potential smolt production capacity of the rivers, with the threshold value defined as 75 % of the production capacity (HELCOM 2018al).
- The indicator ‘Abundance of sea trout spawners and parr’ is based on a comparison of the observed parr densities in rearing habitats with reference potential parr densities in the specified habitats. The indicator is applicable in all HELCOM countries. Good status is achieved when the moving parr densities average over 4-5 years remains above 50 % of the reference parr density (HELCOM 2018am).
The core indicators on salmon and sea trout were not included in the integrated assessment of fish. The endangered European eel (Anguilla anguilla) was assessed descriptively.
All assessed fish indicators focus on aspects relating to the abundance or biomass of fish. 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. A summary on the size structure and key species in the open sea is provided descriptively.
Since the biodiversity assessment includes all fish species in the Baltic Sea area covered by operational indicators and for which data was available, the total list of assessed species differs from that assessed under the assessment of commercial fishing as a pressure (Chapter 4.6 Species removal by fishing and hunting).
Integrated assessment results for fish
The integrated status of fish is generally not good, although with some exceptions.
The status of commercial fish in the open sea is assessed as good in the Bothnian Bay, where only herring is included (Figure 5.3.1). In the other open sea sub-basins, the integrated results reflect a deteriorated status of cod (Gadus morhua), and in some cases also of sprat or herring (Sprattus sprattus, Clupea harengus). The group of demersal fish is only represented by cod and does not show good status in any sub-basin where it is included. The group of pelagic fish is below good status west of Bornholm, in the Bothnian Sea or Gulf of Riga. Results for the different stocks are shown in more detail in Chapter 4.6 Species removal by fishing and hunting.
The integrated status of coastal fish is good in about half of the twenty-one assessed coastal areas. The assessment covers around 75 % of the Baltic Sea coastal areas, but the density of monitoring sites within each assessment unit is low.
At core indicator level, ‘Abundance of key coastal fish species’ shows good status in 13 out of 21 assessed coastal areas. For the core indicator ‘Abundance of key coastal fish functional groups’, the component addressing piscivores achieves the threshold value in most of the assessed coastal areas (13 out of 16), and the group cyprinids/mesopredators achieves the threshold valued in about half of them (7 out of 16; Figure 5.3.2).
Low abundance of predatory fish indicates disturbed food webs. Fishing is one key pressure influencing the indicator, but it may also be affected by changes in pressures affecting recruitment and growth, and may for example benefit from increasing temperatures (HELCOM 2018g). The lower trophic level component is in most cases evaluated based on the abundance of fish within the taxonomic family cyprinids (Cyprinidae), for which high abundances are associated with eutrophication. Cyprinids do not occur naturally in more saline areas, and in those cases, the total abundances of coastal lower trophic level fish species is evaluated.
Over a longer time perspective, a continuously deteriorating status has predominated in both cyprinids and coastal predatory fish during the past three decades, and a slight increase in the share of coastal areas with improving status is seen only 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. The two core indicators ‘Abundance of salmon spawners and smolt’ and ‘Abundance of sea trout spawners and parr’ show different results in different parts of the Baltic Sea (Figure 5.3.3).
The salmon indicator shows good status in the Gulf of Bothnia and the Western Gotland Basin, but not in the Eastern Gotland Basin or the Gulf of Finland. The indicator is not applicable south of the Gotland basins.
The sea trout indicator, on the other hand, shows good status in the southernmost basins that were included, but not in the Gulf of Bothnia, and shows varying statuses in the Baltic Proper. Overall, the seatrout indicator achieves the threshold value in 60 % of the 31 assessment units included in the evaluations. It is estimated that sea trout reproduces in 720 rivers or brooks around the Baltic Sea. About 90 % of these consist fully of wild populations whereas 10 % are mixed rivers where the population is enhanced by stocking.
For both species, there is an additional number of rivers around the Baltic Sea which have lost their salmon and sea trout populations due to damming of rivers for hydropower, or because of dredging. The number of currently unsuitable rivers for salmon and trout reproduction is not reflected in the indicators. Both species are also affected by targeted fishing as well as by being incidental by-catch in other types of fisheries. The restoration of river habitats and management of river fisheries to strengthen Baltic Sea salmon and sea trout is a regional commitment of the Baltic Sea Action Plan.
Internationally assessed commercial fish in the Baltic Sea encompass seventeen demersal and pelagic fishstocks,representing nine species. The stocks were assessed in relation to the objective that both the spawning stock biomass and the fishing mortality should be at levels that are consistent with long term sustainability.
Six of the assessed stocks do not show good status, and three show good status on average during 2011-2016. Eight stocks lack assessment results (Figure 5.3.4). Plaice (Pleuronectes platessa) in the Kattegat is the only demersal stock achieving good status. Its spawning stock biomass has shown an increasing trend over the past decade (Figure 5.3.5). Sole (Solea solea), as well as Western and Eastern Baltic cod (Gadus morhua), does not achieve good status.
Among pelagic stocks, sprat (Sprattus sprattus), herring (Clupea harengus) in the Gulf of Riga, and herring spring spawners in the Western Baltic and Kattegat do not achieve good status. These stocks fail the reference value with respect to fishing mortality, and the herring spring spawners also show too low stock size. Their spawning stock biomasses have been at relatively constant levels over the past decade. The herring stocks of the Gulf of Bothnia and Central Baltic Sea show good status, and increasing spawning stock biomass over the past decade (Figure 5.3.5).
Changes in the size and condition of individual fish are important measures of the overall status of fish populations, in addition to monitoring aspects of abundance or biomass.
Most noticeably in the Baltic Sea, the condition and proportion of larger individuals of Eastern Baltic cod is continuously declining, and the latter has decreased sharply in particular since 2013 (Figure 5.3.6). The condition and mean weight of pelagic fish declined substantially in the 1990s, after which it has remained at a lower level (Casini et al. 2011).
There are many potential reasons for the declines, but so far no conclusive explanation has been identified. A deteriorated size structure has, for example, been attributed to changes in fishing patterns, predation by other species, or a reduced growth rate. The declining condition of Eastern Baltic cod has also been related to changes in feeding opportunities and the spread of areas with poor oxygen conditions in the Baltic Sea, and possibly to factors such as increased parasite infestation, attributed to increased abundance of grey seals, or fisheries selectivity (Eero et al. 2015, Casini et al. 2016).
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 the impact 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.
Further, three fish species are listed as endangered and seven as vulnerable, including sea lamprey (Petromyzon marinus). All shark and ray species in the Kattegat and western Baltic Sea are included in the HELCOM 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.
The status of fish is influenced by several currently acting pressures and ongoing changes in the ecosystem. Overfishing is a main pressure connected with reduced population sizes. Further, fishing targeting certain species and size classes is often connected to a shortage of large predatory fish, and an overrepresentation of smaller fish and fish of lower trophic levels (Pauly et al. 1998). Such effects are also seen in the Baltic Sea, and are likely to influence the long term ecosystem resilience and food web productivity (Svedäng and Hornborg 2017).
Other pressures affecting fish include eutrophication (causing indirect effects on habitat quality and feeding opportunities) and physical alteration of habitats (causing impacts on recruitment, spawning and feeding areas).
A gradual but continued deterioration is a particular concern in shallow coastal areas and river mouths, as desirable areas for development and construction often coincide with important areas for recruitment (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 a very limited contribution to cod recruitment since the 1990s (Köster et al. 2017).
In addition, climate change is expected to have an increasing influence in the future. Climate change can cause changes to fish directly, by effects on recruitment success and growth, or it may influence the distribution range of species, prey availability or other ecological interactions (MacKenzie et al. 2007). For example, changes in temperature and seasonality may affect the reproductive season for fish, or the availability of zooplankton during critical life stages when fish are dependent on these for food. Any decreases in salinity would likely have a strong effect on the open sea fish community in the Baltic Sea, if marine species are disadvantaged and habitats suitable for freshwater species expand.