The status of biodiversity is assessed using several core indicators. Each indicator focuses on one important aspect of the complex issue. In addition to providing an indicator-based evaluation of the abundance of key coastal fish species, this indicator also contributes to the overall biodiversity assessment along with the other biodiversity core indicators.
The core indicator on abundance of coastal fish key species addresses the Baltic Sea Action Plan's (BSAP) Biodiversity and nature conservation segment's ecological objectives 'Natural distribution and occurrence of plants and animals', 'Thriving and balanced communities of plants and animals' and 'Viable population of species'.
The core indicator is relevant to the following specific BSAP actions:
The core indicator also addresses the following qualitative descriptors of the MSFD for determining good environmental status:
Descriptor 1: 'Biological diversity is maintained. The quality and occurrence of habitats and the distribution and abundance of species are in line with prevailing physiographic, geographic and climatic conditions'
Descriptor 3: 'Populations of commercially exploited fish and shellfish are within safe biological limits, exhibiting a population age and size distribution that is indicative of a healthy stock'
and the following criteria of the Commission Decision:
In some Contracting Parties the indicator also has potential relevance for implementation of the EU Habitats Directive.
Coastal fish, especially piscivorous species, are recognized as being important components of coastal food webs and ecosystem functioning (Eriksson et al. 2009; Baden et al. 2012; Olsson et al. 2012; Östman et al. 2016). Moreover, since many coastal fish species are rather local in their appearance (Saulamo & Neuman 2005; Laikre et al. 2005; Olsson et al. 2011; Östman et al. 2017a), the temporal development of coastal fish communities might reflect the general environmental state in the monitoring locations (Bergström et al. 2016b).
Key fish species in coastal ecosystems generally have a structuring role in the ecosystem, mainly via top-down control on lower trophic levels. Also, viable populations of key coastal fish species are generally considered to reflect an environmental status with few eutrophication symptoms and balanced food webs (Eriksson et al. 2011; Baden et al. 2012; Östman et al. 2016). Key coastal fish species are generally piscivores and/or benthivores.
Biological - Extraction of, or mortality/injury to, wild species (e.g. selective extraction of species, including incidental non-target catches)
- Disturbance of species (e.g. where they breed, rest and feed) due to human presence
- Physical disturbance to seabed (temporary or reversible)
- Changes to hydrological conditions Substances, litter and energy
- Inputs of nutrients – diffuse sources, point sources, atmospheric deposition
Substances, litter and energy
- Input of other substances (e.g. synthetic substances, non-synthetic substances, radionuclides)
- Input or spread of non-indigenous species
The state of key coastal fish species in the Baltic Sea is influenced by multiple pressures, including climate, eutrophication, fishing mortality and exploitation of essential habitats, but also by natural processes such as food web interactions and predation from apex predators.
Climate change generally has a large effect on the species considered here (Möllman et al. 2009; Olsson et al. 2012; Östman et al. 2017b) as have alterations in the food web (Eriksson et al. 2009; 2011; Östman et al. 2016). Stressors related to human activities, mainly exploitation of essential habitats (Sundblad et al. 2014; Sundblad & Bergström 2014; Kraufvelin et al. 2018) and fishing (Edgren 2005; Bergström et al. 2007; Fenberg et al. 2012; Florin et al. 2013) also impact the state of coastal fish species. For obligate coastal species such as perch, the outtake comes from both the recreational and small-scale commercial fisheries sector and in some countries to a larger extent in the former (HELCOM 2015b), whereas cod and flounder are exploited both in the offshore and coastal commercial fishery. In some areas of the Baltic Sea, flounder and cod is also targeted by recreational fisheries.
The effect of eutrophication on the state of coastal fish species is also of importance (Bergström et al. 2016b) and might increase with higher latitudes (Östman et al. 2017b).
The abundance of key species of coastal fish (such as perch and flounder) is influenced by recruitment success and mortality rates, which in turn might be influenced by ecosystem changes, interactions within the coastal ecosystem and abiotic perturbations. An increased abundance of perch may, for example, be governed by increasing water temperatures, moderate eutrophication, availability of recruitment habitats, low fishing pressure and low predation pressure from apex predators (Böhling et al. 1991; Edgren 2005; Bergström et al. 2007; Linlokken et al. 2008; HELCOM 2012; Olsson et al. 2012; Östman et al. 2012; Bergström et al. 2016b; Östman et al. 2017b). As for the majority of coastal species, exploitation of recruitment areas has a negative impact on the development of perch populations (Sundblad et al. 2014; Sundblad & Bergström 2014). Changes in the long-term development of the abundance of perch could hence reflect effects of increased water temperature and eutrophication in coastal areas and/or changes in the level of exploitation or predation pressure.
The abundance of flounder is favoured by somewhat increasing water temperatures, moderate eutrophication and low fishing pressure (Olsson et al. 2012; Florin et al. 2013). Increased presence of ephemeral macroalgae due to eutrophication reduces the suitability of nursery habitats (Carl et al. 2008), and increases in the level of predation from avian predators negatively affect the abundance of juvenile flounder with unfavourable consequences to recruitment (Nielsen et al. 2008). Changes in the long-term abundance of flounder thus may reflect effects of eutrophication and/or changes in the level of predation pressure and fishing mortality in coastal areas. Recent studies have also suggested an impact of the invasive species round goby on the abundance of flounder (Ustups et al. 2016).
Natural interactions such as predation pressure from apex predators, foremost cormorants (Phalacrocorax carbo), could at least locally impact the state of coastal fish communities (Vetemaa et al. 2010; Östman et al. 2012; Hansson et al. 2017). In some areas the outtake of coastal fish by cormorants exceeds, or is of a similar magnitude, to that of the commercial and recreational fisheries (Östman et al. 2013). However, the natural mortality from other sources such as predatory fish can be much higher than the mortality caused by cormorants in some areas (Heikinheimo et al. 2016), and compensatory mechanisms may counteract the effects of predation. In the Archipelago Sea, for example, there was no change in the mortality of perch during the period when the cormorants invaded the area, compared to earlier decades (Heikinheimo and Lehtonen 2016). Further, no connection was found between commercial perch and pikeperch CPUEs and numbers of breeding cormorants along the Finnish coast (Lehikoinen et al. 2017).