Prior to its description, this taxon was recognised as an ecologically distinct contingent of P. flesus (Linnaeus 1758), but without any distinct morphological differences (e.g. Florin and Höglund 2008).

Evidence provided by Momigliano et al. (2018) demonstrates that two parapatric species co-occur in the Baltic Sea, and P. solemdali is considered valid for Red List purposes as per Fricke et al. (2023).

Global and European regional assessment: Least Concern (LC)
EU 27 regional assessment: Least Concern (LC)

The Baltic Flounder is a recently described species known only from relatively shallow waters of the Baltic Sea, where it can be common and locally abundant. It is an important commercial species, but catches are primarily taken in demersal fisheries targeting other species (cod, sole, and plaice).

As morphological differentiation of adults and juveniles of this species and the co-occurring European Flounder is difficult, it is not distinguished in fisheries catches and fishery-independent surveys. Despite this limitation, some information is available regarding the relative proportion of Baltic Flounder throughout its range. Landings are reported from several ICES subdivisions in the Baltic, with variable trends in each of the stocks, but overall, there does not appear to be significant declines in the global population of this species.

However, Baltic Flounder has a restricted distribution and is sensitive to fishing pressure and other anthropogenic activities, including those resulting in environmental changes (salinity, turbidity, possibly toxicity). More work is needed to better partition the available fishery data, including landings and discards, to the two flounder species to improve estimates of population size and trend, which, in turn, will benefit fishery management and future assessments.

This species is currently assessed as Least Concern, both globally and for the EU 27 member states.

This species is known only from the northern and central Baltic Sea (Momigliano et al. 2018, Jokinen et al. 2019).

Baltic Flounder co-occurs with European Flounder (P. flesus) throughout much of the Baltic Sea in proportions that vary spatially. In the southern Baltic Sea, European Flounder is the predominant species, whereas in the central and northern Baltic Sea, Baltic Flounder is the predominant species (Momigliano et al. 2018; Jokinen et al. 2019; ICES 2021a,b,c). Juveniles and adults of these two species are morphologically indistinguishable, therefore, they are not separated by species in fisheries or in the fisheries-independent trawl survey (ICES 2021b,c). Some indications of the relative proportion of these two species in different ICES areas are available based on otoliths, molecular evidence, and morphometry (Ojaveer et al. 2017, Joniken et al. 2019), but more work is needed to partition the data by individual species.

Subdivisions 27 and 29–32 (northern central and northern Baltic Sea)
Based on ICES advice, the following information is available for Platichthys spp., of which P. solemdali is the predominant flounder species. Landings declined from the 1980s to the lowest annual levels in the mid-1990s. A slight increase occurred through the early 2000s, followed by a steady decline to the present (ICES 2021a). The stock size indicator, based on a fisheries-independent trawl survey, generally fluctuated without a clear trend; however, an unusually high peak in 2015 was followed by a decline to the present. Fishing pressure is below the length-based F_MSY proxy (ICES 2021a). In this region, flounders are primarily taken as bycatch in sole and plaice fisheries; discarding occurs but is poorly documented. Recreational landings are thought to be significant but are unquantified (ICES 2021a).

In the Gulf of Finland, attempts to disentangle the relative proportions of these two species using DNA extracted from archival otoliths suggested that P. flesus was historically more common than P. solemdali. In the mid-1980s, P. flesus was essentially extirpated from the Gulf of Finland, but it has more recently represented a small (<10%) proportion of the individuals taken from this region. The results suggested that the Gulf of Finland is a sink for P. flesus (Joniken et al. 2019, Momigliano et al. 2019).

Subdivisions 26 and 28 (east of Gotland and Gulf of Gdansk)
Based on ICES advice, the following information is available for Platichthys spp., of which P. solemdali represents approximately 55%. In this region, landings have been variable, with a recent decline (2013-present). The stock size indicator, based on a fisheries-independent trawl survey, fluctuated, but exhibited a significant decline over the time series (2000-2020). Fishing pressure is below the length-based F_MSY proxy (ICES 2021b).

Subdivisions 24 and 25 (west of Bornholm and southwestern central Baltic)
Based on ICES advice, the following information is available for Platichthys spp., of which P. solemdali represents approximately 15%. In this region, landings were stable from 1975-1995, after which landings increased more than 2-fold. Landings peaked in 2013-2016 and declined to the present (ICES 2021c). The stock size indicator, based on a fisheries independent trawl survey, increased 4-fold between 2000 (the beginning of the time series) and 2015, but declined recently by 30-40%. Fishing pressure is below the length-based F_MSY proxy (ICES 2021c). In this region, flounders are primarily taken as bycatch in fisheries targeting other demersal species (primarily Cod, Sole, and Plaice). Discarding ratios vary between countries, gear types, and seasons, and discarding practices are driven, in part, by market price and cod catches (ICES 2021c).

This species occurs on soft and hard substrata in brackish water of varying salinities, typically at depths of 0.5-50.0 m (Momigliano et al. 2018). Baltic Flounder is a benthivore, feeding on various mussels and other benthic invertebrates from its postsettled juvenile stage onwards (Aarnio et al. 1996, Borg et al. 2014).

Adults undertake only limited migrations between deeper wintering grounds and shallower spawning and feeding areas (e.g., Aro and Sjöblom 1983, Momigliano et al. 2018). Spawning occurs in April–June in coastal, nearshore areas and banks at 5 to 20 metres depth with a preferable water temperature of around 8°C (Sandman 1906, Bonsdorff and Norkko 1994, Nissling and Dahlman 2010). Successful reproduction can occur in salinities as low as six psu (Nissling et al. 2002, Momigliano et al. 2018). Mature females lay up to two million relatively small, demersal eggs that after fertilization develop on the bottom (Nissling et al. 2002, Nissling and Dahlman 2010) on stony, sandy, or vegetated substrates (Sandman 1906, Bonsdorff and Norkko 1994). Little is known about the larval or juvenile phases, but it likely has a similar early life history to that of the European Flounder (see Goldsmith et al. 2015). More work is needed to evaluate these life history stages.

No data are specifically available regarding the age and length at maturity for Baltic Flounder. However, around Gotland in the central Baltic Sea, where both P. flesus and P. solemdali co-occur, female flounders recruit to the adult population and attain maturity around the age of three years (range: two to five) at a length of 20–25 cm (Erlandsson et al. 2017), with males often maturing earlier (two to three years) than females (three to four years). As maximum age is unknown for Baltic Flounder the generation length estimated for European Flounder is used as a proxy: about five years, based on a maximum age of seven years and an age at first maturity of about 3 years.

The largest specimen examined by Momigliano et al. (2018) was 28 cm standard length, but this species likely attains a maximum size similar to that of European Flounder, about 50-60 cm standard length (Muus and Nielsen 1999).

This species may be negatively impacted by overexploitation and a number of other anthropogenic activities that affect habitat quality. Across all stocks where Baltic Flounder occurs, the current fishing pressure is less than the length-based F_MSY proxy. However, locally, exploitation has negatively impacted mixed-species stocks of flounders. For example, landings and catch per unit effort in the Gulf of Finland declined substantially since the mid-1970s (Jokinen et al. 2015, ICES 2019).

Previously, this decline was thought to reflect a decline in the local subpopulation of Baltic Flounder, but more recent work suggests that the decline was due primarily to the disappearance of European Flounder, followed by a decline in Baltic Flounder (Jokinen et al. 2019, Momigliano et al. 2019). The factors affecting these declines include environmental changes, such as decreasing salinity (caused by climate change: Vuorinen et al. 2015) and the ongoing deoxygenation of the Baltic Sea (a result of eutrophication: Carstensen et al. 2014). These changes have affected the spawning areas of both species, deep-water areas of the southern Baltic, where European Flounder spawns, and shallow areas of the northern and central Baltic, where Baltic Flounder spawns (Jokinen et al. 2019).

This species may also be negatively impacted by contamination of water and sediments (e.g., Von Westernhagen et al. 1981, Dabrowska et al. 2016).

As environmental conditions continue to change, further range contractions of the two flounders may likely occur (Vuorinen et al. 2015).

Baltic Flounder is an important food source in the Baltic Sea (Wheeler 1969). A directed fishery for the two native flounders (European Flounder and Baltic Flounder) exists in parts of the Baltic Sea. Elsewhere in the Baltic Sea, it is caught incidentally when fishing for other species, particularly in beam trawl, otter trawl, and gill net fisheries. It is a valued bycatch of the cod fishery (Goldsmith et al. 2015). In some areas of the Baltic Sea, recreational landings may be substantial but are currently unquantified.

In the Baltic Sea, local flounder subpopulations are currently managed as single stocks of P. flesus ecotypes (ICES 2021b,c). ICES provides stock status updates but has not been requested to provide advice on fishing opportunities for the stocks that include a substantial proportion of Baltic Flounder. No total allowable catch limits are in place for the management areas where this species occurs (ICES 2021a,b,c).

The genetic test developed and refined by Momigliano et al. (2018, 2019) provides a means for the unambiguous identification of Baltic Sea flounder species and could be employed for real-time monitoring and mixed-stock analyses of catches.