The European River Lamprey is considered to represent the ancestral component of a "species pair" alongside the congeneric European Brook Lamprey (Lampetra planieri).

In genetic analyses, these two species are indistinguishable by single- or few molecular markers, and have been clearly differentiated only through genomic data (Espanhol et al. 2007, Mateus et al. 2013). They often co-occur at breeding sites (e.g., Lasne et al. 2010) and are able to hybridise (Hume et al. 2013, Rougemont et al. 2015). Their larvae are morphologically very similar, but adult European River Lampreys are physically larger than European Brook Lampreys.

These species also differ in their behaviour during the adult life stage in that the European River Lamprey is anadromous and migratory with a parasitic adult stage, whereas the European Brook Lamprey is sedentary in freshwater environments and adults are nonparasitic.

There is evidence that southern subpopulations are genetically more divergent than their northern counterparts, suggesting that the two taxa are undergoing disparate stages of speciation at different locations, with the Iberian Peninsula probably representing a hub of postglacial colonisation (Mateus et al. 2016, De Cahsan et al. 2020).

Their systematics have thus been considered problematic in the past, but the majority of recent publications continue to treat them as separate taxa (e.g., Pereira et al. 2021).

Global and European regional assessment: Near Threatened (NT)
EU 27 regional assessment: Near Threatened (NT)

The European River Lamprey does not approach the range thresholds for a threatened Category under Criterion B (extent of occurrence (EOO) < 20,000 km², area of occupancy (AOO) < 2,000 km²) or D2. The population size significantly exceeds 10,000 individuals, hence it does not approach the thresholds for Criteria C or D, and there exists no quantitative analysis which would permit application of Criterion E.

Although no range-wide population trend data exists, a suspected ongoing reduction based on declining habitat quality might approach or meet the threshold for Vulnerable under Criterion A2 (≥ 30% over the past 12 years = three generations). As a result of this data uncertainty, Least Concern and Vulnerable are equally plausible Red List categories for the present assessment, and this species is assessed as Near Threatened (A2ce) both globally and for the EU 27 member states.

This species is native to the Northeast Atlantic basin in Europe, where its range extends from the Baltic Sea basin to southern Norway, the North Sea and Atlantic coastlines of the British Isles and France. Isolated subpopulations also exist in the Iberian Peninsula (lower Tagus River, Portugal) and the central Mediterranean basin (Magra River system, Italy). Landlocked subpopulations inhabit the Ladoga and Onega lake basins in Russia, a number of lakes in Finland, Loch Lomond in Scotland, and possibly Lough Neagh in Northern Ireland.

It is considered invasive in the upper Volga River, Russia, which it is believed to have entered through artificial shipping canals constructed since the 18^th century.

It should be noted that the upstream limits of this species' range prior to its 20^th century decline (see 'Population' and 'Threats') are often unclear. Moreover, some of the available records are likely to correspond to the European Brook Lamprey (Lampetra planeri), particularly those based on larvae (see 'Taxonomic Notes'). A broadscale approach mostly incorporating entire river systems was therefore adopted to produce the distribution map accompanying this assessment.

This species' population size is unknown, but is understood to far exceed the minimum threshold for Red List criteria (< 10,000 mature individuals).

The current population trend has not been quantified, and the number of subpopulations is unclear. However, the species has declined progressively throughout its range since the mid-19^th century (see 'Use and Trade' and 'Threats'), and this pattern is suspected to be ongoing in some parts of its range. Demonstrable declines in abundance have been reported in several countries since the turn of the century.

In particular, it is today considered to be nationally Extinct in Belgium, Czechia, Switzerland and Spain, with only single subpopulations remaining in Portugal and western Italy. In contrast, the Rhine River subpopulation in the Netherlands and western Germany has reportedly increased in abundance since the late 1980s (see 'Conservation').

This species is a semelparous migratory predator with a life cycle characterised by a blind, microphagous larval (ammocoete) phase, which spends several years buried in freshwater sediments of sluggish river and stream reaches before metamorphosing into the adult form that feeds parasitically on the flesh of other fishes.

Metamorphosis occurs at the end of this growth stage, at which point individuals develop teeth, functional eyes and an osmoregulatory capacity allowing them to enter marine environments. These sexually immature young adults subsequently migrate downstream to the sea, where they spend a further 1-2 years feeding on the flesh of other fish species. Some subpopulations, which have been referred to as "praecox" forms, exhibit a truncated marine stage which may last just a few months.

Once fully mature, individuals cease feeding and migrate back to rivers where they spawn and die. The timing of the annual reproductive period varies depending on location, taking place for example from February to April in France and Germany, March to May in the United Kingdom and May to early July in Finland and Russia. Reproduction generally coincides with water temperatures rising above 9°C.

This species requires well-washed beds of gravel and small cobbles with fast-flowing water and nearby backwaters for successful spawning and larval development. Spawning takes place within a "nest", normally comprising a bowl-shaped indentation excavated from the substrate with a rim of stones deposited immediately downstream. However, in some cases, larger groups of 10-100 individuals participate in mass-spawning and turning of gravel over a patch several metres wide without forming conspicuous depressions.

Spawning itself involves the male attaching to the head of the female and wrapping his tail around her body, at which point the gametes are released. However, alternative spawning strategies such as males exhibiting sneaker behaviour and the release of gametes without physical pairing have been reported. Fertilisation is external, and fecundity has been reported to vary from 650-42,500 eggs per individual female.

Although the nests formed during courtship and spawning have typically been described as shelters constructed to protect the eggs and yolk-sac larvae, recent evidence suggests they may actually function as structures to facilitate downstream egg dispersal.

Hatching occurs c. two weeks after egg deposition, and one to three weeks later the larvae emerge from the substrate and drift downstream to settle in sheltered areas, where they bury into silt beds. These initial movements take place mostly at night and larval distribution is dependant on the morphology and flow characteristics of the individual river. Passive migration during flood events is a regular occurrence.

Once settled at a site, the head of each larva protrudes above the substrate with its mouth directed towards the current in order to trap drifting food particles. The larvae are believed to be generalist feeders, filtering algae such as desmids and diatoms, detritus and microscopic animals from the water column, although some benthic organisms may also be consumed.

The larval stage lasts 2.5 to 4.5 years, depending on location, and tends to be more extended at northerly latitudes. Metamorphosis begins during the summer, and the initial mobile, non-feeding subadult stage is known as the macropthalmia. Downstream migration is initiated by increased discharge and typically occurs from late autumn to spring.

Upstream migration of sexually mature adults mostly takes place at night, and is dependant on latitude, temperature and river discharge. It generally begins during the late summer and autumn of the year prior to spawning, and continues until the following spring.

Both downstream and upstream migration are suspended when water temperatures become too cold, giving rise to the terms "autumn-run" and "spring-run", which are mostly used in reference to adult animals.

Adults do not feed once their upstream migration has commenced, and development of the gonads is accompanied by atrophy of most internal organs and tissues. Field observations suggest they are able to pass smaller artificial and natural barriers.

Although the European River Lamprey is primarily anadromous, some lacustrine subpopulations complete their entire life cycle in freshwater.

This species' decline has been driven by an interacting series of anthropogenic factors.

In particular, large-scale river engineering projects designed to manage seasonal flooding, connect river systems for transport and exploit water for agricultural, domestic and industrial purposes have been ongoing since the 19^th century.

For example, the construction of dams and other barriers has blocked upstream migration routes, caused subpopulations to become fragmented, and hindered natural flow and sedimentation processes. The extent of suitable habitat for egg deposition and larval development has thus been significantly reduced throughout the European River Lamprey's range, e.g., by an estimated 80% in the Iberian Peninsula.

Several other forms of habitat modification are considered to negatively affect this species. Extraction of sand from riverbeds can directly destroy ammocoete habitats, while dredging can drastically modify important spawning sites. Regulation of channels and banks may eradicate entire subpopulations through the loss of vital river features such as riffles and silt beds. Certain land management practices have resulted in increased soil erosion and sedimentation of gravel beds used for spawning.

Widespread pollution from agricultural, domestic and industrial sources has further reduced habitat quality. There exist numerous documented instances of European River Lamprey subpopulation decline or extirpation attributed to poor water quality, e.g., the Thames River in England and Clyde River in Scotland. Adults can be negatively affected by pollution barriers during their spawning migrations, eutrophication due to increased nutrient loads can cause spawning beds to be smothered by algae, and ammocoetes are probably sensitive to high concentrations of environmental contaminants.

Overfishing for commercial harvest may have contributed to its earlier decline, and remains a plausible threat in the Baltic region, where illegal harvest may also be an issue.

Increasing temperatures due to climate change could drive shifts in range, alter migration patterns and impact physiological processes, but this has not been studied in depth.

This species' use as a food fish throughout much of Europe dates back centuries, but it is today marketed as such only in Finland, Russia, Estonia and Latvia, and to a lesser extent in Sweden and France.

However, annual landings in all of these countries have declined considerably since the mid-20^th century and continue to do so, e.g., currently c. 67 tonnes in Latvia vs. 241.3 tonnes during the 1960s and 1970s, c. one million individuals in Finland vs. 2.7-3.0 million in the 1970s.

In the United Kingdom, subadult individuals were from the 1700s sold as bait for use in North Sea long-line fisheries, with large quantities exported to the Netherlands. The only remaining licensed European River Lamprey fisheries in the United Kingdom are on the Ouse and Trent rivers in eastern England, from where the harvest is sold to recreational anglers.

Formerly thriving fisheries in a number of major rivers within this species' range, e.g., the Rhine, Weser, Elbe, Oder and Vistula, crashed entirely because of local stock declines during the late 20^th century. In Poland, for example, annual landings of 80 tonnes were reported during the 1970s, but this had fallen to c. 900 kilogrammes by the late 1980s.

This species is included in Appendix III of the Bern Convention and Annexes II (except in Finland and Sweden) and V of the European Union Habitats Directive. It is nationally-protected and/or assessed as threatened in the National Red Lists or Red Data Books of a number of countries within its range. Numerous subpopulations occur within the boundaries of protected areas, many of which are included in the European Union's Natura 2000 network.

Since the European Union Habitats Directive was adopted in 1992, most member states to which the European River Lamprey is native have designated Special Areas of Conservation (SACs) for its specific protection. In Ireland, for example, these SACs must feature good water quality, clean substrate at known spawning sites, the presence of downstream silt beds and unobstructed access to the sea.

Its conservation has been of interest since at least the 1980s, when proposed measures included lamprey-friendly fishways (because those constructed to facilitate passage of other anadromous fish species such as salmonids are rarely adequate), physical transportation of migrating adults above impassable dams, improved treatment of domestic sewage, pollution control and reduced regulation of water levels in spawning rivers.

In Russia, Finland, Estonia and Latvia there have been attempts to mitigate declines in commercial stocks by releasing hatchery-produced larvae into a number of rivers, e.g., between 1997 and 2010 around 210 million larvae were released into the Perhonjoki River, Finland. Habitat restoration including the creation of new spawning areas, larval habitats, and artificial rapids, modification of fishways, and transportation of gravel to degraded riffles has also been carried out in the Kalajoki, Perhonjoki and other Finnish rivers. In Finland, Latvia and Sweden, migrating adult lampreys have in some cases been physically transported around impassable dams. Despite these efforts, reports suggest that overall abundance in the Baltic region continues to decline.

Elsewhere, the subpopulation inhabiting the Rhine River appears to be recovering from its earlier decline due to improvements in water quality and connectivity that have occurred since the international Rhine Action Programme was launched in 1988.

Fishing bans have been implemented in some countries, e.g., Poland, with bag limits, quotas and/or closed seasons in others, e.g., United Kingdom, Russia, Finland, Estonia, Latvia and Lithuania. The commercial fisheries in Russia and the Baltic nations are also regulated in terms of permitted locations, number of licenses, types of gear and their deployment.

In the Netherlands, recent changes in national fishing regulations stipulate that river lampreys caught as bycatch can no longer be sold to the United Kingdom bait industry and must be returned to the water alive.

It is recommended that future research efforts should focus on establishing this species' population trend across its entire range, understanding the effects of poor water quality on its life cycle, investigating the factors that might result in successful habitat improvements (e.g., barrier removal, passage solutions), and examining the potential effects of climate change. The ecology and behaviour of trophic juveniles is also poorly understood. In practice, such efforts will be most efficiently coordinated at local, national or regional scales.

Some subpopulations exhibit marked genetic divergence, and this should be taken into account when considering conservation efforts, particularly those that involve translocation or use of hatchery stocks.