The river lamprey (Lampetra fluviatilis) is a fish in the lamprey family (Petromyzontidae) with a circular suction disc instead of jaws and a snake-like body. The river lamprey is an endangered migratory fish, with just a handful of spawning grounds in the Netherlands. The species is protected: it is designated as ‘sensitive’ on the Red List and is a Habitats Directive species for various Natura 2000 sites including the ‘Rijntakken’ (Rhine distributaries). From 2019 to 2022, on behalf of the Province of Gelderland and the Water Board ‘Vallei en Veluwe’, we studied migration pathways and bottlenecks to migration within the Grift drainage basin for river lamprey.
Adult river lampreys live in the sea. They are ectoparasites: with their characteristic oral disc they cling to bigger fish by suction and absorb blood and tissue. They swim to the sea from freshwater in the spring as young adults. After about a year and a half they migrate upriver to spawn. Then their entire digestive system is replaced by bigger reproductive organs to make spawning as effective as possible. River lampreys are therefore semelparous: they die after their ascent of the freshwater, regardless of whether or not they have successfully spawned. Conservation of the species is therefore heavily dependent on the accessibility of suitable spawning and nursery habitats.
After spawning, the larvae are briefly carried downstream by the flow, after which they dig themselves into soft brook and river sediments. There they feed by filtering organic waste and micro-organisms from the water (filter feeding). After five or six years they head off downstream to the sea, thereby completing the circle.
Illustration 1. Adult river lamprey are 40 to 45 centimetres long. With their characteristic oral disc they cling fast to the host. With their concentric rows of teeth they tear open the skin and feed on blood and tissue. Photograph: Jesper Berndsen, RAVON.
Impassable man-made barriers
On the way to their spawning grounds, river lamprey and other migratory fish are hindered by many man-made obstacles. Not reaching the spawning grounds or reaching them late has serious consequences, since spawning often takes place in a short time window with much competition for space and partners. The man-made structures are not just physical obstacles; they can also influence sedimentation, flow profiles, water temperature and oxygen level of the water, which may have adverse effects on migratory fish. Therefore it is important to reduce fragmentation and make promising spawning habitats accessible again. Countrywide, habitat fragmentation is a severe problem. In the Grift system, for example, a boat lock in the Apeldoorn Canal and a hydroelectric power plant (HPP) in the Oude Grift prevent free passage, but barriers are also present elsewhere, only some of which have fish passes.
Fish passes and lamprey tiles
Measures to reduce habitat fragmentation are crucial. Fish passes can be effective, but river lamprey cannot jump like salmon or climb vertical ladders like the related Pacific lamprey (Entosphenus tridentatus) of North America. Furthermore, lamprey, like eel (Anguilla anguilla) among others, have low sprint capacity. They can accelerate, but not very strongly or for very long. All these factors make many fish passes ineffective for the river lamprey.
One possible modification is mounting hard plastic studded or lamprey tiles to barriers. The studs slow the flow and provide grip for the river lamprey’s ‘burst-attach-rest’ mode of locomotion to pass barriers. This consists of a short burst followed by attachment to the substrate with the oral disc to aid recovery for another sprint and is repeated until the barrier is overcome. In the laboratory, studded tiles proved to increase passage efficiency at various flow rates and levels of turbulence (Vowles et al., 2017). In the UK the addition of lamprey tiles for facilitating upstream spawning migration of river lamprey has been studied and found to be effective (Tummers et al., 2018; Lothian et al., 2020).
The Grift basin
The Grift flows along the eastern edge of the Veluwe hills ridge and via the Apeldoorn Canal into the river IJssel (see map). This basin has long comprised important habitats for river lamprey. A preliminary study showed that streams and brooks that flow into the Grift are particularly suitable spawning and nursery habitats (De Bruin et al., 2018). Swimming up the river IJssel, the first major barrier for river lampreys on their way to those streams and brooks is the hydroelectric power plant (HPP) in the Oude Grift south of Hattem, with a height difference of four metres. There are some spawning and nursery habitats downstream of the HPP too, but these are smaller and of lesser quality. Furthermore the closely related non-migratory brook lamprey (Lampetra planeri), a Habitat Directive species of the Veluwe Natura 2000 area, resides in the brooks. River lamprey can spawn with brook lamprey and are even attracted towards brook lamprey by their release of pheromones.
Illustration 2. The River Grift and surrounding area. River lamprey migrate from the IJssel (yellow) via the Apeldoorn Canal (blue) and the Oude Grift, which runs parallel to it, towards the Grift (green) and its tributaries (pink). Red dots: PIT tag stations in 2020-2021, at the weir and in the three tributaries. Blue dot next to HPP = release location 2019-2020. Orange dots = release locations 2020-2021. (WKC = HPP; sluis = lock; stuw = weir; monding = mouth.)
In order to investigate how far upstream river lamprey migrate, 95 river lamprey were caught in the Oude Grift (downstream from the HPP) by means of electrofishing in December 2019. We inserted PIT (Passive Integrated Transponder) tags into them and once they had recovered from further handling (determining length and weight) they were released into the Apeldoorn Canal upstream of the HPP (illustration 2). As soon as these tagged specimens came close (about 1 metre) to the antennas of the PIT detection station near the Bonenburg Estate, their unique number was recorded as well as date and time.
The lock in the Grift near the Bonenburg Estate proved to be an impassable barrier for upstream migration lamprey in the winter of 2019-2020. Although 27 of the 95 tagged river lamprey (28%) reached the Bonenburg weir, some of them within 48 hours, not a single one managed to pass the weir. In principle they may have spawned in the stretch of the canal where they were released. Since no individuals were found with a mobile PIT scanner in the 150 metres below the weir, they evidently left the Grift.
Illustration 3. Bonenburg weir is a relatively small obstacle for migratory fish, which sometimes is even under water. To the left on the weir next to the lamprey tiles, the detection station. With two detection antennas we distinguished between the route via lamprey tiles and via the weir. Photograph: Jeroen Tummers, RAVON.
Lamprey tiles as a solution
To investigate measures of improving passage over the Bonenburg weir, in the winter of 2020-2021 it was equipped with a slope with lamprey tiles (illustration 3). This time we caught 101 adult river lamprey in the Oude Grift and tagged them. Of these, 44 and 57 individuals were released respectively downstream and upstream of the Bonenburg weir. Detection stations recorded the passing lamprey both at the weir and further upstream at entrance to the streams and brooks.
In the monitoring period, of the 44 river lamprey released downstream, 19 individuals passed the weir. Of the total of 76 river lamprey that either passed the weir (19) or were released upstream of it (57), 28 were detected in the upstream brooks with suitable habitat. Of these, 21 specimens were found in the Horsthoekerbeek.
The total number of passages of tagged individuals over the weir was 51, of which 30 via the weir itself and 21 via the lamprey tiles. The number of passages is higher than expected because current velocity may displace river lamprey back over the weir, and because lamprey may have been detected both on the lamprey tiles and on the weir.
In the 2021 spawning season, mid-February to mid-April, together with members of a local fish conservation group, we investigated the presence of river lamprey in the brooks and at the catch location in the Oude Grift using a mobile PIT tag scanner. We found 13 river lamprey, all in the Horsthoekerbeek, also upstream of the fixed PIT station operational at its entrance.
In January 2022 a last catch took place. We caught 157 river lamprey in the Oude Grift and released them upstream of the Bonenburg weir in the Grift. Judging by the results of the previous study period, it is likely that a good proportion of these river lamprey reached the spawning grounds in the streams and brooks and successfully spawned.
This study shows where the problems and opportunities lie for bolstering populations of river lamprey and brook lamprey in the Grift basin, and provides some concrete ideas for meeting the KRW objectives and the Natura 2000 conservation goals.
For the conservation and expansion of populations of river lamprey, greater insight is needed into its migration. This multi-year study has shown that river lamprey favour migration from the IJssel to the Grift system, but that the ascent is hampered by (nearly) impassable man-made structures (the lock at Hattem, the HPP in the Oude Grift and the Bonenburg weir). Lamprey tiles proved relatively effective at one location, but higher efficiency is needed. Targeted modification of the man-made structures or the installation of bypasses is necessary to facilitate passage, if possible without facilitating invasion by undesirable exotic species (such as various kinds of goby) via the Apeldoorn Canal.
Elsewhere in the Netherlands life is also difficult for the river lamprey. In the Meuse, river lampreys are hindered by large weir complexes that deny them access to the Grensmaas (‘Border Meuse’) and the ‘Maas bij Eijsden’, where it is designated as a protected species. Furthermore, in the Roer and Geul tributaries, where the brook lamprey is a target species, there are opportunities for conservation of the river and brook lamprey.
In a three-year study in the Grift drainage basin in Gelderland, river lamprey proved either unable to reach their spawning grounds or able to do so only with difficulty. Reasons are poor connectivity by man-made structures such as a boat lock, a hydroelectric power plant and weirs. River lamprey are poor swimmers, for which large barriers in particular but also many fish passes are impassable. A slope with lamprey tiles installed on a trial basis at a low weir proved relatively effective, but for much-needed protection of this Habitat Directive species, targeted adaptations of all man-made structures are necessary.
Bruin, A. de, et al., 2018. Study of river lamprey in the Oude Grift. RAVON Nijmegen, report 2017.144.
Lothian, A.J. et al., 2020. River connectivity restoration for upstream‐migrating European river lamprey: The efficacy of two horizontally‐mounted studded tile designs. River Research and Applications 36(10): 2013-2023.
Russon, I.J. & Kemp, P.S., 2011. Experimental quantification of the swimming performance and behaviour of spawning run river lamprey Lampetra fluviatilis and European eel Anguilla. Journal of Fish Biology 78(7): 1965-1975.
Tummers, J.S. et al., 2018. Enhancing the upstream passage of river lamprey at a microhydropower installation using horizontally-mounted studded tiles. Ecological Engineering 125: 87-97.
Vowles, A.S. et al. 2017. Passage of European eel and river lamprey at a model weir provisioned with studded tiles. Journal of Ecohydraulics 2: 88-98.