Rescuing the ‘jawless’ fish

Water August 04, 2020

RSK Principal Aquatic Consultant Matthew Davison has been rescuing an unusual species from the River Itchen.

Following his addition to the RSK Biocensus aquatic ecology team in early March, Principal Aquatic Consultant Matthew Davison has been carrying out work to support the Winchester Flood Alleviation Scheme on behalf of Hampshire County Council (HCC) and Winchester City Council (WCC). The scheme involves the installation of three penstock sluice gates to support flood defence improvement works within the River Itchen, north of Winchester city centre.

As a classic example of a chalk river, the Itchen supports high densities of protected aquatic species, including white-clawed crayfish (Austropotamobius pallipes), bullhead (Cottus gobio) and Atlantic salmon (Salmo salar), as well as brook lamprey (Lampetra planeri), southern damsel fly (Coenagrion mercurial) and otters (Lutra lutra). Following a walkover assessment by Matthew to characterise the on-site aquatic habitats, it was determined that there was a high likelihood of localised disturbance to brook lamprey, and so a targeted rescue and translocation plan was prepared to safeguard juvenile (ammocoete) lamprey that may be inhabiting the site.


Matthew translocating lamprey ammocoetes

Matthew translocating river/brook lamprey ammocoetes

What are lamprey?

Lamprey belong to a primitive group of fishes, the Agnatha, which lack scales, paired fins and even jaws. Instead of jaws they have a characteristic sucker-like oral disc, which is used for attaching to a host (usually a larger fish) to allow ectoparasitic feeding. There are three species of lamprey in the UK, sea lamprey (Petromyzon marinus), river lamprey (Lampetra fluviatilis) and brook lamprey (Lampetra planeri). Brook lamprey are the smallest of the British species, but they are also the most abundant and widespread and the species of main concern for the project Matthew has been working on.

Lamprey ammocoete

Lamprey ammocoete

Brook lamprey do not feed as adults and are highly cryptic by nature, making it rare to see them until it comes to spawning time. Spawning typically occurs between March and May, when water temperatures reach 10–11oC. During this time brook lamprey may congregate in their thousands, making them vulnerable to predators such as otters, birds and other fish.

Spawning takes place in flowing water, where the current is not too strong, over a bed of sand, gravel and stones within which they form a shallow depression to lay their eggs. These habitats are typically located in upstream environments, in relatively shallow water at the lower end of pools or where the flow starts to form a riffle. Adults then succumb to the energetic costs of spawning and die, thus completing their lifecycle.

After hatching, the ammocoetes drift downstream in search of suitable nursery habitats. These generally consist of stable mud, silt or silty–sandy accretions more than 15cm in depth that are relatively high in organic content and located in partially-shaded areas favourable to diatom (a type of algae) growth. Such habitats are usually associated with areas of little to no perceptible flow in the river and stream channels, such as the inside of tight meanders; the edges of pools, riffles and glides; vegetated or unvegetated bars; and downstream of deposition-causing structures, such as berms, deflectors, weirs and bridge foundations. In addition, rooted macrophytes (aquatic plants), such as branched bur-reed Sparganium erectum, also form nursery habitats by providing substrate stability, shade and reduced water flow.


Lamprey nursery habitat

Nursery habitats

 Ammocoetes may spend up to six-and-a-half years buried in nursery environments before metamorphosing into adults between July and September. After this time, brook lamprey adults emerge from the sediment and begin migrating upstream in search of suitable spawning habitats. The adults continue to burrow in silt or hide under stones during the day until spawning begins, when the water temperature starts to rise in spring and the lifecycle begins again.

Works on the HCC and WCC flood alleviation scheme began in early June and the first successful ammocoete rescue and translocation is now complete. Matthew will return to site to resume mitigation works later in the month.

Electro fishing onsite

Electrofishing on site

Releasing ammocoetes

Releasing ammocoetes