Adult spawning habitat
Lampreys spawn in similar habitats to salmonids; substrate preferred by Sea Lampreys broadly corresponding with Atlantic salmon Salmo salar spawning areas and the precise characteristics of smaller River and Brook Lampreys spawning in areas similar to sea and brown trout Salmo trutta. The most important factors involved in the location of the spawning ground are the presence of a suitable clean substrate for the construction of redds and a relatively stable current flow. Small amounts of sand are also needed to which the eggs adhere to and this in turn will imbed the eggs in the interstices of the gravel in the nest rim, the effect of which is to consolidate the mass while leaving it permeable to the water. The River Lamprey may sometimes share the same spawning grounds as the Brook lamprey; however, River Lamprey can use move stones up to 5cm in diameter and 150g in weight during nest construction.
In discussing the habitat requirements of adult lamprey, it is important to acknowledge that the fundamental passage requirements of both Sea and River lamprey species are met in advance of any suitability assessment of spawning habitat. It is imperative that the upstream passage to the spawning habitat is not blocked or indeed impaired, where impairments and impediments to passage can result in significant timing delays and expenditure of adult energy reserves which are required for successful breeding.
The life cycle of the all three species of lamprey found in Ireland includes a sedentary larval stage during which the juvenile lamprey (ammocoetes) remain buried in depositing areas of watercourses. Suitable juvenile lamprey habitat is typically found at locations on watercourses with no perceptible flow such as at the inside of tight meanders, the edges of pools (sometimes riffles and glides), bars (vegetated and un-vegetated) and depositing stretches of river. Rooted macrophytes are also a positive component of larval lamprey habitat (Maitland, 2003), providing substrate stability, shade and slowing down flow (Maitland, 2003). Hydro-morphologically, the lower river reaches are most likely to contain the best juvenile lamprey habitat. Some artificial structures alter flow regimes and can result in the creation of silt deposits used by ammocoetes. These include bridges, weirs and deflectors.
Where suitable substrates are present Lamprey larvae can occur in streams and rivers upstream as far as the adults are able to migrate. The burrow of the larvae may be detected by the funnel-shaped depression on the mud-surface. Lamprey ammocoetes were found in substrates with a mean particle size of 0.18mm-0.50mm. The habitat occupied by the larvae of all species seems to be very similar. Optimal habitat is defined as stable fine sediment or sand >15 cm deep, low water velocity and the presence of organic detritus. These areas are often partially shaded and thus favourable for the growth of diatoms on which the larval stage of all three species feed on.
Sub-optimal habitat is generally described as shallow fine sediment, interspersed among coarser substrate. It is possible that small numbers of larvae may be supported by patches of suitable silt in comparatively high-velocity, boulder-strewn streambeds. Sub-optimal habitats identified as containing lamprey larvae include areas of shallow organic detritus overlying bedrock; submerged tree roots trapping organic material; silt-dominated cattle drinks; and submerged bankside vegetation rooted in sand/silt. These latter sub-optimal habitats are likely to be temporary refuges due to their unstable nature in the event of floods. However, investigation of their use by lamprey ammocoetes is important during any survey