Demography of sea lamprey (Petromyzon marinus) ammocoete populations in relation to potential spawning‐migration obstructions

Abstract

Abstract Recent advances in the understanding of lamprey migrations have led to concerns over the impacts of obstructions on the demography of many species. This study investigated sea lamprey (Petromyzon marinus) larvae (ammocoetes) in two adjacent but contrasting rivers, both designated Special Areas of Conservation under the EC Habitats Directive (92/43/EEC), one (the River Wye) with a small number of potential migration obstructions in its upper reaches and one (the River Usk) with obstacles along its course. The geographical distributions, densities and age structures of the ammocoete populations were examined in relation to the locations of potential obstructions to the spawning migrations of anadromous adults. A minimum of three age classes was recorded as far as 200 km upstream of the mouth of the River Wye (93% of the length of the mainstem), demonstrating that adults regularly migrate to the upper reaches of the catchment (downstream of a natural waterfall). By contrast, sea lamprey ammocoetes appeared to be absent (in suitable habitat) from 20 km (17%) of the River Usk, and there was a reduction in density, prevalence and the number of age classes upstream of two putative spawning‐migration obstructions. This study highlights some of the potential impacts of habitat fragmentation by obstructions on the spawning migrations of anadromous species, as inferred from ammocoete demography. When used in combination to compare contiguous reaches, ammocoete densities, prevalence and age structure may be a useful indicator of which structures are likely to be important migration obstructions, and where further studies or mitigation efforts should be focused. It is likely that passage past some obstructions is enhanced if high river levels occur during the spawning migration, but there is a need to facilitate passage during all conditions, to improve access to under‐exploited spawning and nursery areas.