Single pass electrofishing method for assessment and monitoring of larval lamprey populations

Our work aims at calibrating and standardizing the single pass protocol of electrofishing to assess and monitor larval lamprey populations, mainly for European populations of sea lamprey (Petromyzon marinus). The method is considered cost-effective and appropriate for use both on the scale of management decisions (from river or watershed to a regional or national scale) and as a routine assessment tool as required by the European Union Habitats Directive. Additionally, the larval surveys may be reconciled with routine electrofishing surveys for fish species, to further reduce costs. In our case, the escapement of larvae from the sampling area when using electrofishing corresponded to 18 % of larvae and 17 % of larval biomass. The general effectiveness of the protocol to determine the density and biomass of age-1 and older larvae in the prospected area was 68 % and 70 %, respectively. Finally, reference categories for density and biomass of larval populations of P. marinus in NW of the Iberian Peninsula are provided.