The brook lamprey ( Lampetra planeri ) is a key species in Danish running waters and is listed as an Annex II species in the European Habitat Directive. Still, its ecological role is very poorly studied, which restricts the use of efficient conservation measures. Larvae (ammocoetes), identified as the genus Lampetra , were sampled and assessed in a small Danish brook, Ovnstrup bæk. We used a 300 × 400 mm bottom sampler, which...