Distribution of brook lamprey (Lampetra planeri) ammocoetes in the sediment of a brook

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 has been found useful to collect ammocoetes in previous studies. This proved efficient in quantifying ammocoetes in a brook where previously conducted surveys, based on electrofishing, did not report catches of lamprey. The method generated 390 samples and 179 ammocoetes were caught enabling mapping of the spatial distribution, along with substrate types and stream parameters, throughout the brook. Larvae were not found in all areas of the brook. Our results suggest that larvae prefer to burrow at the margins of the brook in sediments characterized as mud (< 0.25 mm) and lacking stone (60–300 mm) and gravel (30–60 mm). The average density was 4.2 No./m² with no ammocoetes observed in the upper part where the brook is smallest, and up to 8.5 No./m² in the middle part. No differences in average length between sample-site locations were found, suggesting that ammocoetes size classes are heterogeneously distributed in the brook. Areas of the brook where ammocoetes were not found or where densities were low could be targeted for habitat restoration. A bottom sampler can effectively be used to assess the distribution and habitat use of larval brook lampreys in a small stream.