Length-based assessment of larval lamprey population structure at differing spatial scales

Perspectives on lamprey management contrast between pest control in the US Great Lakes and species conservation in the Pacific Northwest, New Zealand, and Europe. Five lamprey species are listed in the European Union (EU) Habitats Directive (HD) as requiring conservation measures. Assessments of HD 'conservation status' for these lampreys mainly target the larval (ammocoete) stage. Larval lamprey populations can be assessed by evaluating presence or absence, density and demographic structure in riverine samples. Demographic structure has typically been described from length-frequency data using qualitative approaches and the statistical assignment of age classes. Length-based indicators (LBIs) may provide a more rapid and flexible framework. The demographic structure of a fish population can be described by univariate length metrics: length range (L-RANGE) and 90(th) percentile of length (L-90). This study used a pooled data set from seven Irish catchments to estimate a reference point (RP) value for each metric corresponding to a healthy larval lamprey population. Two LBIs were then derived that can be estimated for an observed (OBS) population as LBIRANGE = L-RANGE_OBS/L-RANGE_RP and LBI90 = L-90_OBS/L-90_RP. Simulated lamprey population length structures, representing a range of status values, were used to develop a reference gradient (RG) to support population assessment using LBIs. The assessment framework was applied to each of the seven Irish catchments, and also to subcatchment and sampling-site scales within the Barrow system. The LBIs and RG suggested that most catchments had favourable conservation status overall, whereas smaller assessment scales revealed a spread of status values. The proposed framework allows the rapid assessment of demographic structure and comparison across systems. There is potential to track states across sampling sites, events, and legislative reporting cycles, and to interpret change in relation to the local environment. The general approach may be adaptable for other fish species monitored during juvenile stages.