Population size estimation of larval coregonids in large lakes: Stratified sampling design with a simple prediction model for vertical distribution

The lake-specific vertical distribution of larval coregonids was assessed annually in Finnish lakes by bongo net sampling. The effect of inter-annual and inter-lake variation in vertical distributions on population estimates of larval coregonids were analysed in 1999–2008. The vertical distribution of newly hatched vendace (Coregonus albula (L.)) and European whitefish (Coregonus lavaretus L. s.l.) larvae was analysed and high inter-annual and lake-specific variation in the larval vertical distribution was found. Generally, in both littoral and pelagic areas, larvae were aggregated near the water surface, mostly in the top 30 cm layer. We compared observed and predicted density estimates in vertical water column and found high correspondence. The lake-specific annual vertical distributions of coregonid larvae had a marked effect on the annual estimates of their population. Thus, reliable assessment of the annual vertical distributions of coregonid larvae in each lake must be included in the population estimation procedure.