Rapid methods for quantitative fish larvae community assessment using metabarcoding. Ichthyoplankton metabarcoding

1. Climate change stressors greatly impact the early life-stages of many organisms, however, the cryptic morphology of larvae renders them difficult monitor using morphological identification. High-throughput sequencing of DNA amplicons (metabarcoding) is potentially a rapid and cost-effective method to monitor larvae for management and environmental impact assessment purposes. Nonetheless, there is conflicting information about whether metabarcoding is sufficiently quantitative for such applications.2. We compared metabarcoding with traditional morphological identification to evaluate taxonomic precision and reliability of abundance estimates, using fish larvae from 14 offshore sites in the Irish and Celtic seas. To improve abundance estimates, biomass input for each specimen was standardised and 12S mitochondrial primers with conserved binding sites were used and family level correction factors were applied. 3. Family level relative abundances, richness (Rs=0.81, P=0.007) and diversity (Rs=0.88, P=0.003) were positively correlated across the study. Where amplification bias occurred, family level correction factors further improved abundance estimates. Community composition analysis using both metabarcoding and morphological assessment detected similar patterns of spatial distribution, independent of assessment method.4. We demonstrated that, by using a conserved single marker, standardising biomass input and applying correction factors, bulk tissue metabarcoding can be used to accurately estimate abundance in a field context. This represents a feasible and efficient alternative to the much more time consuming morphological identification for monitoring change in larval communities.